41 
On the Classification of Nemertes and Planarice. 
Art. IV.—An Essay o?i the Classification of Nemerles and 
Planarice: Preceded by some general considerations on the 
primary Divisions of the Animal Kingdom; by Charles 
Girard. 
Read before the American Association for the Advancement of Science, at NeVf 
Haven, August, 1850. 
I. 
I am gathering materials for a monograph of the Nemertes of 
this side of the Atlantic. I have already said on another occa- 
sion that 1 was doing the same with regard to the Planariae. 
It will not be anticipating the final results of these investiga- 
tions to present here some general considerations respecting the 
place that may conveniently be assigned to these animals in our 
zoological system. 
The chronological history and the position that the various 
authors have assigned to the Nemertes and Planarioe having been 
fully brought out by Mr. de duatrefages,* I shall not dwell upon 
that part of the subject. 
The question now under consideration bearing merely upon 
the fundamental groups of the animal kingdom, I need only go 
back to Cuvier, since he established these groups and since the 
foundations of our classification are the results of his labors. 
Nor shall I at present enter upon the secondary divisions to be 
established among the Nemertes and Planariae ; this aspect of the 
question cannot be settled until we are better acquainted with the 
organization of the group of Rhabdocoela or freshwater Planariae. 
II. 
Before however I treat of the Nemertes and of the Planariae 
in a more particular manner, I have a few words to say on the 
subordination of characters in the primary groups of the animal 
kingdom. 
There are in the animal kingdom but two main systems of 
organs to which all the others are related, being in a word, mere 
dependences or branches of them. These two systems of organs 
are : 1. the nutritive system or systern of vegetative life, and 2. 
the sensitive system or system of animal life. 
Now which of these two systems of organs is the most import 
tant; in other words, which ranks highest as a zoological char- 
acter. The nervous system, some physiologists will answer. 
But let us examine this point. What is the distinguishing charac- 
ter of the animal kingdom as a kingdom? A digestive cavity of 
some kind, into which are introduced nutrient substances neces- 
* Ann. Sc. Nat., 3d Serie, iv, 1845, p. 
VoL XI, No. 31.—Jan., 1861. 42 
On the Classification of Nemertes and Planar ice. 
sary for the maintenance of living bodies; necessary to their 
growth anterior to the period of full development and to theii 
equilibrium after having reached their complete size. The 
digestive system then is the universal character, common to all 
animals, and this character gives us the animal kingdom: it 
ranks then highest. 
Next to the entire animal kingdom we have the four great pri- 
mary divisions, the division of Vertebrata, the division of Articu- 
lata, the division of Mollusca and the division of Radiata, and 
these four divisions are characterized by the nervous system 
chiefly. The plan of structure of the nervous or sensitive system 
giving these divisions and these divisions exclusively, its impor- 
tance is of a secondary degree. For the nervous system has not 
yet been materially demonstrated in all the Radiata, whilst the 
nutritive system is to be seen everywhere. Not that I deny the 
sensitive system to any animal even where it has not been 
shown. There exists among the lower Radiata a homogeneity of 
substance which is perhaps the only obstacle to its discernment; 
nevertheless the digestive system being everywhere distinct, this 
latter must have the preeminence. 
It has the preeminence because it gives us the unity of the 
kingdom as we have also this unity in the perfect resemblance of 
all eggs at an early period of their history. In the animal the 
first substance which is formed is the vitellus or yolk; it is the 
foundation of the future being, it is, as Prof. Agassiz has observed, 
the being itself. Out of the yolk the nervous system originates 
as well as all other parts of the organism, so that in an embryo- 
logical point of view the nervous system holds a second rank. 
It is of secondary rank as a material organ and character of 
classification of animals. It plays the highest part by its imma- 
terial essence in the human species. But here we leave the 
boundary of the animal kingdom and therefore the classification, 
which is our object, to enter another kingdom, the kingdom of 
Thought. 
The nutritive system being the index of animality we see all 
animals equally compelled to take food; this is the essential con- 
dition of their existence. 
The nervous system being the fundamental basis of the pri- 
mary divisions, it gives to each division a special immaterial ten- 
dency so long taught by Prof. Agassiz. Now as there are four 
divisions there are also four of these tendencies. And as soon 
as there are four immaterial tendencies, there is an antagonism 
amongst them. This is a natural consequence since the nervous 
system overrules the division and its dominion is of a spiritual 
character. 
The nervous system stamps upon the divisions their zoological 
form as the symbol of their diverse tendency. We shall see On the Classification of Nemertes and Planar ice. 
43 
further an apparent anomaly of this kind in the beings placed at 
the boundaries of two divisions where the material form, to use 
the words of Milne Edwards, escapes the supremacy of the nervous 
system. The principle however remains always the remote 
cause. 
Besides the antagonism of the divisions between themselves, 
there is an antagonism between the instinct and the intellect, that 
is to say, three of the divisions against the fourth: the Articu- 
lata, the Mollusca and the Radiata, collectively known under the 
appellation of invertebrate animals, all of which have only in- 
stincts, against the Vertebrata which possess, beside instincts, 
intelligence to a certain degree. 
Now between the invertebrate animals and the vertebrata the 
struggle is latent, passive, because the two principles, the instinct 
and the intellect, do not coexist. But among Vertebrata where 
we find both the instincts and the intellect in the same indi- 
vidual, the struggle is active and direct, and we are indebted to 
the observations of Fred. Cuvier, for the revelation of this aston- 
ishing law, that wherever the instincts command, the intellect 
is actionless, and wherever the intellect governs, the instincts are 
silenced or nearly so. There is a struggle and an open struggle ; 
the victory of one of the principles involves the subjection of 
the other. 
Morally speaking, we might thus establish two series in the 
animal kingdom; an instinctive series, stationary and sightless, 
and an intellectual series, progressive and seeing ; the first in- 
cluding three divisions, (Radiata, Mollusca, Articulata,) and the 
second only one (Vertebrata). 
And now, if we go back to the origin of the animal kingdom, 
and trace its history in the past, we see the two series appearing 
simultaneously from the first manifestation of life upon the sur- 
face of the earth: the instinctive series with its three divisions 
and all the classes ; while the intellectual series is represented by 
one class only, that of fishes. 
Their forms are renewed during a succession of periods and 
each great revolution of the globe adds one class to the division 
of Vertebrata or the intellectual series, in the order of their zoolog- 
ical gradation: Reptiles, Birds and Mammals:—then Man crown- 
ing the whole work. Thus a real progress is manifested in the 
division of Vertebrata while the Invertebrate animals remain 
what they were, although undergoing a renewal of forms. The 
reason of this is, as Fred. Cuvier states, that the instinct is innate, 
always sightless, necessary and unchangeable, whilst the intellect 
is progressive, conditional and susceptible of modifications.* 
* Flourens. Resume des travaux de F. Cuvier, sur l’instinct et des 
animaux. Paris, 1844. 44 
On the Classification of Nemertes and Planarice. 
In the intellectual series there was an aim, a design, and this 
was, to arrive at man, the true domain of intelligence. This aim 
realized, the creation would stop and it did stop. Zoological forms 
had acquired all that diversity with which the sphere of activity 
of each division was endowed. To the immaterial principles 
nothing was left except a limited play, a contest for supremacy. 
To intelligence alone was given the power to arrive at the 
knowledge of the actual world, to look back in time, to contem- 
plate itself in the past in view of the future, finally to study 
itself,—in a word, to reflect.* 
The power of reflection belongs exclusively to man, the last 
being created. Man being the converging point of the material 
creation, in him were also to be concentrated in our time the 
struggles of the two spiritual principles of all past time. 
One word more on the intellectual series. The fishes, reptiles, 
birds and mammals belong to this series, but the fishes, the rep- 
tiles, the birds and most of the mammals in their natural condi- 
tion of life have no intelligence,—have no intellect. 
The intellect resides within the brain, and the vertebrata alone 
have a true brain. The brain is composed of several parts. 
There is the base of the brain which sends nerves to the organs 
of sense, and the hemispheres, the special seat of the intellect. 
Now of the hemispheres the fishes have only a rudiment, and 
this is the reason why they have no intellect. There exists a 
well defined progression from the fishes to the mammals with 
respect to the development of the hemispheres ; placed anteriorly 
jn the fishes, they rise degree by degree in the other classes over 
the base which is gradually covered and concealed under them. 
Here we see the organ reflected upon itself, reminding us of its 
function in its full activity, reflection. To this gradual develop- 
ment corresponds a position of the head more and more raised 
which becomes vertical in man,—where it forms a right angle 
with that of fishes. One step more would have been retrograde : 
the development there stopped. 
Thus by a gradation almost imperceptible we have beings be- 
longing to the intellectual series which have the intellect only in 
a virtual state. They have the organ without having the princi- 
ple, or at least admitting the principle virtually present, the organ 
is not sufficiently developed to allow its manifestation. 
These general considerations, although a brief resume, will 
perhaps appear out of place in this paper ; but my object is 
the discussion of the value of the nervous system as a zoological 
character, and to show that while this system of organs gives 
only the divisions, these latter are governed by it in an absolute 
manner. 
I now come to the special topic of my communication. 
* Flourens. Resume des travaux de F. Cuvier, sur l'instinct et l’intelligence des 
Paris, 1844, On the Classification of Nemertes and Planarice. 
45 
III. 
The place assigned by Cuvier to the Nemertes and to the Pla- 
narise in his “Animal Kingdom,” is entirely provisional as ac- 
knowledged by the illustrious naturalist himself. 
The Nemertes are placed in the division of Radiata immedi- 
ately after the Intestina cavitaria. 
Between Nemertes and the intestinal worms of this order there 
are only analogies. The extraordinary length of the body of 
some of them, for instance N. Borlasii (Borlasia angliae), a 
length reminding us of the class of worms, and above all of some 
of the intestinals, such as the tape worm, had prevailed over all 
other considerations. Their affinities were not acknowledged 
because their organization was unknown. 
At that time the intestinal worms were regarded as Radiata, 
for the reason that their nervous system had not been found, and 
the Nemertes, as well as Planariae, were regarded as intestinal 
worms because all of them reminded us by their forms, of the 
forms of these last. 
When the more recent labors of some zoologists had estab- 
lished beyond any doubt that the intestinal worms belonged to 
the division of Ad’ticulata, on account, first, of their having a 
nervous system, and a nervous system constructed on the plan of 
that group, secondly, by the structure of their body, which is 
composed of a series of articulations or rings movable upon each 
other, then the Nemertes were carried with the Intestina cavitaria, 
into the division of Articulata where they remained as little known 
as before. It is but of late that they have been made the sub- 
ject of a special study by a skillful zoologist, Mr. de Qmatrefages, 
and I am surprised that this author has not pointed out the close 
affinities which they bear to Mollusca. 
Cuvier was well aware of the space which separated Nemertes 
from intestinal worms, inasmuch as he foretold that they would 
one day constitute a new order. In spite of these external re- 
semblances, their structure which as he says 11 is of an extreme 
softness,” caused him to doubt. Nevertheless it did not, on this 
account, enter into his mind to compare them with Molluscs. 
At that time, as indeed now, the idea of a mollusc corresponded 
with the idea of a shell-bearing animal, with the form of a body 
more or less drawn together into itself, while the lengthening of 
the body involved by analogy the idea of a worm. 
Now if abstracting the form, which is not the characteristic of 
the divisions, we look at the intimate structure, if we give up 
also the shell as circumscribing the division of Mollusca, we shall 
find in the Nemertes all the principal characters of Molluscs: a 
soft body, entirely smooth, covered with a glutinous mucosity ; a 
very simple nervous system reduced to a small number of cephalic 46 
On the Classification of Nemertes and Planarice. 
ganglia, whence nervous threads depart to distribute themselves 
in the body. If we further state that, as in the greater number 
of Molluscs, the surface of the body is covered with vibratory 
cilise which help their movements, movements generally slow, 
deprived of energy, then we directly arrive at the idea that 
Nemertes are really Molluscs,—Molluscs of a low rank, being 
parallel with the worms of the division of Articulata by the 
analogy of their forms. 
Having discussed above the value of the nervous system as 
the predominant character of the division, exclusively of any 
other character, it remains only for me to state that the nervous 
system of Nemertes is constructed upon the plan of the nervous 
system of Molluscs: there exists a cephalic mass more or less 
lobed, representing either the superior assophageal ganglion of 
other Molluscs, or that same ganglion to which, on account of 
the peculiar form of the body, are added the two or three abdom- 
inal ganglia. Nervous threads are distributed in all directions; 
two of them more voluminous than the rest, but uniform in 
structure, run along the sides of the animal, and sending off 
thinner threads without showing in their course those ganglia or 
swellings which distinguish the nervous system of Articulata, 
such as it is in Malacobdella, Peripates, &c. 
The disposition of the nervous system of Nemertes, then, is 
merely analogous to that of Annelids; its structure is that of the 
nervous system of Molluscs. 
IY. 
The position of Planarim in the division of Radiata is not less 
curious than that of Nemertes. Included in the second order of 
intestinal worms, the Parenchymata, they are brought near the 
Trernatodes, to which they have only analogies, in the same 
sense as those that Nemertes have to the Intestina cavitaria. 
The Intestina parenchymata have been withdrawn from the 
division of Radiata and brought into that of Articulata, and for 
the same reasons as that of the cavitaria. The Planarise, of 
course, have thus been compelled to follow them in the same 
manner as Nemertes have followed the latter. But also little in- 
vestigated at that time, their affinities with Molluscs have escaped 
the eyes of zoologists. 
The Planarias are not parasitical as Trernatodes are, and it is 
important that this fact should be noted, parasitism existing most 
extensively among Articulata. The investigations of Mr. de 
Qmatrefages and others have moreover made us acquainted with 
their structure which, although not yet entirely understood, as I 
believe, is however of the highest interest. Indeed in Distoma 
the digestive system is constructed upon an analogous plan with 
that of Planaria; but the digestive system, as we have said, can- On the Classification of Nemertes and Planarice. 
47 
not characterize the primary division. It gives but the -pfeesefer 
rfatswawo n 
As soon as it is acknowledged that the division rests upon the 
structure of the nervous system, the fact that the nervous system 
of Distoma is that of Articulata, and the nervous system of 
Planarias is that of Mollusca, there is no ground for further 
hesitation. 
In Distoma there are two cephalic ganglia, whence nervous 
threads part for the anterior region of the body. Prom each one 
of these ganglia arises a nervous thread which takes its course 
backwards; this thread presents on its length a series of small 
ganglia, scarcely distinguishable upon the middle region, it is 
true ; but very apparent towards the posterior region, where they 
are seen sending off smaller threads distributing them to the 
body. This arrangement of the nervous system of Distoma be- 
comes especially distinct in Malacobdella where the .same arrange- 
ment is found, with the ganglia of the lateral threads more de- 
veloped. Thus, taking the nervous system into consideration, 
Malacobdella is one degree higher than Distoma, then Clepsine 
would follow in which the two threads are brought so close to- 
gether that they combine into one single thread. Above Clepsine 
would rank the other Hirudines. 
In Planarias we have a cephalic ganglion more or less lobed on 
its circumference which sends nervous threads to all the regions. 
There are two more voluminous lateral ones (one on each side of 
the body) as in Distoma, but uniform as in Nemertes, still recalling 
here by analogy the nervous system of Articulata. The funda- 
mental difference, although lest apparent at first sight, consists in 
the absence of ganglia upon their lengths, and this fact decides all. 
The body of Distoma, indeed, is not articulated and this may 
perhaps lead to a belief of a closer affinity between Planarias and 
worms. Their broad and flattened form would be better adapt- 
ed to their mode of life without that structure. Moreover the 
articulation of the body, although a character of the division, is 
subordinate to the nervous system. Consequently there is no 
ground for surprise at seeing it vanish or even entirely disappear 
in those groups placed at the boundaries of two divisions as a mate- 
rial property in litigation, disputed by antipathic vital tendencies, 
each endeavoring to appropriate it to itself. 
Considering now the softness of Planariae, that glutinous mu- 
cosity which surrounds them, that body of a uniform shape with- 
out articulations and deprived of articulated limbs, that general 
apathy, all these are so many characters which they partake with 
Mollusca and do not partake with Articulata. 
Howevfer, I do not know that any one has proposed to con- 
sider these animals/Molluscs, although Baer and Duges had al- 
ready compared the inferior disk of Planariae to the foot of Gas- 48 
On the Classification of Nemertes and Planarice. 
t'eropoda without thinking of bringing them together in the same 
natural group. Mr. de Qmatrefages refutes this comparison. • We 
shall come to it again presently. 
For, if the shell does not characterize Mollusc.a, inasmuch as 
all living Cephalopoda are naked, and among Gasteropoda we 
have the whole group of Nudibranchiata deprived of a shell, that 
of Pteropoda, and among Acephala that of Tunicata, it requires 
no effort of imagination to admit in that division, animals such as 
Planariae. They are flattened Molluscs in the same manner as 
Nemertes, are elongated or stretched Molluscs, as are also Denta- 
lium, which nobody would place elsewhere than among Molluscs. 
Many authors have spoken of the organization of Planariae, 
from Duges to MM. de Quatrefages and Blanchard. All have 
viewed them as worms, doubtless prepossessed by the idea that 
Cuvier who had established the divisions could not have been 
mistaken so far as to place side by side in the same order animals 
belonging to two different divisions. 
But this error of the author of the “ Animal Kingdom” is easily 
accounted for. At that time he was in want of the essential 
datum to settle such a question in its details, the knowledge of 
the nervous system. 
The plan of structure of the nervous system, we have already 
said, and we cannot repeat it too often, gives only the division 
and nothing but the division. Now Cuvier did not know it when 
he laid the foundation of his classification,- although he foresaw 
the four plans of organization of the whole kingdom ; and as he 
said there are but four of these plans. 
He who has done most after Cuvier to establish the doctrine 
of these four plans of organization, is Prof. Agassiz. Fie has 
the unquestionable merit of having followed out the traces of 
these plans beyond the existing creation, and in ascending through 
past ages of the world’s history he has been enabled to recon- 
struct the biological phases to which I have alluded above. In 
reading the history of the earth on the strata which compose 
its crust as so many pages of a book written by the hand of 
the Creator, we then find again the thought of these four plans 
of organization preconceived by Cuvier, demonstrated as a doc- 
trine by Prof. Agassiz. 
These four plans of organization would acquire a far greater 
importance if embryology should ratify them. Now embryology 
does so. All embryological investigations past and contempora- 
neous lead towards four plans of structure. I shall not treat 
of this question more in detail here; it is sufficient merely to 
mention the fact. 
Now the question respecting the class among Molluscs to which 
Planarise belong is easily settled. They crawl on the inferior On the Classification of Nemertes and Planarice. 
49 
surface of their body; that they are Gasteropods, there can be 
no doubt. l)o we not see them, as well as Pulmonata, Nudi- 
branchiata and others, creeping along the walls of a basin and 
when near the surface of the water, reversing their position and 
walking in that manner with the same facility ? Do we not witness 
the same undulatory contractions in that foot? Therefore were 
not Baer and Dnges guided aright when they compared the infe- 
rior surface of the body of Planariae to the foot of Gasteropods? 
I am not aware that any of the Planariae I have studied moves 
with the same facility on the back as on the belly as Mr. de 
Q.uatrefages states. Whenever I placed them in that position I 
always saw them changing it as quickly as possible. Besides in 
most cases I have seen the upper surface differing widely from 
the inferior one. 
In Planocera we have on the dorsal surface two cylindric tenta- 
cles, analogous to the cephalic tentacles of Doridians. In Thyza- 
nozoon or Eolidieera, the cephalic tentacles are flattened and are 
brought entirely forwards while other tentacles appear on the 
back, reminding us of the dorsal appendages of Eolidians proper. 
Then come Proceros and Procerodes with but the flattened ante- 
rior tentacles of Eolidieera and in the same place as in these last. 
There exist then in the marine PlanariaB one group which re- 
minds us of Doris, another of Eolis, and still another group in- 
termediate between Eolis and Planaria proper which lead to the 
freshwater species. 
This shows that it is near Nudibranchiata that Planariae will 
find a natural place, and on several accounts we should be tempt- 
ed to consider them as a degradation of that type. 
But for us who do not admit the so-called degradations, inas- 
much as each being appears to us as to Cuvier, a perfect whole 
by itself we consider Planariae as a family nearly allied to that of 
Nudibranchiata, bearing in itself the reason of its existence as 
strongly defined as this latter, and representing merely a variation 
in the thought of the Creator. 
For, in Nudibranchiata we find a number of types all as much 
diversified. These are: Doridians, Eolidians, then Canthop- 
sis analogous to Procerodes which leads to the Acteonians, then 
finally the genera Pelta and Chalidis which constitute another 
group almost Planarian, deprived of external appendages of any 
kind. 
There exists a striking parallelism between the zoological forms 
of these two families. 
Viewed in the light of their organization nothing is more alike. 
The nervous and digestive systems scarcely differ. I have already 
spoken of the first. With regard to the second I'shali recall the 
fact that it is ramified in Nudibranchiata as in Planariae; the 
ramifications being diversified according to the groups. 
Second Series, Yol. XI, No. 31.—Jan., 1851. 50 
On the Classification of Nemertes and Planarice. 
The organs of generation do not differ much more. Nudi- 
branchiata are androgynous like Planariee. The fecundation 
takes place by mutual fecundation as is the case in Pulmonata. 
But in Planarire we have cases where an individual fecundates 
itself, the hermaphroditism being here complete. 
It still remains for me to make some general remarks upon Ne- 
mertes and Planarise. 
Mr. de Quatrefages tells ns himself, “ neither Nemertes nor 
Planarise have externally a resistant and tough layer, similar to 
that which is found with Annelids, for example, or even with 
Rotatoria.”* 
We have then two groups of Gasteropodous molluscs parallel 
with two groups of annulated Articulata; the group of Planarise 
reminding us of Helminthes, and the group of Nemertes remind- 
ing us of the Hirudines. 
At the extremes of these two groups, at the bottom of the two 
classes, we witness in some sort a strange and opposed struggle 
of the two immaterial principles of the divisions which exert 
themselves to take from each other some portion of their mate- 
rial property. As examples we have : 
In Molluscs,—the Nemertes which elongate and become worm- 
like ; the Planarise which remain shorter but pressed down, spread 
out, flattened in thin leaves. 
In Articulata,—the softness of Helminthes in general, the flat- 
tening of their body in Trematodes in which the articulation of 
the body vanishes, analogous to Planarise;—the softness still of 
the Leeches with a distinct articulated structure, being parallel 
with Nemertes. 
These groups do not oppose each other in an exact parallelism, 
for Nemertes which form a low type of Gasteropods are opposed 
to the Leeches of a higher grade of worms, and Planarise a higher 
grade among Gasteropods are opposed to Trematodes a lower 
type among worms. In this manner: 
Y. 
Hirodines, 
Planariae, 
Trematodes, 
N'emertes, 
In the elongation of the body of Nemertes there is nothing to 
surprise us. Placed at the bottom of the class to which they be- 
long, they assume a form analogous to a group of the division of 
Articulata which attract them but to which they do not belong. 
Now when a Mollusc, whose body is elongated beyond all pro- 
* Ann. Sc. Nat., 3d series, vol. iv, 1845. On the Classification of Nemertes and Planarice. 
51 
portion, and obliged sometimes to move by the contraction of 
transverse muscular fibres, accelerates its progress, then we see 
that Mollusc assuming certain transverse, irregular and unequal 
folds,—shadows of articulations which in reality do not exist. 
At the bottom of the division of Articulata we observe similar 
facts. The Trematodes lose insensibly that elongated form of 
the body which constitutes the prominent character of the worms ; 
they are flattened, spread out, meanwhile the articulation of the 
body, the characteristic of their type, vanishes completely,—thus 
foreshadowing the type of Planariae. 
The position and number of eye-specks in Nemertes and Pla- 
nariae indicate also a greater resemblance to the same organs in 
Molluscs than to those of Annelids. When eyes exist in Anne- 
lids, they are arranged in pairs on both sides of each articulation 
or else form a crown on one of the anterior rings. In Planariae 
we find the eye-specks irregularly grouped on the upper surface 
near the anterior region of the body. The same arrangement is 
observed in Nemertes. This arrangement forcibly reminds us of 
what we see in Gasteropods. In Planariae alone they are more 
numerous and distributed with less constancy, a fact which is ac- 
counted for by the lower position of that family in the class. 
The habits of Nemertes and Planariae speak rather in favor of 
Molluscs than worms. Most of them live concealed under stones 
which is the case with many Molluscs, while I do not know 
any which lives within a tube constructed like those of worms. 
The embryonic development of both Nemertes and Planariae 
takes place according to the laws we witness among Gasteropods. 
The larval condition of both Gasteropods and Planariae is very 
similar. 
It, is plain enough, Nemertes and Planariae are only analogous 
to Articulata; by their affinities they are Molluscs. The divis- 
ion of Articulata hence appears to us as a type more natural and 
rational, as well as that of Mollusca; this latter including all an- 
imals which are soft, slimy and flabby, whatever may be their 
form. 
The Rhabdoccelae or freshwater Planariae will be the connecting 
link by which the Nemertes approach Planariae proper, as mem- 
bers of the same group. We may say of them that they are the 
freshwater representatives of both Nemertes and Planariae. And 
now observe the transition: the group, Rhabdocoelae, which is 
distinguished at once from Planariae and Nemertes, this group, I 
repeat, lives in fresh waters, while the other two groups which 
they connect together are scarcely found beyond the boundaries 
of the seas. If some of them ascend the rivers it is within the 
limits where the water still retains a part of its marine character. 52 
On the Classification of Nemertes and Planarice. 
So that the connection although materially expressed, exists es- 
sentially in their immaterial essence. 
VI. 
I now conclude by a few words on Annelids and Gasteropods. 
Having withdrawn from the first of these classes a certain 
number of its representatives to include them among the second, 
it is to be expected that I should present in a synoptical manner 
the systematic modifications resulting from it. 
If we admit the two sections of worms proposed by Milne Ed- 
wards, the Pleuronera and Annelids proper, we should place them 
one above the other to form a single series instead of one. 
Opposite we should have the series of Gasteropods beginning 
with Nemertes; above these the Planaria?, then the Nudibran- 
chiata; 
Annelides. 
Annelids proper. 
Scoleides. 
Hirudines. 
N udibrancliiata. 
W. 
s 
Vh 
o 
l> 
Peripatus. 
Malacobdella. 
Polycladus. 
Helmintlies. 
cr! 
o 
£ ■ 
C 
in 
ci 
O 
Planarise. 
Pleuronera. 
JTemertes 
What would come next to the Nudibranchiata I am not prepared 
to say. The series cannot go on, on that footing, for the very 
simple reason that embryology assigns a lower rank to all Gaste- 
ropods provided with a shell, inasmuch as Nudibranchiata, when 
hatched, have a shell, which they lose at an early period of their 
life. The attempt at forming organic series in the animal king- 
dom is one of the most difficult of labors, and a work of a rela- 
tive value. Unless several series are established in the class of 
Gasteropoda, the position of Nudibranchiata in the above synop- 
sis cannot be accounted for. I have already made the remark 
that Planariae were rather parallel to Nudibranchiata than of a 
lower rank, finding in these two families groups of equal im- 
portance. 
Respecting the serial arrangement of worms, as given above, 
and resting upon the morphology of their nervous system, I have 
also to remark, that it does not coincide with another series allu- 
ded to in a remarkable paper of late publication ;* and indeed if 
lumbricine Annelids and Leeches are higher in their class, as I be- 
* The natural relations between Animals and the Elements in which they live ; 
by Prof. L. Agassiz. This Journal, 2d ser., vol. ix, p. 369. J. Lawrence Smith on Emery. 
53 
lieve it, this fact corroborates the idea that the nervous system 
characterizes only the primary divisions. And our position would 
turn out to be the right one. 
If instead of a single series, or several series with a starting 
point and an end, we construct a circle or an ellipse, and place 
the Gasteropods on its circumference, then the arrangement of 
each group would meet with less difficulty and perhaps satisfy 
completely the mind. 
Cambridge, Mass., October, 1850.