From the Proceedings of the Boston Society of Natural History. Vol. xiv. Oct. 18, 1871. ON THE EARLY STAGES OF AN ASCIDIAN, (Cynthia ^yriformis Rathke.') By EDWARD S. MORSE, Ph.D. BOSTON: PRESS OF A. A. KINGMAN. 1871. From the Proceedings of the Boston Society of Natural History, Vol. xiv, Oct. 18, 1871. ON THE ) • EARLY STAGES OF AN ASCIDIAN, (^Cynthia J^yriformis Rathke.') By EDWARD S. MORSE, Ph.D. BOSTON: PRESS OF A. A. KINGMAN. 1871. Notes on the Early Stages of an Ascidian (Cynthia pyriformis, Rathke). By Edward S. Morse, Ph.D. PLATE I. In the year 1866, Kowalevsky published a remarkable series of observations on the embryology and early stages of several Ascidians,1 in which a structure similar, if not identical, with the type characters of the vertebrata was demonstrated. Professor Kupffer at first doubting, then not only confirms the validity of Kowalevsky's observations, but adds additional facts show- ing that the nerve mass actually penetrates the tail of the embryo to a considerable length. 2 The important facts revealed in these investigations have led the eminent naturalists above mentioned, as well as Haeckel, Schultze, and others, to believe that the connecting link between the Vertebrates and Invertebrates had at last been established. Darwin, with prompt recognition has incorporated the facts in his last work on the "Descent of Man." Since Kowalevsky's memoir above referred to was published, he has traced out the embryology of Amphioxus 3 in which the closest resemblance is seen between this low vertebrate and similar stages of the Ascidian. In these unsuspected relations between the Vertebrates and Ascid- ians through Amphioxus, it is interesting to remark that long ago Goodsir called attention to the resemblance between the phryangeal sac of the Ascidians, and that of Amphioxus. 4 He says "The Lance- let respires by receiving sea water into the anterior compartment of the intestinal tube-this cavity is kept dilated by the elasticity of the numerous filamentous ribs, and this dilatation may be increased by the action of the super-imposed ventral bundles of the lateral muscles. It is contracted by the action of the abdominal muscle. 'Kowalevsky, Mem. Acad. Imp. St. Petersburg, Series vii, Tom. x, 3, 1866. 'Kupffer. Schultze's Archiv. fur Mikrosk. Anatomie, Bd. 6, 1870. 3 Mem. Acad. Imp. des Sci. St. Petersbourg, vii Series, Tome xi, No 4. * Goodsir on the Anatomy of Amphioxus. Koyal Society of Edinburg, Vol. xv, part i, page 259. 4 This is a mode of respiration similar to that which prevails in the tunicated mollusks. It is interesting to observe that the branchial membrane of the Lancelet is exactly similar in its peculiar vascular- ity (ramifications at right angles) to that which lines the branchial cavity of the mollusks just specified As in Ascidiae the entrance of the intestino-respiratory canal is guarded by filaments." In the recent removal of the Tunicates from the mollusks, and incorporation of the same with the Vermes by Gegenbaur and others, it is interesting to recall from the memoir of Goodsir above cited, the fact that he calls attention to certain resemblances between Amphi- oxus and the Annulosa. "The plan of circulation is simple and in accordance with the primitive condition of the respiratory apparatus, both functions being performed in a manner closely resembling that observed in certain annulose animals. The dorsal vessel corresponding to the heart, or branchial artery, and the abdominal vessel to the aorta of the Lance- let." Ibid., p. 260. Any scrap of information connected with a subject so profoundly interesting justifies me in bringing forward a single observation made upon the young stages of a sessile Ascidian, Cynthia pyriformis, at Eastport, Maine, July, 1870. John E. Gavit, Esq., of New York, kindly placed the larvae in my hands for examination. The eggs were noticed in all stages, as well as free swimming larvae in active motion. These were easily seen with the unassisted eye and looked like gigantic spermatozoa. A special series of observations in another line limited me to a single examination of these interesting forms. Kowalevsky in the paper above referred to represents on plate n, fig. 26, a delicate membrane bordering the tail like a fin. This is represented as structureless. In the unfolded tail no traces of cartil- agenous centra are shown, though he represents these parts as more or less conspicuous while still enclosed in the egg. Kupffer repre- sents the same condition of things in his figures.1 In the many free larvte examined by me, the axial segments were perfectly defined. Fig. 2, plate I, represents two as they rested on the slide. In these, forty segments were counted, four of these seg- ments extending into the body proper, the anterior segment sending off three diverging processes toward the haemal, or ventral region. Beside the persistence of these segments after the animal was freed Ubid./raf. ix, fig. 16. 5 from the egg, a remarkable structure not hitherto figured, as far as I am aware, was noticed in the caudal fin, which vividly recalled the fine diverging rays as seen in the embryo fish. These rays were extremely delicate though plainly marked. They ran off nearly parallel to the longitudinal axis of the tail, and were confined to the last five segments, reaching beyond the last caudal seg- ment, to a distance equal to eight segments, as shown in figs. 3 and 4. Those who have seen the caudal fin of the embryo trout with its closely crowded ray lines, will bear witness to the strong similar- ity between the two. At the junction of the tail with the body, a series of rays of various lengths converging in pairs at the outer border of the membrane, and running off at right angles to the longi- tudinal axis, were also marked, though of extreme tenuity. This peculiar feature is represented in fig. 2. All of the segments were nucleated, and the- tail appeared of uniform length. They were enclosed in a continuous investing sheath which disappeared at the caudal tip. This latter region was granulated. The peculiar black spots, the lsinnesorgane' of which nothing is known, save that they are supposed to be sense organs of some sort, were seen, but nothing defi- nite was made out in regard to them. In fact the structure of the fin only was studied. Since the above lines were written, Mr. Gavit has placed in my hands for examination a number of embryos, from the same lot studied, which he preserved in a saturated solution of salt and water. These he has since mounted in cells with the same preservative fluid, and though a year and a half has elapsed since these specimens were immersed in the fluid, they are remarkably preserved and reveal cer- tain features not recognized in the living specimens. Fig. 6 represents one' of these preserved specimens. All traces of segments have disappeared, the segments being irregularly broken in a series of short, cylindrical masses. Lbwig and Kblliker1 repre- sents the tail of a larval Botryllus with the segments divided in a median line, making a double row of segments running parallel, as well as a double row of smaller cells representing the investing mem- branes. This condition may represent a later stage, though all the specimens preserved by Mr. Gavit present this peculiar appearance. With the breaking up of the segments, a contraction of the axis has taken place at the tail, leaving a distinct cavity, this is more plainly shown in fig. 5, where double transverse lines are faintly seen indi- 1 Annates des Sciences Naturelie, in Series, Tome v, pl. 7. 6 eating the former presence and position of the segments. The fin is seen as a continuous membrane bordering the entire outline of the animal. The fin rays are very conspicuous and commence just at the junction of the tail with the body .and start off at right angles with the longitudinal axis. At the tail they rapidly diverge and finally run parallel with the axis. The wider marks of the fin rays correspond nearly in number to the number of segments in the axis. As these were invisible in the living specimens it is impossible to say whether they agree as to number and position with the segments. In conclusion it is interesting to add that Savigny, Milne Edwards, Van Beneden, Sars, Kolliker, Dalyell, Agassiz and many others have added their testimony in regard to the existence in many gen- era of Ascidians of active tailed larvae, till it was supposed that this appearance of the embryo was characteristic of all Ascidians. Lacaze Duthiers,1 however, describes the young of Molgula as pre- senting a remarkable exception ; the young not having a tail, nor showing any signs of activity, but escaping from the egg with the appearance of Amoeba, by flowing out of the egg, a rounded plastic fluid mass, and remaining sedentary at the bottom of the vessel. Albany Hancock 2 on the contrary has observed the embryos of two species of Molgula in which they present all the features of the usual active tailed larvae, and questions whether Lacaze Duthiers had Mol- gula at all; for certain reasons, which he presents, he thinks Duthiers, had another genus, Eugyra, under examination. Prof. A. E. Verrill however, in a series of valuable papers on the Ascidians of New England,3 states in regard to Lissoclinum tenerum V. (gen. et sp. nov.) that "the eggs are few and relatively very large. The develop- ment of such eggs is direct, without passing through a tadpole- shaped larval state," with the following note, "with the alcoholic specimens it is not possible to trace completely the early stages of this development, or to be perfectly certain that these egg-like bodies are genuine eggs, although some of them appear to contain at first, a germinal vescicle." Further investigation will probably show that eggs passing through such an anomalous development are different in their nature, and it is not improbable that the simple Ascidian Eugyra and the compound Ascidian Lissoclinum will also present a kind of egg which passes 1 Comptes Rendus, Tome lxx, p. 1154. 'Annals and Magazine Nat. Hist, iv Series, No. xxxv, p. 353. ' American Jour. Science and Arts, Jan. to June 1871, p. 445. 7 through that course of development supposed to be typical of the class. Prof. Verrill, with his usual caution, expresses a doubt whether the egg-like bodies he observed, were genuine eggs, in the note just quoted. EXPLANATION OK PLATE I. The embryos from which these were drawn were about eight one-hundredths of an inch in length. Fig. 1. Showing body of embryo and anterior end of axial, cartilaginous rod Fig. 2. Two embryos showing fin rays at the junction of the tail with the body. Figs. 3, 4. Showing caudal fin. Fig. 5. Tail of preserved specimen showing contraction of axial rod, leaving cavity. Fig. 6. Entire drawing from preserved specimen, showing breaking up of axial rod with fin rays. Proc.Bost. Soc.Nat.Hist.Vol. XIV. Plate 1. MORSE, ON EARLY STAGES OF AN ASCIDIAN.