The Development of Ophiothrix Fragilis. 557

THE DEVELOPMENT OF OPHIOTHRIX FRAGILIS. 557 The Development of Ophiothrix fragilis. By E. W. ftlacBridc, M.A., D.Sc, F.R.S., Professor of Zoology in McGill University, Montreal. With Plates 31—36, and 4 Text-figures. CONTENTS. PAGE Introduction ..... 557 Historical Sketch . .558 Material and Methods .... 567 Normal and Abnormal Development . 570 Tlie Development of the Full-grown Larva . 575 Metamorphosis of the Larva into the Brittle-star . 5S0 Comparison with Development in other Classes of Echino- dermata . 590 Summary and Conclusion .... 596 INTRODUCTION. THE present work, which has occupied my attention for the last four years, was begun with the object of extending to other classes of Echinoderms the researches which I had already made on the development of Asteroidea (18) and Echinoidea (19). It has proved to be a task of extraordinary difficulty owing to the minute and refractory character of the larvas oE Ophiuroidea. Nevertheless, the results obtained will, I think, bear fair comparison with those which I have already published concerning the development of Asteroidea and Echinoidea, whilst a number of new and unexpected facts have disclosed themselves which possess interest for a 558 E. W. MAOBRIDE. wider range of students than specialists in the class Echino- dermata. The most interesting result which was obtained from the study of the development of Asteroidea and Bchinoidea was the discovery that the ccelom of the larva showed distinct traces of metameric segmentation, a division into three somites being clearly indicated. The pioneer in this work is Bury, to whose stimulating papers (4, 5, and 6) I wish on this occasion, as formerly, to express my deep indebtedness. Bury, however, only recognised two somites, whilst the main point of my previous researches was that the hydrocosle, or rudiment of the water-vascular system, which Bury regarded as essentially a single organ, was in reality paired, the left rudiment developing whilst the right remained small and insignificant. This interpretation of the facts has been challenged by other observers such as Goto (11) and Masterman (20), who interpret what I regard as the right hydrocoele in a different manner. I may summa- rise the results of the present paper by saying that the study of the development of Ophiuroidea has afforded me a com- plete confirmation of my views. The state of affairs in the Ophiuroidea is much cleai*er and simpler than in the other classes of Echinodermata, and I hope to convince every unprejudiced reader of this paper that the Echinoderm larva really possesses three metameres. HISTORICAL SKETCH. Our knowledge of the development of Ophiuroidea dates back to Johannes Muller. At the meeting of the Academy of Sciences of Berlin, held on December 4th, 1845, this celebrated naturalist described a number of new marine animals which he had observed whilst at Heligoland in the autumn of that year. Amongst these was one which Muller named Pluteus paradoxus, from its fancied resemblance to a painter's easel. This description was published in the ' Archiv fur Anatomie und Physiologie,5 in 1846 (24), and on October 29th of that year Muller read before the Academy THE DEVELOPMENT OF OPHIOTHMX PRAGILIS. 559 of Sciences a paper entitled " £f ber die Larven und die Meta- morphosen der Ophiuren und Seeigel" (25), in which he describes the metamorphosis of Pluteus paradox us and shows that it is the larva of an Ophiurid. A somewhat similar organism also referred to the genus Pluteus is shown in this paper to be the larva of an Echinoid, and Miiller adopted the regrettable custom of using the same word "Pluteus" to designate these two quite different larva); this custom has persisted until quite recently and has been the source of much confusion. In the case of Pluteus paradoxus the change into an Ophiurid is traced step by step, and Miiller figures without describing it a specimen which has a five-lobed rosette—the form assumed by the rudiment of the water-vascular system—on the right as well as on the left side of the oesophagus. In this figure also the metameric division of the coolom is beautifully shown, and, indeed, Muller's drawing could have been used to illustrate .this paper. Of course, Miiller was unable to interpret his results fully, and since the coelomic rudiments have extremely narrow lumina it is not surprising that he did not recognise them as the forerunners of the body-cavities of the adult. A second form of Ophiurid pluteus with more slender arms is described in the same paper, and in a later paper (27) read before the Academy in 1851, Miiller describes two further forms of Ophiurid Jarvse which he discovered during a visit to Trieste, one of which he named Pluteus bimaculatus from the brown pigment spots with which it was ornamented, the other he recognised as the subject of the present paper—the larva of Ophiothrix fragilis. In Pluteus bimaculatus Miiller discovered the larval anus of Ophiuroidea, thus showing that the absence of the anus in this class of Echinodermata is a secondary, not a primary affair. The hydrocosle or rudiment of the water-vascular system is described as the palmate organ, since it possesses five lobes like the five fingers of the human hand. Miiller saw that it was formed late in larval life, and was at first round in shape. In its later stages it extends ventral to the oesophagus, and each lobe becomes VOL. 51, PART 4.—NEW SERIES. 42 560 E. W. MACBRIDE. five-lobed, so that each somewhat resembles the whole organ in an earlier stage. Miiller was led astray by this resemblance and gave to each primary lobe the name " palmate organ/' and he imagined that the palmate organ of the earlier larva was identical with the first of these, and that the rest were formed de novo. He described also a semi- circular wavy fold of skin, from which the rudiments of the adult arms grew out, to which the " palmate organs " became co-adapted later. These latter gave rise to the radial canals and tentacles of the water-vascular system. The larva of Ophiothrix fragilis was recognised by Miiller owing to its possessing on the first tentacles of the developing brittle-star the papillae characteristic of this species. Miiller only obtained late metamorphosing stages, and his description concerns itself chiefly with the elaboration of the details of the brittle-star. The last Ophiurid larva described by Miiller is one which he called the " worm-like larva," which he imagined to belong to the Asteroidea (28). It was re-discovered by Krohn (15), who proved that it was the larva of an Ophiurid. In 1900 Caswell Grave (12) found a similar larva on the American coast, and showed that it was the young stage of Ophiura brevispina. Owing to the opacity of this larva neither Miiller nor Krohn made out much of its structure. The uext great advance in our knowledge of Ophiurid development was made by Metschnikoff in 1869 (21). In a paper on the development of Echinoderms and Nemertines he records the results of investigations on all four classes of Echinoderm larvte, as well as on the embryos of the viviparous species, Amphiura squamata. Three kinds of Ophiurid plutei are described, two of which are identified with larva . described by Johannes Miiller. Of only one form, however, did he obtain sufficient stages to describe the development in any detail, and this is the Pluteus bimaculatus of Miiller. In the youngest stage which he obtained he was able to make out the right and left coolomic cavities lying at the sides of the larval oesophagus. In the next stage he discovered a THE DEVELOPMENT OF OPHIOTHB1X FEAC4ILIS. 561 pair of discs lying at the sides of the stomach, and concluded that they had been segmented off from the cesophageal cavities, which he could still make out. This is the first assertion of a trace of metameric segmentation in any Echinoderm larva. In subsequent stages he discovered that the left anterior cavity assumed the five-lobed form characteristic of the hydroccele, whilst the right grew smaller and ultimately disappeared. In the next stage he showed that the hydroccolic rudiment had by growth changed its longitudinal position for a transverse one, and was, in fact, beginning to surround the larval oesophagus. Miiller had supposed that, as in Echiuoidea, the larval oesophagus disappeared, but Metschnikoff shows that it persists. He further corrected Miiller's error about the " palmate organs," and showed that each of these corre- sponded to one lobe of the original palmate organ or hydrocoele. He showed also that the primary xnadreporic poro, although formed on the left side, rotates during metamorphosis to the right along with the left prae-oral arm of the Plnteus. In a word, Metschnikoff made ont almost everything that it is possible to discover in a Pluteus without the use of sections. The viviparous species, Amphiura squamata, offered an opportunity of studying the embryonic type of development in an Ophiurid. Its viviparity was discovered by Quatrefages in 1842. He communicated his results to Milne Edwards, by whom they were published. Krohn published in 1851 a paper on the subject (14), dealing with the relation of the embryo to the maternal tissues. In 1852 Schultze (30) discovered a transitory larval skeleton, which he compared to the skeleton of the Pluteus. In the paper referred to above Metschnikoff records most valuable observations on the development of Amphiura squamata, although the younger embryos are difficult to obtain. He found a thick-walled blastula, and then a later stage, in which the coelom was represented by two thick- wnlled bodies lying at the sides of the archenteron.

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