236 



a somewhat more detailed study as regards the development and gradual 

 transformation of the inner structures. The results acquired from the study 

 of these objects, though they cannot claim to be of more general value, 

 are in themselves of very great interest and throw important light on 

 several problems connected with the developmental history of Echino- 

 derms. 



The most interesting of the facts discovered by the study of the said 

 forms with abbreviated development would appear to be that of the am- 

 nion developing from the pharynx in Peronella Lesueuri, while other- 

 wise it develops as an invagination from the ectoderm on the left side of 

 the larva, above the hydrocoel. (In Heliocidaris erythrogramma it could not 

 be decided whether it likewise develops on the left side, there being appar- 

 ently no means of identifying the right and the left sides in this perfectly 

 cylindrical larva). This may perhaps give some indication as to the way 

 in which the amnion of Echinoid-larvae originated. MacBride has come 

 to the conclusion that the amniotic cavity of the Echinopluteus may be 

 interpreted as a portion of the stomodaeum which is formed separately 

 from the rest of it^), because in the metamorphosing larva of Ophiothrix 

 fragilis he has found the primary tubefeet protruding into the stomodaeum. 

 The fact that the amnion of Peronella Lesueuri does originate as an out- 

 growth from the pharynx would seem to lend an emphatic support to 

 MacBride' s theory. Still I would not take it as having been definitely 

 proved as yet by these facts. Peronella Lesueuri is a very highly special- 

 ized type; may we then really rely upon this remarkable development of 

 the amnion in the much reduced larva as meaning a reversion to the pri- 

 mitive mode of development? It is perhaps not quite safe to draw this 

 conclusion as yet. If other similar cases were found, the conclusion would 

 be very considerably strengthened; but this isolated, evidently very spe- 

 cialized case does not seem to me to afford sufficient proof. Neither does 

 the Ophiothrix-lsirysi appear to me to be a sufficient proof of the theory. 

 It is the only Ophiuran with a typical pelagic larva the metamorphosis 

 of which has been adequately studied as yet. But the little we know about 

 the metamorphosis of other Ophioplutei does not point towards the stomo- 

 dseum as having generally the function of an amnion in Ophiurid-larvse. 

 In Ophiopluteus bimaculatus, so carefully studied by J oh. Miiller^) and 

 Metschnikoff 3) — though not by means of sections, of course — it ap- 

 pears that the suboral cavity has got the function of an amniotic 



1) MacBride. Textbook of Embryology. I. Invertebrata. 1914. p. 522 

 ') Job. Miiller. Uber die Ophiurenlarven d. Adriatischen Meeres (V. Abhaudl 1852) 

 laf. II — III. ■ " 



») E. Metschnikoft. Studien uber die Entw. d. Ecliinod. u. Nemertinen. Taf. VI— VII. 



