MUSEUM OF COMPARATIVE ZOOLOGY. 107 



and Balfour's observations on Ophiothrix may be settled by those whose 

 good fortune it may be to study the embryology of this genus, but 

 since the archenteron is shown in the account of observations here pub- 

 lished to be formed by an invagination in Ophiopholis, we may still 

 adhere to our acceptance for some genera of brittle-stars of a general law 

 of Echinoderm development, known to apply to the gastrulte of some 

 genera of the Holothurioidea, Echinoidea, and Asteroidea. "While, how- 

 ever, my observations are believed to show that in at least this genus an 

 invaginated gastrula occurs, they do not prove that the opening into the 

 primitive infolding becomes the anus of the pluteus. 



Our most accurate knowledge of the young stages of Ophiurans relates 

 to a viviparous genus, Amphiura. 



Although the development of Amphiura has been studied by several 

 observers, we find in their accounts of the subject so many discrepancies, 

 that a call is made for a new study of the first stages of this and related 

 genera. ]\[etschnikoff* supposes that in Amphiura the stomach, " Ver- 

 dauungsapparat," is formed by invagination. According to Apostolides 

 tlie endoderm is formed by delamination, and there is no such invagina- 

 tion, although he describes a primitive opening in the lai-va, which he 

 considers the anus.t "Why lie should give this name to the opening in 

 question does not appear, and if he has gi'ounds for such an interpreta- 

 tion he does not make them evident in his account. 



Another opening into the digestive tract, of the origin of which he is 

 equally reticent, he calls the mouth. The endoderm or wall of the diges- 

 tive cavity, according to this author, is formed in Amphiura by delamina- 

 tion, and not by invagination of the blastoderm. 



As bearing upon the question of whether the primitive opening of 

 the larva becomes a mouth or not in Ophiurans, an observation of Sir 

 "Wyville Thomson on Ophiacanfha j,'ivipara, Ljn., is important. He 

 says : J " Although I had not an opportunity of working the matter out 



* Studien iiber die Entwickelung der Echinoderraen und Nemertinen. M^m, 

 de I'Acad. Imp. des Sci. St. Pctersh., VII. Ser., XIV. 8, p. 14. 



t Metschnikoff, in a later publication {Zeit.f. Wiss. Zool, XXXVII., p. 307,) ex- 

 presses an opinion against the idea of Apostolides that the endoderm is formed by 

 delamination in Ophiothrix, and explains the error into which he supposes Apos- 

 tolides has fallen, by the supposition that he (Apostolides) has confounded the 

 mesoderm with the entoderm. In a note on the same page he takes occasion, 

 however, to express his agreement with Apostolides that an intestine and anus is 

 present in the embryo of A. squamata. 



X Notice of Peculiarities in the Mode of Propagation of certain Echinoderms of 

 the Southern Sea. Journ. Linn. Soc. XII., pp. 77 and 78. Here mentioned as 



