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 gastrule 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. Metschnikoff* supposes that in Amphiura the stomach, “ Ver- 
dauungsapparat,” is formed by invagination. According to Apostolides 
the endoderm is formed by delamination, and there is no such invagina- 
tion, although he describes a primitive opening in the larva, which he 
considers the anus.t Why he should give this name to the opening in 
question does not appear, and if he has grounds 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 Ophiacantha vivipara, Ljn., is important. He 
says: { “ Although I had not an opportunity of working the matter out 
* Studien iiber die Entwickelung der Echinodermen und Nemertinen. Mém. 
de’ Acad. Imp. des Sci. St. Pétersb., VII. Sér., XIV. 8, p. 14. 
+ Metschnikoff, in a later publication (Zeit. f Wiss. Zool., XXXVIL., p. 807,) 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. syuamata. 
t 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 
