GENERAL PART. 63 



there is no reason to think that this manner of entoderm formation should 

 occur in any Crinoid with total cleavage. 



A characteristic feature of both Tropiometra and Antedon is found in the 

 shape of the archenteron, which is curved in its upper end. Seeliger has 

 ascertained that in Antedon the curvature is directed against the ventral 

 side; in Tropiometra the orientation could not be ascertained, but there is, 

 evidently, no reason to doubt that it is in conformity with what occurs in 

 Antedon, In Isometra vivipara such curvature of the archenteron is not distinct. 



It seems very probable that this feature is of some morphological import- 

 ance. It is a general feature in Echinoderm larvae that the upper end of the 

 archenteron curves towards the ventral surface in order to meet the invagi- 

 nation, which forms the larval mouth. The suggestion then occurs that the 

 curvature of the archenteron in Comatulids is a morphological reminiscence 

 of a stage in the Crinoid phylogeny when a larval mouth did really develop 

 in that position. The morphological meaning of the vestibulary invagination 

 is, doubtless, that it represents the larval mouth. 46 That it does not open till 

 later, at the metamorphosis, is due to the fact that the egg contains sufficient 

 nutrition for the developing embryo. If true pelagic Crinoid larvae really 

 exist, comparable to the Pluteus, Bipinnaria, and Auricularia larvae, of other 

 Echinoderms, self-feeding like these, not subsisting alone on the yolk sub- 

 stance contained in the egg, it will doubtless be found that the larval mouth 

 occurs on the ventral side as in other Echinoderm larvae, in the place where 

 the vestibulary invagination occurs in the Comatulid larvae. The fact that 

 the mouth of the Pentacrinoid does open in the bottom of the vestibulum is 

 in itself not a direct proof of the homology with the larval mouth. Also, in 

 the bottom of the amnion cavity of the Echinoid larva the final mouth opens, 

 but the amnion is evidently not at all homologous with the vestibulum of the 

 Crinoid larva. 



A noticeable difference between Antedon and Tropiometra is found in the 

 shape of the blastopore, which is an elongate, transverse slit in Antedon, 

 while it is a small, round opening in Tropiometra. At the present state of 

 our knowledge of Crinoid embryology, it is of course impossible to say which 

 of these shapes represents the more primitive condition or which is the more 



general in Comatulids. 



4. THE ENTERQOEL. 



In the three forms in which the formation of the enterocoel has been 

 studied, Antedon, Tropiometra, and Isometra, it proceeds exactly in the same 

 way. Immediately after the closure of the blastoporus the archenteron 

 divides into an upper and lower part, the upper forming the entoderm and the 

 hydroccel, the lower giving rise to the enterocoel. The latter becomes trans- 

 versely elongated, the middle part narrowing into a thin canal connecting 

 the two widening ends and soon the connecting canal atrophies. The two 



"Thus far it is not so "sehr unzweckmassig Larvenmund genannt." (Seeliger, p. 195.) 



