64 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



vesicles, the right and left enterocoel vesicles, are thus separated from one 

 another, and gradually occupy their final place, the right at the dorsal side, 

 the left at the posterior, which later becomes the anterior end. 



Although the process of the enteroccel formation has not been followed 

 in Compsometra and Notocrinus, the arrangement of the enterocoel vesicles 

 in the larval stages studied of these, identical with the arrangement found in 

 the corresponding stages of the three other types, leaves no doubt that the 

 whole process is the same in all. There is, then, fair evidence that this will 

 prove to hold good of the Comatulids as a whole. 47 



In one point, however, Antedon differs very markedly from both Tfopio- 

 metra and Isometra. In Antedon there is a pair of posterior prolongations 

 from the anterior part of the archenteron embracing the narrow connecting 

 canal of the two enterocoel vesicles, sometimes even uniting below this canal, 

 which thus goes through a ring (Bury, Seeliger). In Tropiometra and Iso- 

 metra I have found no trace of such posterior prolongations from the anterior 

 part of the archenteron. This complicated structure is, then, not of general 

 occurrence in the embryology of Comatulids. It is hard to see the reason of its 

 occurrence in Antedon, and upon the whole, its morphological value. It may 

 be a morphological ancestral reminiscence; at least it is hardly conceivable 

 that such a complicated structure, apparently without any special importance, 

 should have originated in this genus, while on the other hand it is not so 

 inconceivable that an ancestral reminiscence from it should have been retained 

 in one form and dropped in others. In any case it is highly interesting to 

 note this rather conspicuous difference in the early developmental processes 

 between such nearly related forms as Antedon and Tropiometra. In Isometra, 

 which is still nearer to Antedon than Tropiometra, belonging to the same family 

 as the former, the absence of the posterior prolongations might perhaps be 

 due to the unusual embryological conditions, and it would certainly be unwise 

 to lay any stress thereon if that were an isolated case; but together with 

 Tropiometra, in which the embryological conditions are as normal as possible, 

 this case also counts. 



5. THE HYDROCCEL. 



Although the development of the hydrocoel could not be followed in detail 

 from its origin in any of the forms studied in the present memoir, such stages 

 as were observed are in perfect accordance with the corresponding ones in 

 Antedon as known especially through the researches of Seeliger, and there 

 is no reason to doubt that the hydroccel in the Comatulids as a whole originates 

 in the same way as a pouch from the upper part of the divided archenteron. 48 

 From the vesicle thus separated off, lying on the ventral side, proceeds an 

 anterior prolongation, the parietal canal, which in its turn is separated off 



In Antedon pelasus the enteroccel formation appears to be quite different (comp. the author's paper 

 quoted on p. 54). 



48 Antedon pctusus perhaps makes an excepiion also in this regard. 



