118 GEORGE W. FIELD. 



At the earliest stage iu which I have yet found this it is close 

 to the raesenchymatous lining of the body-wall ; whether it 

 arises from the lining of the wall or not I cannot yet positively 

 say. Later a lumen is formed in the midst^ and by the increase 

 of the lumen the cells of the wall become flattened. The 

 earliest stage which I have yet seen is shown in fig. 34, s. v. 

 Successive stages in the growth are shown in figs. 27 and 31. 

 In a Bipinnaria like fig. 29 the schizocoel vesicle, figured in 

 fig. 31, measured '04 mm. in its antero-posterior diameter. 

 Of the ultimate fate of this schizocoel, or of its identification 

 with the schizocoels hitherto described, I am not certain. 



General Considerations. 



Morphological opinions upon the significance of the larval 

 form of the Echinoderms fall into two diametrically opposite 

 classes : (1) that the larval form has been coenogenetically 

 acquired, or (2) that it is ancestral in character. 



The arguments which make for the former are chiefly based 

 upon the necessity for better means of distribution, and in 

 the free-swimming larval form is found a means secondarily 

 acquired for the purpose of eff'ecting this distribution. This 

 view necessarily presupposes that the ancestor of the Echino- 

 derms was sedentary ; but this fundamental supposition does 

 not seem to be well grounded. Even though the ancestors 

 of the Crinoids appearing in the Cambrian seem geologi- 

 cally to be oldest, it by no means follows that we find in 

 paleontology a correct phylogenic record. From the nature 

 of the case, too, we could gain from palaeontology little know- 

 ledge of the phylogenetic stages previous to the time when the 

 hard calcareous skeletal parts appeared, and these skeletal 

 parts are seen to be structures which have undergone exceed- 

 ingly great coenogenetic modifications. Palaeontology, there- 

 fore, far from giving us a record of the phylogenetic series of 

 ancestral forms, furnishes little more than a history of the 

 skeletal parts of some of the later descendants. No transi- 

 tional adult forms uniting the Echinoderms with the other 

 animal groups are known ; as a group they stand widely iso- 



