310 



COMPARISON BETWEEN TYPES OF LARVAE. 



described now as the ' enteric 'axis; whilst the new main axis, that parallel 

 with the plane of progression, passes through the dorsal region of the body 

 running obliquely in relation to the enteric axis. Only one lobe or out- 

 growth of those radially disposed in the telostomiate organisms now per- 

 sists. This lobe lies dorsally to the mouth, and through it runs the new 

 main axis. This lobe is the Prostomium, and all the organisms which 

 thus develop a new main axis, oblique to the old main axis, may be called 

 prostomiate." 



It will be seen from this quotation that the aboral part of the body is 

 supposed to elongate to form the trunk, while the prseoral region is derived 

 from one of the tentacles. 



Before proceeding to further considerations as to the origin of the 

 Bilateralia, suggested by the Pilidium type of larva, it is uecesssary 

 to enter into a more detailed comparison between our larval forms. 



A very superficial consideration of the characters of these forms 

 brings to light two important features in which they differ, viz. : 



(1) In the presence or absence of sense organs on the prasoral 

 lobe. 



(2) En the presence or absence of outgrowths from the alimentary 

 tract to form the body cavity. 



The larvae of the Echinodermata and Actinotrocha (?) are without 

 sense organs on the pras- 

 oral lobe, while the other 

 types of larva? are provided 

 with them. Alimentary 

 diverticula are character- 

 istic of the larva? of the 

 Echinodermata and of Tor- 

 naria. 



If the conclusion al- 

 ready arrived at to the 

 effect that the prototype 

 of the six larval groups 

 was descended from a ra- 

 diate ancestor is correct, it 

 appears to follow that the 

 nervous system, in so far 

 as it was differentiated, 

 had primitively a radiate 

 form ; and it is also pro- 

 bably true that there were 

 alimentary diverticula in 

 the form of radial pouches, 

 two of which may have 

 given origin to the paired 

 diverticula which become 

 the body cavity in such 

 types as the Echinoder- 



a/t 



CCft 



FIG. 229. Two STAGES IN THE DEVELOPMENT 

 or TORNABIA. (After Metschnikoff .) 



The black lines represent the ciliated bands. 



m. mouth; on. anus; br. branchial cleft ; Jit. 

 heart ; c. body cavity between splanchnic and so- 

 matic mesoblast layers; w. so-called water-vascular 

 vesicle; r. circular blood-vessel. 



