376 



COMPARISON -BETWEEN TYPES OF LARWE. 



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

 posed to elongate to form the trunk, while the praeoral 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 

 necessary 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. : 



(i) In the presence or absence of sense organs on the praeoral 

 lobe. 



FlG. 229. TWO STAGES IN THE DEVELOPMENT OF TORNARIA. 



(After Metschnikoff.) 



The black lines represent the ciliated bands. 



m, mouth; an. anus; br. branchial cleft; ht. heart; c. body cavity between 

 splanchnic and somatic mesoblast layers ; iv. so-called water-vascular vesicle ; v. 

 circular blood-vessel. 



(2) In 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 prseoral lobe, while the other types 



