372 



TORNARTA. 



Paired excretory organs, opening to the exterior and into the 

 body cavity, are often present (fig. 226 np/i). 



This type of larva is found in the Rotifera (fig. 217) (in which 

 it is preserved in the adult state), the Chaetopoda (figs. 225 and 

 226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and 

 the Polyzoa (fig. 228) \ 



A. B 



FIG. 223. A. THE LARVA OF A HOLOTHUROID. 

 B. THE LARVA OF AN ASTEROID. 



m. mouth; st. stomach; a. anus; I.e. primitive longitudinal ciliated band; pr.e. 

 proeoral ciliated band. 



4. Tornaria. This larva (fig. 229) is intermediate in most 

 of its characters between the larvae of the Echinodermata (more 

 especially the Bipinnaria) and 

 the Trochosphere. It resembles 

 Echinoderm larvae in the posses- 

 sion of a longitudinal ciliated 

 band (divided into a praeoral 

 and a postoral ring), and in the 

 derivation of the body cavity 

 and water-vascular vesicle from 

 alimentary diverticula ; and it 

 resembles the Trochosphere in 

 the presence of sense organs on 

 the prseoral lobe, in the existence 

 of a perianal ring of cilia, and in 

 the possession of a contractile 

 band passing from the praeoral lobe to the cesophagus. 



FIG. -224. A LARVA OF STRONGY- 

 LOCENTRUS. (From Agassiz.) 



vi. mouth ; a. anus ; o. oesophagus ; 

 d. stomach ; f. intestine ; 



and ?'. 



ciliated ridges ; w. water- vascular tube ; 

 r. calcareous rods. 



1 For a discussion as to the structure of the Polyzoon larva, vide Vol. II. p. 305. 



