ENTEROPNEUSTA . 



483 



cc. 



an. 



FIG. 273. YOUNG TORNARIA. 

 (After Miiller.) 



m. mouth; an. anus; w. water- 

 vascular vesicle; oc. eye-spots; c.c. 

 contractile cord. 



As the larva grows older the original bands of cilia become more 

 sinuous, and a second transverse 

 band with small cilia is formed (in 

 the Mediterranean larva) between 

 the previous transverse band and 

 the anus. The water-vascular vesicle 

 is prolonged into two spurs, one on 

 each side of the stomach. A pul- 

 sating vesicle or heart is also formed 

 (fig. 274 B, lit}, and arises, according 

 to Spengel (No. S7 2 )> as a thicken- 

 ing of the epidermis. It subsequently 

 becomes enveloped in a pericardium, 

 and is placed in a depression in the 

 water- vascular vesicle. Two pairs of 

 diverticula, one behind the other, 

 grow out (Agassiz, No. 568) from the 

 gastric region of the alimentary canal. 

 The two parts of each pair form 

 flattened compartments, which to- 

 gether give rise to a complete in- 

 vestment of the adjoining 

 parts of the alimentary 

 tract. The two parts of 

 each coalesce, and thus 

 form a double-walled cylin- 

 der round the alimentary 

 tract, but their cavities re- 

 main separated by a dorsal 

 and ventral septum. 



Eventually (Spengel) 

 the cavity of the anterior 

 cylinder forms the section 

 of the body cavity in the 

 collar of the adult, and that 

 of the posterior (fig. 274 B, 

 c) the remainder of the 

 body cavity. The septa, 

 separating the two halves 

 of each, remain as dorsal 

 and ventral mesenteries. 



The conversion of Tor- 

 naria (fig. 274 A) into Ba- 

 lanoglossus (fig. 274 B) is 

 effected in a few hours, and 

 consists mainly in certain 

 changes in configuration, 

 and in the disappearance 

 of the longitudinal ciliated band. 



FIG. 274. Two STAGES IN THE DEVELOPMENT 

 OF TORNARIA. (After Metschnikoff.) 



The black lines represent the ciliated bands. 



m. mouth; an. anus; br. branchial cleft; fit. 

 heart; c. body cavity between splanchnic and 

 somatic mesoblast layers; w. water-vascular vesi- 

 cle; v. circular blood-vessel. 



