580 



ENTEROPNEUSTA. 



FIG. 273. YOUNG TORNARIA. 

 (After Miiller.) 



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

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

 contractile cord. 



established, and on it are placed 

 two eye-spots (fig. 273 oc, and 

 fig. 274 A). A deep bay is 

 formed on the ventral surface of 

 the larva. 



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 pulsating vesicle or heart is 

 also formed (fig. 274 B, /if), and arises, according to Spengel 

 (No. 572), as a thicken- 

 ing of the epidermis. 

 It subsequently be- 

 comes 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 

 together give rise to a 

 complete investment of 

 the adjoining parts of 

 the alimentary tract. 

 The two parts of each 

 coalesce, and thus form 



ten- 



FlG. 274. 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. water- vascular vesicle; 



v . circular blood-vessel. 



