420 



SUB-THYL UM HZMICHORDA. 



m- 



c.r. 



move about at the bottom in shallow water. It elongates 

 and becomes more wormlike ; there is an anterior tuft and 

 a posterior ring of cilia; the primitive gut forms five 

 coelomic pouches ; a mouth and an anus are perforated, 

 there seem to be no fore-gut nor hind-gut invaginations. 

 The regions of the body are defined at a very early stage. 



The Tornaria larva found in other species is at first bell-shaped. 

 A ventral mouth opens into the curved gut, which is furnished with 

 a posterior terminal anus. A "dorsal pore" leads into a thin-walled 



sac which becomes the proboscis cavity 

 of the adult. There are external bands 

 of cilia, something like those of an 

 Echinoderm larva, and also an apical 

 sensory plate (like that of many Annelid 

 trochospheres), with two eye spots. The 

 Tornaria is a pelagic form. During its 

 period of free pelagic life it gradually 

 loses its distinctive bands of cilia, be- 

 comes diffusely ciliated, acquires a 

 proboscis and two gill-slits, and thus 

 approaches the form of the larva first 

 described. The further development is 

 the same in both cases. The Tornaria 

 must be regarded as the more primitive 

 larval form ; the temporary absence of 

 mouth and anus in the other type is 

 probably an adaptation acquired after 

 the pelagic habit was lost. 



Johannes Miiller ranked the Tor- 

 naria larva, whose adult form was not 

 then known, beside the larvre of 

 Echinoderms, and the resemblance has 

 been recently emphasised by Willey. 

 The ciliated bands of the Tornaria 

 resemble those of Echinoderm larva. 1 , 

 but this is only a superficial character- 

 istic. The anterior pouch, which 

 forms the cavity of the proboscis 



and communicates with the exterior, has also been compared with 

 the beginning of the water vascular system in Echinoderms, and it is 

 true that in both several independent coelom pouches grow out from 

 the primitive gut. The anterior body cavity in Balanoglossus com- 

 municates with the exterior by a pore, which becomes the proboscis-pore 

 of the adult, and this has been compared with the water-pore, or 

 outlet of the water vascular system of Echinoderms, which similarly 

 opens from an anterior enteroccel to the exterior. On the other 

 hand, the presence of an apical plate a structure almost invariably 

 absent in Echinoderms, suggests an affinity with an Annelid 

 trochosphere. 



s.c.r 



FIG. 208. - - Tornaria larva, 

 from the side. After Spen- 

 gel. 



M., mouth; g., gut; a., anus; 

 /?., heart ; p., pore entering 

 proboscis cavity ; c. r. , anal ring 

 of cilia ; s.c.r., secondary anal 

 ring. The dark wavy line in- 

 dicates the margin of the lobes 

 of the larval body with their 

 bands of cilia. Note also the 

 apical spot with cilia and sense 

 organ. 



