6 



ZOOLOGY 



SECT. 



ca. rd. ,s 



cil.i 



Fig. 667. Tornaria. Dorsal view. an. anus ; card. s. cardiac 

 sac ; cil. r. post-oral ciliated band ; cil. r 2 . posterior ciliated 

 ring ; eye, eye-spots on apical plate ; prob. car. proboscis cavity; 

 prob. po. proboscis pore. (After Spengel.) 



being the beginnings 

 respectively of the 

 proboscis, the collar 

 and the trunk. The 

 branchial region be- 

 comes marked off 

 by the appearance 

 of a pair of apertures 

 -the first pair of 

 branchial slits (g. si.) 

 -and other pairs 

 subsequently de- 

 velop behind these. 

 In the species 

 that undergo a 

 metamorphosis the 

 embryo assumes a 

 larval form termed 

 Tornaria. This 

 (Figs. 667 and 668) 

 is somewhat like an 

 Echinoderm larva, 

 with a pair of cili- 

 ated bands, one of which is considered prse-oral, and the other 

 post-oral, and an inde- 

 pendent circlet of strong 

 cilia at the posterior 

 end. At the anterior 

 end, in the middle of 

 the prse-oral lobe, is an 

 ectodermal thickening 



-the apical plate 

 containing nerve-cells 

 and eye-spots and, like 

 the apical plate of a 

 trochosphere, constitut- 

 ing the nerve-centre of 

 the larva : this disap- 

 pears in the adult. 

 There is a short ali- 

 mentary canal with 

 mouth and anus. The 

 ciliated bands become 

 lost ; an outgrowth is 

 formed to give rise to 

 the proboscis, and a 



onTHstri'ptirm ivofo Fig. r>68. Tornaria. Lateral view. Lettering as in 



Fig. 607 ; in addition, mo. mouth. (After Spengel.) 



card.s 



-ft rob. fie 



