ZOOLOGY 



SECT. 



rob. 



card s 



and endoderm become completely separate. The embryo 

 elongates and a transverse groove (gr.) appears (A) : the mouth 

 is formed by an invagination in the position of the groove. 

 The anus is developed in the position formerly occupied by 

 the blastopore. Before the mouth appears there are formed 

 two diverticula of the archenteron which become completely 

 separated off, their cavities subsequently giving rise to the coelomic 

 cavities of the proboscis and of the collar, and the body-cavity of 

 the trunk. By the appearance of a second transverse groove (B] 

 the body of the embryo becomes divided into three parts an 

 anterior, a middle, and a posterior these being the beginnings 



respectively of the 

 proboscis, the collar, 

 and the trunk. The 

 branchial region is 

 marked off by the 

 appearance of a 

 pair of apertures 

 the first pair of 

 branchial slits (g.sl.) 

 and other pairs 

 subsequently de- 

 velop behind these. 

 In the species 

 that, undergo a 

 metamorphosis the 

 embryo assumes a 

 larval form termed 

 Tornaria (Figs. 709 

 and 710). This is 

 somewhat like an 

 Echinoderm larva, 

 with a looped cili- 

 ated band some- 

 times lobed, some- 

 times produced into 



tentacles, running along its anterior part, and a ring of mem- 

 branellae (cil. r.\ in some cases with a ring of smaller cilia (cil. r 2 .), 

 round the posterior (anal) end. At the anterior end, in the 

 middle of the pre-oral lobe, is an ectodermal thickening the 

 apical plate containing nerve-cells and eye-spots, and, like the 

 apical plate of a trochophore, constituting the nerve-centre of the 

 larva : this disappears in the adult. There is a short alimentary 

 canal with mouth and anus. The ciliated bands are lost ; an 

 outgrowth is formed to give rise to the proboscis, and a con- 

 striction separates it from the collar ; the hinder part becomes 

 elongated and narrow to form the body of the animal ; a series of 



<x,ns 



FIG. 709. Tornaria. Dorsal view. an. anus ; card. s. cardiac 

 sac ; cil. r. post-oral ciliated band (membranelke) ; cil. r 2 . 

 posterior ciliated ring ; eye, eye-spots on apical plate ; prob . cav. 

 proboscis cavity ; pro'), po. proboscis pore. (After Spengel.) 



