ZOOLOGY 



SECT. 



rob COM 



cord- s 



ob bo 



and endodcrm become coniYjletely separate. The embryo 

 elongates and a transverse groove (.yr.) appears {^A) : the mouth 

 is formed hy 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 iheccElomic 

 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 

 ye- 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 (f/.sl.) 

 — and other pairs 

 subsequently de- 

 velop loehind 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- 

 branella; {cil. r.), m some cases with a ring of smaller cilia {cil. r.) 

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

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

 apiml plate— contammg nerve-cells and eye-spots, and, like the 

 apical plate ..f 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 arc lost ; aii 

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

 striction separates It fr„m the collar; the hinder part becomes 

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



:il.r 



'cil r^ 



Fici. 709.— Tornaria. licireal view. ,:„. muis: nn-.i 



.sac; cil. r. pust-onil ciliated band (mouiliranelliu) ■ ','(/ 

 posterior ciliated ring ; , ,/c, eye-sijots on apical plate ; prob ra 

 proboscis cavity ; 2>ro',. -po. proboscis pore. (After Spengel ) 



■diae 



