8 



ZOOLOGY 



SECT. 



The course of the development (Figs. 717-719) differs in different 

 species. In some it is comparatively direct ; in others there is a 

 metamorphosis. Impregnation is external. Segmentation is com- 

 plete and fairly regular, resulting in the formation of a blastula, 

 which is at first rounded, then flattened. On one side of the 

 flattened blastula an invagination takes place. The embryo at 

 this stage is covered with short cilia, with a ring of stronger cilia. 

 The aperture of invagination closes and the ectoderm 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 sepa- 

 rated off, their cavities 

 subsequently giving rise 

 to the ccelomic 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 em- 

 bryo becomes divided 

 into three parts an an- 

 terior, a middle, and a 

 posterior - - these being 

 the beginnings respec- 

 tively 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. si.) and other pairs subsequently develop 

 behind these. 



In the species that undergo a metamorphosis the embryo assumes 

 a larval form termed Tornaria (Figs. 718 and 719). This is some- 

 what like an Echinoderm larva, with a looped ciliated band, some- 

 times lobed, sometimes produced into tentacles, running along its 

 anterior part, and a ring of membranellse (cil. r.),m 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- 



Fio.~719. Tornaria. Lateral view. r Lettering as 

 in Fig. 718 : in addition, int. intestine"; mo. mouth. 

 (After Spengel.) 



