VIII 



PHYLUM MOLLUSCOTDA 



363 



S f tag cisieua 



(Argiope). b. provisional setae; 

 W. blastopore ; me. mesen- 



teron ; p v . cceiomic pouches. 



are passed through in brood-pouches ; in the fourth the eggs are 



only attached temporarily to the setae of the parent and become 



free in a comparatively immature condition. Segmentation is 



regular and complete, and results in the 



formation of a ciliated blastula, the wall 



of which is composed of cells of like 



character throughout. This is converted into 



a gastrula by invagination (Fig. 302, A) : 



the blastopore narrows to a slit and gradu- 



ally completely closes. The stomodaeum 



arises as an invagination of the surface 



ectoderm apparently in the position of the 



anterior end of the former blastopore. An 



ectodermal thickening in front forms the 



apical plate from which the supra 



cesophageal ganglion is derived, and a 



second thickening behind the mouth on the 



ventral surface subsequently gives origin to 



the infra-cesophageal ganglion. Meanwhile 



the CCelome Originates in a pair of SaCS 

 /T ,. \ i 



(Fig. 302, pv), or a single sac subsequently 

 dividing into two the cceiomic pouches- 

 growing out from the archenteron, and 

 eventually becoming completely closed off from it to give rise to 

 the ccelome, which is thus of the enteroccele-type as regards its 

 derivation. A groove, the mantle-groove, running transversely 

 divides the embryo into two regions, a broad 

 anterior and a narrower posterior. Folds 

 which are formed in front of the mantle-groove 

 are the beginnings of the dorsal and ventral 

 mantle-lobes. A groove which appears in 

 front of these separates off a head segment from 

 the rest, and the embryo is now superficially 

 divided into three segments or regions head 

 region, body- or mantle- region and foot- or 

 peduncle- region (Fig. 303). The mantle-lobes 

 te increase in size and grow backwards over the 

 Sf m and two peduncular region : in them are developed four 

 bundles of setae (From groups of provisional setCB which project back- 



the Cambridge Natural c , r /T7V * 0/AO \ /-v , n i i .1 



History, after Kowai- wards (Fig. 303) i in Cistella, in which there are 

 no setae in the adult, these are thrown off subse- 

 quently : in the genera in which setae are present in the adult they 

 are also thrown off, but are replaced later by the permanent setae. 

 The head-region has in the meantime become broadened out and 

 in Cistella soon assumes the form of an umbrella-shaped disc 

 bordered with cilia and usually bearing eye-spots (Fig. 304, A), 

 and sometimes an apical tuft of long cilia. In this condition the 



