Ill 



PHYLUM MOLLUSCOfDA 



(Ari/iojjc). b. provisional setae; 

 bl. blastopore ; me. mesen- 

 teron ; i>v. m'loniic pouches. 

 (From Balt'oiir's Embryology, 

 after Kowalevsky.) 



are passed through in brood-pouches ; in the fourtli 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 imagination (Fig. 302, A) : 

 the blastopore narrows to a slit and gradu- 

 ally completely closes. The stomodseum 

 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 



flip infra-rpsrmhao-pal p-a no-lion Meanwhile FlG - 3()2 - Two stages in the 

 .opildgec m. lut development of Cistella 



the ccelome originates in a pair of sacs 



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



dividing into two the coelomic 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 enterocoele-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- reqion (Fig. 303). The mantle-lobes 



FIG. 30:?. Young larva of '-, -, j ,, 



Cisteiia, with the increase in size and grow backwards over the 

 eye^spotef m and two peduncular region : in them are developed four 

 bundles of setae. (From groups of provisional setO3 which proiect back- 



the Cambridge Natural , . - r onox -re n L- i, 1 



uistoru, after Kowai- wards (.big. <303) : in Cistella, in which there are 

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

 quently : in the genera in which seta3 are present in the adult they 

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

 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 



