66 



BRACH10P0DA. 



pv. 



may be divided into two groups, the one including the genera Argiope, 

 Terebratula and Terebratulina, and the other being represented by 

 Thecidium. The distinctions between these two groups, however, 

 are only in points of secondary importance, and can be explained by 

 the crowding of the blastomeres in Thecidium. 



In Argiope, the mature eggs pass first into the body-cavity and 

 thence into the nephridial canals that function as oviducts. The 

 latter open into brood-pouches* lying on 

 either side of the body ; these are to be 

 regarded as invaginations of the body- 

 wall, and in them the eggs pass through 

 the first stages of their development. The 

 embryos are attached to the wall of 

 the brood-sac by a tough filament at their 

 anterior ends. It has not yet been 

 definitely ascertained where fertilisation 

 takes place, but it is probable that it 

 occurs after the egg has reached the 

 brood-sac. Cleavage is total and almost 

 equal, and leads to the development of 

 a regular coeloblastula which is followed 

 by a gastrula-stage resulting from invagina- 

 tion. During this stage, the plane of 

 symmetry of the body seems already to 

 be defined. The last point at which the 

 blastopore closes seems to correspond to 

 the anterior part of the ventral side, 

 perhaps to the position of the future oral 

 aperture (ef. similar conditions in Phoronis, p. 2). While the 

 blastopore is closing, two lateral coelom-sacs become separated from 

 the archenteron by ingrowths of its walls (Fig. 29 A); this abstriction 

 takes place in such a way that the last vestige of communication 

 between the three cavities is retained in the most anterior part of the 

 body. In this last point only is there a distinction between this 

 process and that by which mesodermal folds arise in Sagitta (Vol. i., 

 p. 367) ; in other respects the two processes are somewhat alike. 



After these coelomic sacs have become completely cut off, we 

 find in the now lengthening embryo an archenteron closed on all 



* In certain fossil forms, the embryos seem to pass through the whole of their 

 development within these brood-sacs, or at least within the mantle-cavity of 

 the parent, M is indicated by the discovery by Suess of quite young shells 

 enclosed in a Stringocephalus (c/. ZlTTBL, No. 17). 



PP- 



Fig. 29.— Two ontogenetic stages 

 of Argiope (after Kowalevsky, 

 from Balfour's Text-book). 

 A, late gastrula stage showing 

 the origin of the coelomic sacs 

 (pv). B, stage after the sepa- 

 ration of the three regions of 

 the body, b, provisional setae ; 

 bl, blastopore ; me, mid-gut ; 

 pv, coelomic sacs. 



