370 



ZOOLOGY 



SECT. 



sending prolongations into the pallial sinuses. Some genera are 

 dioecious, others hermaphrodite, the epithelium of the gonads 

 producing, in the latter case, both ova and sperms. 



The development of the Brachiopoda is best known in Cistella, 

 in which the first stages of development are passed through 



in a pair of cavities, the . hrood-pouches, 

 A nf situated at the base of the lophophore. 



Segmentation is regular and complete, 

 and results in the formation of a blas- 

 tula, which is converted into a gastrula 

 by invagination (Fig. 296, A). Paired 

 sacs, the ccelomic pouches (p.v), grow out 

 from the archenteron, and the blastopore 

 closes. The ccelomic sacs separate from 

 the mesenteron (B, me) or middle portion 

 of the archenteron, and extend between 

 it and the ectoderm, forming the right 

 and left divisions of the ccelome : their 

 outer walls thus become the somatic, 

 their inner walls the splanchnic layer 

 of mesoderm. The mesenteron remains 

 closed and surrounded by the ccelomic 

 sacs during the whole of larval life. 



The embryo now elongates and be- 

 comes divided by an annular groove 

 into two divisions, an anterior and a posterior : a second groove 

 soon appears in the anterior division, the embryo then consisting 

 of three regions (B), which, from a superficial 

 point of view, might be looked upon as meta- 

 meres. But as the segmentation affects only 

 the body- wall and not the internal parts, the 

 process is not one of metamerism, and the 

 three apparent segments are called respect- 

 ively the head-region, the oody-region, and 

 the peduncular region (Fig. 297). 



Next the head-region grows out into an 

 umbrella-like disc surrounded with cilia and 

 bearing four eye-spots (Fig. 298, A), and on 

 the body-region a backwardly-directed an- 

 nular fold (m) appears, bearing four groups 

 of provisional setae. In Cistella, which has 

 no setae in the adult condition, the pro- 

 visional setae are subsequently lost, and are 

 not replaced. In forms which possess setae in the adult condition 

 the provisional setae are likewise lost, but are replaced by the per- 

 manent setae. Soon this mantle-fold divides into dorsal and ventral 

 lobes, which, being directed backwards, cover the peduncular region. 



I>v^ 



Fio. 2%. — Two stages in the 

 development >of Cistella 

 (Argiope). b. provisional set* ; 

 bl. blastopore ; me, mesen- 

 teron ; pv. ccelomic pouches. 

 (From Balfour's Embryology, 

 after Kowalevsky.) 



Fig. 297 



Young larva of 

 Cistella, with the 

 three segments, two 

 eye-spots, and two 

 bundles of seta; (From 

 the Cambridge Natural 

 History, after Kowal- 

 evsky.) 



