VIII 



PHYLUM MOLLUSCOIDA 



occurs in all known genera except Rhynchonella, in which there are 

 two pairs, one dorsal and one ventral. Besides discharging an 

 excretory function they act as gonoducts. 



The nervous system always takes the form of a circum-oeso- 

 phageal ring with ganglionic enlargements, the largest of which 

 is ventral or sub-oesophageal in position. Otocysts have been 

 described in Lingula, rudimentary eyes in Megerlia, and patches 

 of sensory epithelium in Cistella : with these exceptions sensory 

 organs are unknown. 



There are usually four gonads, two dorsal and two ventral, 

 sending prolongations into the pallia! 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 brood- 

 pouches, situated at the base of the 

 lophophore. Segmentation is regular 

 and complete, and results in the forma- 

 tion of a blastula which is converted 

 into a gastrula by invagination (Fig. 

 272, A). Paired sacs, the coelomic pouches 

 (p.v), grow out from the archenteron, 

 and the blastopore closes. The coelomic 

 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 be- 

 come the somatic, their inner walls the 

 splanchnic layer of mesoderm. The 



mesenteron remains closed and surrounded by the coelomic sacs 

 during the whole of larval life. 



The embryo now elongates and becomes 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 con- 

 sisting of three regions (B), which, from a superficial point ot view, 

 might be looked upon as metameres. But as the segmentation 

 affects only the body-wall and not the internal parts, the proces 

 is not one of metamerism, and the three apparent segments are 

 called respectively the AW region (Fig. 273, vs), the body-region, 

 (ms) and the peduncular region (Its). 



Next the head-region grows out into an umbrella-like disc sur- 

 rounded with cilia and bearing four eye-spots (A), and on the 

 body-region a backwardly-directed annular fold (>/*) appears, bear- 



FIG. 272. Two stages in the 

 development of Cistella 

 (A.-ij.rtp. ), >. provisional setaj ; 

 bl. blastopore ; nit. mesen- 

 teron ; }/<. ccelomie pouches. 

 (From I5alfonr's Embryology, 

 after Kowalensky.) 



