674 



ZOOLOGY 



SECT. 



mes 



which is apparently to test the purity of the water entering by the 

 respiratory current. Close to each pedal ganglion a minute statocyst 

 (" otocyst ") (Fig. 580) is sometimes found, the nerve of which is 

 said to spring from the cerebro-pedal connective, being probably 

 derived from the cerebral ganglion. Sensory cells probably 

 tactile also occur round the edge of the mantle, and especially 

 on the fimbriae of the inhalant siphon. 



Reproductive organs. The sexes are separate. The gonads 

 (Fig. 576, gon.) are large, paired, racemose glands, occupying a 

 considerable portion of the visceral mass amongst the coils of 

 the intestine : the testis is white, the ovary reddish. The gonad 

 of each side has a short duct which opens (g. ap.) on the surface 

 of the visceral mass just in front of the renal aperture. 

 In ^the^breeding season the _ eggs, extruded from the genital 



rk aperture, pass into the 



mes supra-branchial chamber 



and so to the cloaca. 

 There, in all probability, 

 they are impregnated by 

 sperms introduced with 

 the respiratory current. 

 The oosperms are then 

 passed into the cavities 

 of the outer gill-laminae, 

 which they distend enor- 

 mously. Thus the outer 

 gill-laminae act as brood- 

 pouches, and in them 

 the embryo develops 



TIG. 581. Early embryo of Anodonta. eh, vitelline into the peculiar larval 



membrane ; ent. archenteron ; m. micropyle ; mes. meso- r -i 



derm ; rk, polar cells ; s&, shell-gland ; sz, lateral cells; lOrm presently to DC 



w, cilia. (From Korschelt and Heider's Embryology.) described. 



Development. Segmentation of the oosperm is complete, but 

 unequal. A gastrula is formed by the invagination of the mega- 

 meres into the micromeres, but the archenteron (Fig. 581, ent.) 

 thus formed is quite small and insignificant, and has no physiological 

 importance until a late period of larval life. Certain of the cells of the 

 gastrula are budded off into the blastocosle, where they accumulate 

 and form the mesoderm (mes.). At about the same time a deep 

 invagination (sd. 2 ) is formed, which might easily be mistaken for 

 the archenteron, but is really a very characteristic molluscan organ, 

 the shell-gland : it marks the dorsal surface of the embryo. The 

 posterior end is distinguished by a tuft of long cilia. 



The shell-gland becomes converted into a plate of long, cylin- 

 drical cells (Fig. 582, sd.), from which an impaired shell (s.) is 

 secreted. This is replaced before long by a bivalved shell of 

 triangular form, its ventral angles produced into incurved hooks 



