OVIPOSITION AND CAKE OF THE BROOD. 23 



Icihlv also /'>rtt>u, Fullarton, No. 14). This form of oviposition is 

 common among marine forms, but among fresh-water and a few 

 marine forms the female takes considerable care of the brood. In 

 these latter cases, fertilisation occurs either in efferent genital ducts 

 or in the branchial cavity, into which the spermatozoa have passed 

 from outside. In U/u'n and Anodonta, for instance, the eggs which 

 are discharged into the inner division of the supra-branchial cavity 

 are driven by the stream of respiratory water filled with sperm into 

 the cloaca, and thence into the external division of the supra- branchial 

 cavity, and so into the interlobar cavity of the outer gill-lamella whei'e 

 they pass through their embryonic development. In Pisidium, the 

 eggs lie in special brood-pouches at the base of the gills, and, in 

 C)/rf>is also, brood -capsules are formed in the gills by growths of the 

 epithelium between the septa, and in each of these an egg or embryo 

 lies. Embryos have even been observed in such pouches nourishing 

 themselves by swallowing the branchial epithelial cells (Stepanoff, 

 No. 54, Ziegler, No. 60). Dreisseyisia is peculiar among fresh- 

 water Lamellibranchs in this respect, and it discharges its eggs direct 

 into the water like the marine forms mentioned above (Korschelt, 

 No. 27). On the other hand, in some marine Lamellibranchs, care is 

 taken of the brood. The eggs of Teredo, for example, are retained 

 in the branchial chamber (Hatschek), and in Odrea edulis they are 

 found, up to the time when the free-swimming larva develops, 

 within the mantle-cavity (Mobius No. 37, Horst, No. 19). Ento- 

 valva mirabilis forms a bell -shaped brood-cavity at the posterior end 

 of the body through the fusion of the two halves of the mantle, in 

 which the ernbryos remain till the Trochophore stage is reached 

 (Voeltzkow, No. 57). 



The spherical eggs are loosely surrounded by a thin, structureless 

 membrane (vitelline membrane), which may be lost even during 

 embryonic life (e.g., in Teredo). Sometimes the egg-integument is 

 exceedingly delicate, and disappears even during the earliest stages 

 of development, and then the eggs pass out direct into the water 

 (Dreissensia, Mytilus, Ostrea). On the other hand, the envelope may 

 be thicker and multilaminar as in Cardinal exiguum, the eggs of which 

 with their vitelline membranes have a lenticular form and are 

 attached to firm objects by the mother (Loven, No. 33). In some 

 genera (Anodonta, U/rio, Ci/cIuh), a chimney-like appendage, the 

 micropyle, is found on the egg-integument (Fig. 22, m^jjuJK)). 



