XII 



PHYLUM MOLLUSCA 



691 



developed organs which occur in the bases of the most anterior 

 filament of the inner lamina of the gill in Mytilus and some other 

 genera. 



Reproduction and Development. Most Pelecypoda are 

 dioecious, but several hermaphrodite forms are known. Some of 

 these, such as some Oysters, 

 are protandrous, the gonad 

 producing first sperms and 



afterwards ova : in others 

 part of the gonad serves as 

 an ovary, part as a testis, 

 the two opening into a 

 common duct : in others 

 again there is a distinct 

 ovary and testis on each 

 side opening by separate 

 ducts. There are never any 

 accessory organs of repro- 

 duction, such as sperma- 

 theca, penis, &c. Fertili- 

 sation frequently takes 



place in the Water after the FIG - 604. Veliger larva of Ostrea. a. anus ; dm 



, . j dorsal longitudinal muscle ; 1. " liver " ; m. mouth 



eggS are laid. Segmentation ma. stomach ; s._shell ; sm. adductor muscle ; ss. 



is total and follows the 

 spiral type. One of the 

 four megameres is very much larger than the other three. The 

 gastrula is formed either by invagination or by epiboly. A shell- 

 gland (Fig. 603, sd.) is formed as an invagination of the dorsal 

 surface, a stomodseum (m) as an invagination of the ventral surface, 

 and the larva of most forms, unlike that of Anodonta or Unio, 



H 



hinge of shell; Vel. velum; vm. ventral longitu- 

 din t! mus cie. (From Korschelt and Heider.) 



FK; OU5. Two embryos of Cyclas. a. anus ; by. byssus-gland ;/. foot ; g. gonad ; k, gill ; 

 m. mouth ;m + l. stomach and " liver " ; mr. edge of mantle ; n. kidney ; p. pericardium ; 

 *', unpaired shell ; s", rudiment of paired shell ; sd. shell -gland ; vd. gullet ; vel. velar area. 

 (From Korschelt and Heider.) 



