190 



INVERTEBRATE ZOOLOCY 



are highly sensory. The small projections from that part of the 

 mantle which forms the siphons are especially sensitive to touch. 

 Eyes (Fig. 90) are distributed over the margins of the mantle in 

 some forms such as the scallops. 



Reproduction. — Most of the Acephala are dioecious. In 

 many of the marine forms the gonads discharge their germ cells 

 directly into the water where they undergo fertihzation and pass 

 through cleavage and early larval development to form free- 

 swimming larvae. In the fresh-water mussels, however, the eggs 



Fig. 90. 



-A section through an eye from the mantle margin of Pecten. {Slightly 

 modified from Patten). 



after discharge from the ovary are passed into brood pouches 

 or marsupia located within the gills. Either in the marsupia, 

 or en route to them, sperm cells, brought in with the water 

 currents by way of the siphon, fertilize the eggs. Each egg 

 undergoes cleavage to form a larva known as a glochidium (Fig. 

 91 A-D). This larval form is provided with a thin bivalve shell 

 operated by a single adductor muscle. Other soft parts within 

 the shell are not clearly organized except for the presence in some 

 species of a "larval thread" of uncertain significance and of 

 minute sensory hairs. The glochidia, when discharged from the 



