PELECYPODA 



347 



OLAflB I 



for a time, by the uncalcified portion of the original pellicle, which develops 



into the ligament of the adult. The paired embryonic shell, corresponding to 



the protoconch of Cephalopods, has been named by 



Jackson the prodissoconch (Figs. 588, 589). In 



general these valves are very uniform in character, 



as seen on the tips of the uneroded valves in the 



adult. They are usually rounded or slightly pointed 



at the umbonal end, and have in their earliest stages 



a straight, rather long hinge line. In Solemya the 



prodissoconch is elongate, rounded at the ends, with 



the ventral and dorsal margins nearly parallel, much 



as in the adult shell. In Pinna the prodissoconch 



is globular, as in most bivalves. In Unto, Anodon, 



and Philobrya, a second or nepionic stage is traceable, 



owing to a semi-parasitic habit of the young, which leave the mother and 



become encysted on the fins or gills of fishes ; during this period the shell 



Fio. 5S8. 



tli' anti-rifir finl (-I). and from th>- 

 ri-ht tipi.'-r .si. I.- (/;). ":/, (after 

 Jackson). 



FIG. 589. 



A, Avicula sterna. Young specimen, viewed from the left (a) and right (1>) sides, the latter showing byssal 

 sinus. U/i. B> Area pexata. Very young, showing prodissoconch (p), succeeded by early dissoconch growth. 

 *Vi (after Jackson). 



remains stationary, though some development of the contained soft parts is in 

 progress. 



The bivalve shell reduced to its lowest terms comprises two convex pieces 

 (the valves), attached to one another dorsally (1) by an elastic ligament 

 external to the cavity of the two valves ; and (2) by muscles and connective 

 tissues which pass from the inner surface of one valve to the inner surface of 

 the opposite valve. The contraction of the muscles brings the margins of the 

 valves into close contact, thereby forming a hollow receptacle in which the 

 soft parts of the animal are enclosed, and from which all obnoxious foreign 

 matters may be excluded. The elasticity of the ligament, acting on the 

 principle of the C spring, tends to separate the valves when the tension of 

 the internal adductor muscles is relaxed. The substance of the valve is 

 secreted by marginal glands around the edge of the investing tissue or mantle, 

 and is subsequently reinforced by material supplied by secretion from the 

 general surface of the mantle. As the animal grows and the original pro- 

 dissoconch becomes too small to cover the soft parts, the valves are enlarged 

 around the margins, so that each of them represents, fundamentally, a hollow 

 cone. Since growth progresses more rapidly along some portions of the 

 mantle than at others, the cones necessarily become oblique, arched, or 

 cycloidally curved. The apex of the cone is formed by the beak or umlo of 

 the shell, the base is the entire margin of the valve. 



The shell of most Pelecypods is composed of several layers of distinct 



