CLASS I PELECY PODA 347 
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 P 
the ventral and dorsal margins nearly parallel, much CE ME Os OC 
as in the adult shell. In Pinna the prodissoconch the anterior end (4), and from the 
is globular, as in most bivalves. In Unio, Anodon, Tere upper side (©). %h Ger 
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 


fi ( 

( 

Fic. 589. 
A, Avicula sterna. Young specimen, viewed from the left (a) and right (6) sides, the latter showing byssal 
sinus. 19/;. B, Arca pexate. Very young, showing prodissoconch (p), succeeded by early dissoconch growth. 
44/; (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 wmbo of 
the shell, the base is the entire margin of the valve. 
The shell of most Pelecypods is composed of several layers of distinét 
