424 



MOLLUSCA 



PHYLUM VI 



with a saddle-shaped, single shell gland, which secretes a pellicle of the same 

 form, upon which, at two points corresponding to the valves, calcification sets 



in independently. These rudiments remain con- 

 nected across the dorsum for a time, by the uncal- 

 cified portion of the original pellicle, which develops 

 into the ligament of the adult. The paired em- 

 bryonic shell, corresponding to the protoconch of 

 Cephalopods, has been named by Jackson the fro- 

 dissoconch (Figs. 637, 638). In general these valves 

 are very uniform in character, as seen on the tips 

 Completed of the uucroded 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 Unio, Avodon and Philohnja, a second or nepionic 



Fig. 637. 



Ostrea virginiana. 

 prodissoconcii stage, vie\ved from 

 the anterior end (.1), and from 

 the right upper side (B). s7/j 

 (after Jacl^son). 



Fig. G38. 



A, Avicula sterna. Young specimen, viewed from the left («) and right (b) sides, the latter sliowiiig byssal 

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

 ^^/i (after Jackson). 



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 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 usually 

 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 extension of 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 prodissoconch becomes too small to cover the soft parts, the valves 

 are enlarged around the margins, so that each of them represents, fundament- 

 ally, a hollow cone. Since growth progresses more rapidly along some 

 portions of the mantle than at others, the cones necessarily become oblique. 



