THE PHYLUM MOLLUSCA 



eventually become free, burrow in the bottom, and develop to maturity with- 

 in a few years (of. Fig. 13.5). 



The majority of the larger fresh-water mus.sels, such as Lampsilis, Anodonla, 

 (hmdrula, and others, exhibit a very interesting life cycle which involves a 

 larval stage parasitic on fresh-water fishes. In the.se mussels the spermatozoa 

 released by the male are drawn into the mantle cavity of the female. After 

 passing through the gills, they encounter ova which have been discharged into 

 the suprabranchial chamber, and fertilization occurs. The zygotes are trans- 

 ferred into specialized parts of the gills of the female, varying in diflferent 

 species, which serve as brood chambers, and the early developmental stages are 

 passed within these pouches. The result of these changes is a larval stage 

 called the glochidium, a microscopic form possessing a bivalve shell, with 

 hooks and well-developed adductor muscles, and often a long, thread-like 

 byssus (Fig. 13.6). These larvae are emitted from the body of the female and 

 lie on the bottom, with their valves gaping and the byssus extending upward 

 in the water. Contact with the proper species of fish stimulates the con- 

 traction of the adductor muscles, and the valves of the shell clamp shut upon 

 fin rays, gill filaments, or other exposed parts of the fish. The young parasite 

 is subsequently overgrown by tissues of the host and remains in this situation 

 for a variable period of time, eventually leaving the host to begin its free 

 life upon the bottom. Like the majority of parasitic animals, glochidia are 

 specialists in the sense that they can live and develop in only one or a few 

 related species of hosts. The glochidia of most species parasitize fishes; one 

 species, however, is known to develop on the mud-puppy, Necturus. 



Life cycles involving parasitic larval stages are not the invariable rule 

 among fresh-water mussels. In some species, as in members of the genus 

 Sphaenum, the eggs are fertilized and develop into miniature adults within 

 brood pouches formed in the gills of the female. 



Other Functions: Shell Formation. One of the chief functions of the 

 mantle in bivalve mollusks concerns the secretion of the calcareous shell. 

 The shell itself consists of three layers diflfering in composition and appear- 

 ance. Externally, the valve is covered by a thin, proteinaceous periostrocum, 

 often tan, greenish, brown, or black in color. Beneath the periostracum lies 

 the prismatic layer, composed of spicules of calcium carbonate laid down 

 within an organic matrix. The prismatic layer is chiefly responsible for the 

 bulk and strength of the shell. The valve is lined internally by a so-called 

 nacreous layer, generally smooth and highly polished and often showing 

 the beautiful, iridescent coloring characteristic of "mother of pearl." The 

 nacreous layer contains a higher proportion of organic material than 

 the prismatic layer. The margins of the mantle appear to be most active in the 

 secretion of the shell. The periostracum is laid down by special cells in 

 this region, and in normal growth the calcareous layers are added to the 

 margins of the valve by this part of the mantle. Recent investigations have 

 shown that the edge of the mantle and adjacent regions are more active in 

 the deposition of calcium salts than other parts. Damage to the shell, how- 



377 



