80 



BRANCH MOLLUSCA 



The siphon is incomplete and the pallial line is entire, that is, without 

 sinus or indentation. The foot is long and compressed. The valves of 

 the shell are held together by the strong adductor muscles, and opened, 

 when these relax, by the elastic spring or hinge ligament. The shells are 

 a dull black on the outside, and pearly white, tinted with iridescent hues, 

 on the inside. The shell of the Unio is not so large and strong as that of 

 Anodonta, while the latter genus has no hinged teeth. Clams are found 

 in ponds and large streams (which do not dry 

 up in the summer), distributed along the direction 

 of the strongest currents to insure food supply. 

 They are partly buried in the mud, the open edge 

 of the shell down and the valves slightly apart, 

 with the fleshy foot protruding from the anterior 

 ventral margin. When disturbed, the foot and 

 edges of the mantle-lobes are retracted and the 

 valves tightly closed. 



The shell is the mussel's principal means of 

 defense. It has many enemies besides man, such 

 as the musk-rat, raccoon, mink, otter, and other 

 mammals that live in and about the streams 

 where the clam is found. Such animals as the 

 musk-rat gnaw off the hinge ligament to get the 

 shell open. 



The young clams are carried in the gills, and 

 were formerly mistaken for parasites, and are 

 called glochidea. They differ much in shape from 

 the adult. The glochidea, or young clams, pass 

 out through the exhalant siphon and attach them- 

 selves by hooks on the valves to the gills or fins of 

 fishes, by which they are protected from enemies 

 and kept supplied with fresh water until suffi- 

 ciently mature for independent existence, when 

 they detach themselves from their host and drop 

 to the bottom of the stream. 



The giant clam (Tridac^na gi'gas) of the tropics 

 has a shell from 2 to 4 feet long, which may 

 weigh from 300 to .")00 pounds. 



The soft-shelled clam {Mi/'n arena^ria) abounds 

 in the mud flats of the Atlantic coast north of 

 Cape Cod. The young clams swim about on the 

 surface of the water. After the shell appears, 

 they sink to the bottom and attach themselves by 

 the byssus. When the clam is about \ inch long, 

 the byssus disappears and the animal buries itself 

 in the mud. As it grows, it keeps enlarging and 

 deepening its burrow until it may extend from 

 8 to 12 inches below the surface of the mud. The long siphons are extended 

 up to within reach of the sea-water, whose currents bring to the clam food 

 and air. The water enters through the ventral siphon, is driven through 

 the gills, and finally passes out through the excretory tube, the dorsal 

 siphon. 



Another form much used for food is the " Quahog " {Venus mercenaria) , 

 which is characteristic of warmer waters, and is found from Cape Cod to 

 Texas. It burrows a little way below the surface, but is often found with 



Fig. m.—Tere'dona- 

 vaVis, removed from its 

 calcareous tube, with 

 elongated siphons. 



(Quatrefages.) 



