MUSSELS. 



189 



;;» c 



- «. 



>a 



are provided witli one or two siphons, which can be 

 extended or contracted. Their foot is long, beardless, and 

 capable of great extension. The SpTiseriidx are ovoviparous, 

 and are divided into two genera: (1) Sphsgrium (a sphere), (2) 

 Pisidium (pea-shaped). 



The Sphssria possess two siphons, one longer than the 

 other. The former is called 

 the incurrent siphon, and is 

 used for respiration and nu- 

 trition, and the latter is 

 known as the excurrent 

 siphon, and serves to caiTj 

 away that for which the 

 animal has no further use. 

 The valves of the shell are 

 of equal size, but of slightly 

 difEerent shape. The hinge 

 possesses what are called 

 cardinal or anterior teeth and 

 lateral or posterior teeth. 

 The Sphaeria are active in 

 their habits, and many of 

 them, if not all, are more 

 or less " thread-spinners." 

 They breed readily in con- 

 finement, and sometimes be- 

 fore they reach maturity. 

 They feed principally upon 

 animalcula, and when winter 

 comes on they bury them- 

 selves in the mud at the 

 bottom of the water, and 

 remain there in a state of 

 toi-pidity until the spring. 



Fig. 124. Outline of the Dorsal 

 Part of both Valves op Unio 

 TUMIDUS. CM, Thickness ; bb, Umbo ; 

 c, Lunule ; d. Ligament ; e. Anterior 

 Extremity ; /; Posterior Extremity. 



They seem to be imtable 



or timid little moUuscs, for when they have ascended the 

 sides of the aquarium, the slightest touch or startling 

 will frequently cause them to quit their hold . and sink to 

 the bottom. 



