188 FEESH-WATER AQrAEIA. 



the same ease that the Planorbis corneus perforins the like 

 feat, or they will suspend themselves in the water by means 

 of a " moUnscan thread," while some of the Unionidas will 

 seem ±o remain for days in the same place without moving. 



Th^ soft parts of a " shell-bearing " mollusc consist chiefly 

 of a mantle having two lobes or divisions, leaf-like gills, 

 siphons, or a siphon or orifices, and a tongue-shaped toot 

 which can be considerably elongated and sometimes possesses 

 a tuft of muscular threads called a " byssus,^" by means of 

 which the animal can attach itself to foreign bodi«s. There 

 are also various muscles, a he|irt, a liver, a stomach, and 

 intestines. There is no distinct head. 



The two valves or shells of these molluscs are drawn to- 

 gether by means of muscles called the " adductor muscles." 

 Some of -the Conchifera have only one adductor nauscle, which 

 is/'situated near the centre of the body, while others possess 

 two, one of which is placed towards the anterior extremity of 

 the animal, and the other towards the posterior. The former 

 molluscs aye called Monomyarian (having one muscle), the 

 latter Dimyarian (having two muscles). The places inside the 

 shellg^ to which these muscles are attached are marked by an 

 impression, more' or lessl distinct, called the " muscular scar." 

 Another scar, the "pallial," runs from the anterior muscular 

 scar, following at a short distance from the edge the course 

 of the ventral margin of the shell, and then joins the 

 posterior muscular scar. This pallial soar shows where the 

 mantle <jif the animal was attached to the shell. The two 

 valves are joined together by a hinge, which often possesses 

 teeth. The ligament, as the hinge is sometinaes called, is 

 either external or internal. The male and female of these 

 molluscs are united in the same individual. Figs. 124 and 125 

 wiU perhaps help to make plain the following description of 

 some of the bivalves suitable for the fresh-water aquarium. 



The Lamellibranchiata (possessors of leaf-like gills), as the 

 Conchifera are often called, are now divided into three 

 families, viz : (1) Sphxriidce, (2) Vnionidse, (3) Dreiasenidx. 



The members of the Spheeriidss family have the shell either 

 oblong or oval, and its hinge possesses teeth. They 



