56 ACEPHALA LAMELLIBRANCHIATA. 



tubes, forming the so-called siphons, capable of being re- 

 tracted by sj)ecial muscles. Through one of the tubes ^vater 

 is inhaled, through the other ejected. In such of the La- 

 melUhrancMata as have only one adductor muscle, there 

 are no tubes, and in several of those with two adductors. 

 The mass of the body is placed in the deeper and central 

 part of the cavity of the shell, and consists of variously 

 modified digestive, reproductive, and secretory organs, 

 vessels, and nerves. The liver and generative glands con- 

 stitute the greater part. In a majority of species, a single 

 linguiform muscular organ is developed for locomotion on 

 the ventral side of the viscera. This is the foot. It has 

 been observed that the development of the nervous and re- 

 spiratory systems corresponds in degree with that of the 

 locomotive organ. The degree of the development of the 

 nervous system varies much among these bivalves. The 

 principal ganglion (the branchial) is present in all, so are 

 usually two labial ganglia. The presence of a pedal gan- 

 glion depends on the presence of a foot. The organs of 

 sense are very variously developed. There are almost al- 

 ways around the mouth, which is to be sought for at the 

 opposite extremity of the body from that where the siphons 

 project, more or less developed lips, usually four in number. 

 Ocelli, imperfect organs of sight, are present in the majority 

 of bivalves, and very conspicuous in some genera, arranged 

 along the margin of the mantle, or dotting the edges of the 

 siphonal orifices, exactly as in the Ascidife. Minute sacs, 

 with vibrating otolites (rudimentary organs of hearing), 

 have been observed in several species, and are possibly pre- 

 sent in all. The sexes are either separate or combined. 

 All the lamellibranchiate bivalves undergo an imperfect 

 metamorphosis. They live upon infusorire and microsco])ic 

 plants. 



