200 STRUCTURE OF THE MOLLUSCA. 



taliina, in which the animal is much elongated, conical, and 

 covered with a conico-tubular shell, open at both ends. 



CLASS IV.-ACEPHALA, OE LAMELLI- 

 BEANCHIATA. 



The great class of MoUusca known by the names of La- 

 mellibranchiata, derived from the form of their respiratory 

 organs, or Tropiopoda, from the compressed form of the 

 foot, are readily known by their having a shell formed of 

 two valves, connected by a hinge, fig. 5. The animals of 

 this class are of a compressed form, with the head not dis- 

 tinct ; the mouth, fig. 9, e, situated anteriorly, between two 

 pairs of flattened labial appendages, fig. 9, /; the oesophagus, 

 fig. 9, e, generally short ; the stomach pyriform, fig. 9, g ; 

 the intestine, fig. 9, i, convoluted within; the liver and 

 ovary, fig. 9,j; the circulatory apparatus a ventricle and an 

 arterial system, and a venous system with two auricles ; two 

 pairs of laminiform transversely striated branchiae, fig. 9, 7, 

 situated, a pair on each side, between the mantle and the 

 body. The nervous system, fig. 9, r s, is very simple; there 

 being no cerebral mass, or head ; nor any organs of sense, 

 besides those of taste and touch. Continuous with the lower 

 part of the body is a compressed muscular foot, fig. 9, k, not 

 capable of being used for crawling. The mantle, fig. 9, 6 6 

 is very large, thin, laminiform, induplicate, and enclosing the 

 body. There are generally two tubes behind, fig. 9, op, 

 one for allowing a passage to the water, p, the other for the 

 expulsion of the excrement, 0. These tubes are often united 

 with a common envelope, fig. 5, h. The shell consists of 

 two valves, covering the mantle, and having at its upper 

 part an elastic ligament, which throws them open, when the 

 adductor muscles, of which there are two, generally distant 



