BIVALVE MOLLUSCS 479 



colourless blood circulates in the body cavity. There are two nephridial 

 apertures, one on each side of the anus ; and two nephridia. The 

 sexes are separate ; the reproductive organ is simple and dorsal in 

 position ; the elements pass out by the right nephridium. The 

 gastrula is succeeded by a free-swimming stage, in which there is a 

 hint of a velum and a rudimentary shell gland. 



Examples. — Dentalium, Enialium. About forty widely distributed 



species are known. Dentalium entale occurs off British coasts. 



The genus occurs as a fossil from Devonian strata onward. 



Class IV. Lamellibranchiata or Bivalves 



{Synonyms — Acephala, Conchifera, Pelecypoda, 

 Lipocephala, etc.) 



Examples. — Cockles, Mussels, Clams, and Oysters 



Lamellihranchs are bilaterally symmetrical Molluscs, in 

 which the body is compressed from side to side and the foot 

 more or less ploughshare-like. The head {or prostomium) 

 region remains undeveloped, and without tentacles ; radula, 

 horny jaws, and salivary glands are absent, but there is a 

 pair of labial palps on each side of the mouth. The mantle 

 "skirt is divided into two flaps, which secrete the two valves of 

 the shell, now lateral instead of dorsal in position. The 

 valves are united by a dorsal elastic ligament, and closed by 

 two transverse adductor muscles or by one. Internal bilciteral 

 symmetry is marked by the paired nature and disposition of 

 the nephridia, auricles, gills, digestive gland, and reproductive 

 organs. The gills {ctenidia) consist of numerous gill filaments , 

 which typically grow together into large plates {hence the title 

 Lamellibranch). There are usually three pairs of ganglia : 

 (a) cerebro-pleurals in the head ; (b) pedals in the foot ; (c) 

 viscerals at the posterior end of the body. The heart consists 

 of a ventricle and two auricles, and is surrounded by a 

 pericardium which is coelomic in origin, and communicates 

 with the exterior by means of the two nephridia. Repro- 

 ductive organs are always simple, and the sexes are usually 

 separate. The typical development includes trochosphere and 

 veliger stages. Most Lamellibranchs feed on microscopic 

 organisms and particles ; the distribution is very wide, both 

 in salt and fresh water ; the general habit is sedentary or 

 sluggish. 



