LAMELLIBRANCHIATA. 425 
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, Entalium. 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 
(Synxonyms—Acephala, Conchifera, Pelecypoda, 
Lipocephala, etc.) 
Examples.—Cockles, Mussels, Clams, and Oysters 
Lamellibranchs are bilaterally symmetrical Molluscs, in 
which the body is compressed from side to side and the foot 
move or less ploughshare-like. The head (or prostomium) 
region remains undeveloped, and without tentacles ; radula, 
horny jaws, and salivary glands are absent, but there ts a 
pair of labial palps on each side of the mouth. The mantle 
skirt ts divided into two flaps, which secrete the two valves of 
the shell, now lateral instead of dorsal in position. The 
values are united by a dorsal elastic ligament, and closed by 
two transverse adductor muscles or by one. Internal bilateral 
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) cerebropleurals 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 ts sedentary or 
sluggish. 
