LAMELLIBRANCHIATA. 363 



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

 region remains undeveloped, ivithoiit tentacles or eyes ; the 

 mouth is without radula, horny jaws, or salivary glands, but 

 there is a pair of labial palps on each side. 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 bilateral 

 symmetry is marked by the paired nature and disposition of 

 the nephridia, auricles, gills, digestive and reproductive organs. 

 The gills {ctenidia) consist of numerous gill filaments which 

 typically grow together i?tto large plates [hence the title 

 La?nellibranch). 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 surrou?ided by a peri- 

 cardium which is coslomic in origin, and commtitticates with 

 the exterior by means of the tivo nephridia. Reproductive 

 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 ivide both in salt and 

 fresh water ; the average habit is sedentary or sluggish. 



Type of LAMELLIBRANCHIATA : The Fresh water Mussel 

 {Anodonta cygnea). 



The fresh water mussel lives in rivers and ponds. It lies 

 with its head end buried in the mud, or ploughs slowly 

 along by means of its wedge-like foot. Its food consists of 

 minute plants and animals, which are wafted in at the 

 posterior end by the currents produced by the ciliated gills. 



External Appearance, 



The bivalve is four to six inches long ; its valves are 

 equal and united in a dorsal hinge by an elastic ligament, 

 an uncalcified part of the shell ; on the ventral surface 

 when the valves gape the foot protrudes ; the anterior end 

 is rounded, the posterior end is more pointed, and it is 

 there that the water currents flow in (ventrally) and out 

 (dorsally). In bivalves the ligament is generally posterior 



