682 ON MOLLUSCS 



the body. Capillaries are wanting, and the veins are replaced by 

 sinuses, which have no proper Avails, but are excavated in the 

 different parts of the body. The blood of molluscs is commonly 

 white or whitish blue. Some molluscs are bisexual, and require 

 mutual impregnation; in others the sexes are distinct. The most 

 are oviparous. The eggs are surrounded by a thin shell (Chorwn), 

 between which and the yolk in some an albuminous fluid is inter- 

 posed, and which is sometimes horny, and but seldom covered with 

 calcareous incrustation. Often the eggs, when laid, are connected in 

 bunches, or adhere to each other by a gelatinous mass. The num- 

 ber of species that live on land is small in comparison of the much 

 greater quantity of species that live in fresh, and especially in salt 

 water. 



Before we proceed to the division of the molluscs, we would 

 treat shortly of the shells which cover the body in most. The uni- 

 valve shells are called cochlea, the bivalve, as those of mussels, 

 conchce. There are also some molluscs that are covered by many 

 pieces of shell. Such a multivalve shell (testa multivalvis) has the 

 genus Chiton, where transverse calcareous plates, lying behind one 

 another in a row, cover the back. The bivalve shells are thicker 

 at the part where they are connected with each other. This part is 

 named the point (apex}. At the point there are on the margin, within 

 the shell, usually projections and hollows, which mutually fit into 

 each other, and to which the name of hinge (car do) is given. When 

 this margin is smooth, there is said to be no hinge (testa acardis). 

 In front of the point a slight depression is seen on the shell, which 

 is named the male depression (lunula, by LINNAEUS anus) ; behind 

 the point is a chink, ordinarily smaller and more elongate, the fissure 

 (fissura, in French ecusson, with LiNN^US vulva). On this the liga- 

 ment is usually situated which, formed of elastic horny fibres, runs 

 transversely from one shell to the other. Where this ligament, as in 

 most of the bivalve molluscs, is attached to the outside of the shells, 

 it is obvious that they will be opened by its contraction. Yet 

 even where the ligament is placed internally the two shells are 

 separated from each other by its elasticity, because in this case the 

 fibres are forcibly compressed by the shell when closed. In those 

 bivalve molluscs which move freely, the opening of the shell is 

 turned downwards, the point upwards, and the ligament backwards. 

 LINNAEUS, in his description, placed the bivalve shell with its point 



