682 ON MOLLUSCS 
the body. Capillaries are wanting, and the veins are replaced by 
sinuses, which have no proper walls, 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 (Chorzon), 
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 cochlew, the bivalve, as those of mussels, 
conche. 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, lyimg 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 (cardo) 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 LiINN&US anus); behind 
the point is a chink, ordinarily smaller and more elongate, the fissure 
(fissura, in French écusson, 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. 
Linnxvs, in his deseription, placed the bivalve shell with its point 
