xcviii Introduction. 



in which they are provided with a unilobar ciliated velum. The 

 fresh-water species, as is often the case, go through less complex 

 changes than the marine ; and in one instance, that of Cyclas Cor- 

 nea, the foot, and not, as usual, the mantle with its shell, has been 

 stated to have been the first organ which differentiated itself in the 

 germinal membrane. The unilobar velum may be armed with a 

 flagellum, and in the marine species come to resemble the homo- 

 logous organ of the Dentalkim. 



The Lamellibranchiata are mostly marine. They may be either 

 fixed or free. They are never social in the sense of being organically 

 connected, but the peculiarities of their reproductive functions render 

 it necessary in their case, as in those of many similarly conditioned 

 creatures, that they should be massed in considerable numbers upon 

 the same spot. 



Class, Brachiopoda. 



Molluscoidea with bivalve shells, which admit of being opened, 

 though usually not widely, either by means of a hinge acted upon 

 by muscles, or by muscles alone, but which are not provided with 

 an elastic ligament as are those of the Lamellibranchiata, to which 

 the term ' bivalve' is sometimes exclusively applied. The shell of 

 the Brachiopoda diifers from that of the Lamellibranchiata further 

 in being almost always equilateral, but not equivalve, and in 

 having its valves articulated across and not along the dorsal ridge. 

 They are in the adult state always fixed ; either, as ordinarily, by a 

 peduncle which is attached to the internal surface of a ' ventral ' 

 shell, placed in the living* animal superiorly to a shell called, 

 from its relation to the heart, '^ dorsal,' or by the attachment of 

 the ventral shell, then placed inferiorly, to some marine object. 

 Larval Brachiopoda have been observed to move from place to 

 place in two ways ; viz. either by means of the ciliated epithelium 

 covering their arms, which are then protruded as is the lophophore 

 of a Polyzoon, or by means of spines implanted in the ventral lobe 

 of the mantle. The Brachiopoda are ordinarily said to be dioecious, 

 and in Thecidium the sex can be predicated from an inspection of 

 the shell ; but observations exist to show that hermaphroditism also 

 exists in this class, as in some representatives of every molluscan 

 Class, except the Cephalopoda. They are never social, though, as is 



