182 INVERTEBRATE ANIMALS. 
high type of organisation. The common Bivalve shell-fish, 
such as the scallop, possess numerous simple eyes placed along 
the margins of the mantle, but in many cases even these are 
absent. Locomotion is very variously effected, but seldom with 
much vigour or activity. The lowest classes of the AZo//usca 
are in the great majority of instances fixed when adult. The 
common Univalve shell-fish, such as whelks, snails, slugs, &c. 
creep about slowly by means of a flattened disc, developed 
on the under surface of the body, and known as the “foot.” 
Other Univalves and many Bivalves can effect short leaps by 
means of the foots but many of the latter are permanently fixed 
to solid objects, or buried in the sand. The minute A/o/usca, 
known as the Prerofoda (fig. 108), swim freely at the surface of 
the ocean by means of two fins, formed by a modification of the 
foot, and attached to the sides of the head. The only Molluscs 
which enjoy really active powers of locomotion are the preda- 
cious cuttle-fishes, which swim rapidly by means of fins, or by 
ejecting a jet of water from the cavity of the mantle, and which 
can also creep about by means of the “arms” placed round the 
mouth (fig. 110). 
The last feature in the J/o//usca which requires to be men- 
tioned is the “shell.” The shell is not invariably and univer- 
sally present in the A/o//usca, many being either destitute of a 
shell altogether, or having one so small that it would not com- 
monly be recognised as such. In these cases, asin the common 
slugs, the animal is said to be “naked.” In all the 17/o//usca 
which possess a shell, this is secreted by the integument, or by 
what is technically called the mantle; and.in all cases it is com- 
posed of carbonate of lime. The méthods in which the lime is 
arranged differ in different cases; but all living shells have an 
outer covering of animal matter, which is known as the “ epi- 
dermis.” In a great many of the higher J/o//usca, such as the 
whelks, periwinkles, snails, and others, the shell consists of only 
a single piece, when it is said to be “univalve.’? In many 
others, such as oysters, mussels, scallops, &c., the shell is com- 
posed of two pieces, and is then said to be “ bivalve.” Ina few 
forms the shell consists of several pieces, and it is then said to 
be “multivalve.” The more important variations in the shells 
of the Mollusca will be noticed in speaking of the different 
classes of the sub-kingdom. 
In accordance with the nature of the nervous system, the 
