92 ZOOLOGY. 
rare in the Mollusca. The shell with which many species are protected, 
has not in the least degree the nature of an external skeleton, no muscles 
being attached to it but those which are necessary for the opening and 
‘shutting of its own valves. It is a mere calcareous exudation from the 
mantle, a thick, soft, flexible skin, with which, in all the Mollusca, the 
whole body is invested. Many Mollusca have no shell; many have the 
shell in a single piece, which is often a spiral tube closed at one end, and | 
gradually widening to its open extremity. Such shells are called wni- 
valves. Snails, whelks, and periwinkles afford familiar examples of 
the most common form of univalve shells. Somewhat different forms 
may be seen in limpets, cowries, &c. The shells of many molluscs 
are composed of two valves, as those of oysters, mussels, &c., which 
the animal is able to open and shut at pleasure. Such shells are 
called bivalve.2 In a small number of molluscs, the shell is composed of a 
greater number of pieces, and such shells are said to be multivalve,? but 
the multivalve molluscs differ little from univalves in their general 
organisation. 
All molluscs have the power of locomotion, at least in the earliest 
stage of their existence ; although many of the lower tribes soon lose 
it, and become permanently fixed in one spot. The modes of locomotion 
are various. Many molluscs move by means of a muscular structure 
in some particular part of the mantle, termed the foot, which in some, as 
in the cockle, is an organ for springing, in others for burrowing, in many 
for crawling, and in some for swimming. In the highest of all orders of 
molluscs, the Cephalopoda*—of which the cuttle-fish and squid are 
examples—the head is furnished with tentacles armed with suckers, and 
by means of these the animal drags itself along, whilst the body is also 
often provided with fin-like expansions, which serve for swimming. The 
respiration of Mollusca generally takes place by gills; but slugs and snails, 
which do not inhabit water, have a pulmonary sac or cavity lined with a 
vascular network. 
Vertebrata. 
The highest primary division of the animal kingdom is that of 
Vertebrata, comprising the five classes of Fishes, Amphibians, Reptiles, 
Birds, and Mammals, the first-named being the lowest, and the last the 
highest of ajl. All the classes of Vertebrata agree in what may be 
called a general type of structure, exhibited in its highest perfection 
in man, and of which some account has already been given under 
1 From Latin wnus, one, and valva, a valve. 
2 From Latin dis, twice, and valva, a valve. 
3 From Latin multus, many, and valva, a valve. 
4 From Greek cephalé, the head, and pous, podos, a foot. 

