173 THE WORLD OF THE SEA. 
according to the number of pieces of which the shell was com- 
posed. But further examination proved this simple division to 
be very unsatisfactory, inasmuch as some mollusks, very nearly 
allied in their characters and structures, differed widely in their 
shells; and, on the other hand, some whose shells were very 
similar, were in reality far separated by more essential differences. 
In addition to this, as many of the mollusks are shell-less, or 
naked, they were altogether excluded from the classification. 
The shell, although a very valuable adjunct, is discarded as a 
foundation for the divisions of which the sub-kingdom is evidently 
capable. The structure of the animal has finally been decided 
upon as the best means of subdivision. And at once the mollusks 
are divided into cephalous—those having a head, or a prominence 
in which is situated the mouth—and the acephalous, or those 
without a head. 
The cephalous mollusks present three types:—1, the Cepha- 
lopoda, which have their feet or tentacles arranged around the 
head : to this class belong the cuttle-fish, which approaches nearest 
to the vertebrata, and therefore the cephalopoda are the kings of 
the mollusks; 2, the Péeropoda, a small class, which is charac- 
terised by the possession of a pair of wing-like expansions of the 
mantle, which supply the place of fins; and 3, the Gaséeropoda, 
which is the most extensive group of the three, and derives its 
appellation from the fact that, from the under surface of its body 
is protruded a muscular foot. 
The Acephala, as might be expected, do not enjoy so high an 
organisation ; they occupy the humblest position in the kingdom of 
the mollusks, and in their case we appeal to the shell to aid us in 
their subdivision. The Conchifera are those which are encased in 
a calcareous covering, whereas in the Zzzcata the shell is wanting, 
but a leathery tunic encloses the whole body. 
We trust that this general outline of the sub-kingdom will give 
the reader some idea of the territory of the ocean world which 
we now enter. 
Mollusks are solitary or gregarious. We shall treat of the latter 
first, since their constitution is more simple, and they are the next 
link in the great chain of life to the polypiers and the polyzoaries. 
