PART If. 
SYNOPSIS OF THE GENERA. 
CHAPTER I. 
OLASS I.—CEPHALOPODA. 
THE cephalopoda are represented by the common squid, the 
nautilus, and the ammonite; forms with which most of us are 
more or less familiar. They possess a more complicated struc- 
ture than any other group of the mollusca; but in this respect 
they are much inferior to the vertebrate animals, in whom 
the setting apart of particular organs for the performance of 
distinct functions is developed to so high a degree. We cannot 
trace a series of gradativnal forms between the highest cepha- 
lopod and the lowest vertebrate; but we can descend from the 
more to the less specialised forms of mollusca, which ultimately 
merge in one direction in such creatures as Fasciola, among 
entozoa; and in another direction, to forms like Vorticella, 
through the intermediate genera — Pedicellina, among the 
Bryozoa, and Perophora among the Ascidians. It is conse- 
quently much easier to define the higher than the lower 
boundaries of a great primary group. The points of analogy 
between the cephalopods and the vertebrates are the internal 
skeleton, the similarity in the form of the blood corpuscles, and 
in the capillary structure of the portion of the circulatory 
system situated between the arteries and veins. 
The cephalopods move partly by means of a series of long 
muscular arms arranged round the mouth, partly by means of 
fins, or flaps, attached on each side of the body, and partly by 
the forcible expulsion of water through a tube or siphon. 
Unlike most of the mollusca, they are symmetrical animals, 
haying their right and left sides equally developed. Their shell 
is usually straight, or coiled in a vertical plane. The nautilus 
and argonaut alone (of the living tribes) have external shells ; 
the rest are termed ‘‘ naked cephalopods,” because the shell is 
internal. They haye powerful jaws, acting vertically, like the 
