THE CEPIHALOPODS. 271 
the senses are as highly developed as in the fishes, in others they 
are confined to the narrow perceptions of the polyp. Many 
are individually so small as to escape the naked eye, others of 
a size so formidable as to entitle them to rank among the giants 
of the sea; some are perfectly harmless and unarmed, others 
fully equipped for active warfare. It is evident that creatures 
so variously gifted, and consequently so widely dissimilar in 
structure, cannot possibly be grouped together in one description, 
and that each of the four orders, Cephalopoda, Gasteropoda, 
Pteropoda, and Acephala (Lamellibranchiates, Brachiopods, 
Polyzoa, and Tunicata), into which they have been subdivided, 
must be separately brought before the reader, in order to give 
him a clear and faithful picture of their organisation and mode 
of life. 
The Cephalopods are the most perfect specimens of the 
molluscan type, as the decapods are the first among the 
crustaceans. These remarkable creatures consist of two dis 
tinct parts: the trunk or body, which, in form of a sack, 
open to the front, encloses the branchize and digestive organs, 
and the well-developed head, provided with a pair of sharp- 
sighted eyes, and crowned with a number of fleshy processes, 
arms or feet, which encircle and more or less conceal the mouth. 
It is to this formation that the cephalopod owes its scientific 
name, for as the feet grow from the circumference of the mouth, 
it literally creeps upon its head. 
All the cephalopods are marine animals, and breathe through 
branchiz or gills. These are concealed under the mantle, in 
a cave or hollow, which alternately expands and contracts, and 
communicates by two openings with the outer world. The one 
in form of a slit serves to receive the water; the other, which is 
tubular, is used for its expulsion. 
According to the different number of their gills, the 
cephalopods are divided into two groups. The first, to which 
the poulp and common cuttle-fish belong, and which comprises 
by far the majority of living species, has only two sets of gills; 
while the second, which, in the present epoch, is only 
represented by a few species of Nautilus, has four, two 
on each side, according to the number of their arms or feet— 
for these remarkable organs serve equally well for prehension 
or locomotion. The first group is again subdivided into two 
