156 MANUAL OF THE MOLLUSCA. 
mandibles of birds. The tongue is large and fleshy; part of its 
surface is sentient, whilst the rest is armed with recurved spines ; 
their eyes are large, and placed on the sides of the head. Inall 
probability they possess the faculty both of smelling and hearing. 
All are carnivorous, and live in the sea. 
The nervous system is more concentrated than in the other 
mollusca, and the brain is protected bya cartilage. The respira- 
tory organs consist of two or four plume-like gills, placed 
symmetrically on the sides of the body, in a large branchial 
cavity, opening forwards on the wnder* side of the head: in the 
middle of this opening is placed the siphon or funnel. The sexes 
are always distinct. The cephalopoda are divided into two 
orders, the names of which are derived from the number of the 
branchice. 
ORDER I.—DIBRANCHIATA, Owen. 
Animal swimming; naked. Head distinct. yes sessile, 
prominent. Mfandibles horny (Pl. I., fig. 2). Arms eight or 
ten, provided with suckers. Body round or elongated, usually 
with a pair of jins; branchie two, furnished with muscular 
ventricles ; ink-gland always present; funnel a complete tube. 
Shell internal (except in argonauta), horny or shelly, with or 
without air-chambers. The shell of the argonaut does not 
correspond with the ordinary shell of mollusks. (See p. 39.) 
The typical forms of the cuttle-fishes were well described by 
Aristotle, and haye been repeatedly examined by modern 
naturalists; yet, until Professor Owen demonstrated the exist- 
ence of a second order of cephalopods, departing from all the 
above-mentioned characters, it was not clearly understood how 
inseparably the organisation of the cuttle-fishes was connected 
with their condition as swimming mollusca, breathing by two 
gills. There are two types of lung structure among the dibran- 
chiates. Thus, in Octopus and Sepia the gills form a cylinder, 
while in Loligo and other genera they form a half cylinder. 
The characters which co-exist with the two gills, are the 
internal rudimentary shell, and the substitution of other means 
of escape and defence, than those which an external shell would 
haye afforded ; viz., powerful arms, furnished with suckers ; the 
* According to the established usage, we designate that the under or ventral side of 
the body, on which the funnel is placed. But if the cuttle fishes are compared with 
the nucleobranchs, or the nautilus with the holostomatous gasteropods, their external 
analogies seem to favour an opposite conclusion, There are many terms in use which 
are apt to mislead, such as fins, arms, &c.; they have a definite meaning when applied 
to the vertebrata, but not so when applied to the invertebrata,. 
