2 EOCENE MOLLUSCA. 
and they are endowed with the sense of smell. The eyes are placed on the sides of 
the head, and in one of the two orders into which the animals are divided (the Dibran- 
chiata) are, generally, lodged in an orbital cavity in which they move freely ; in some 
genera, however, they are united to the outer integument, and are then incapable of 
motion. When lodged in orbital cavities, they exhibit two distinct modifications, of 
which M. d’Orbigny has availed himself for the subdivision of the order into two 
groups (called respectively J/yopside* and Ovgopsidet). In the first modification, 
which is found among the littoral Cephalopods, the eyes are wholly covered by the 
skin of the head, which becomes thin where it passes over the ball of the eye; in the 
other modification, which characterises the pelagic species, the orbital cavity is largely 
open, and the eyes are in contact with the water. 
In the tetrabranchiate Cephalopod, whose food is found principally at the bottom 
of the sea, and to whom enlarged vision would be comparatively useless, the eye is 
not lodged in an orbital cavity, but pedicillated, and assumes a simpler structure, 
approaching that of the inferior mollusc. 
These animals possess an external auditory opening, generally protected by an 
external ear more or less complicated in structure. The organs of smell are supposed 
to reside in certain pores or sacs, opening externally, termed by M. dOrbigny 
aquiferous pores (ouvertures aquiferes), which are divided into cephalic, oral, anal, and 
brachial pores according to their position, and are used by him as generic distinctions. 
The mouth is terminal, and is furnished with two strong, horny beaks or jaws termed 
mandibles, working vertically upon each other like the bill of a parrot, with which they 
are usually compared. 
The body is inclosed in a thick membranous skin or mantle, united along the belly 
of the animal so as to form a muscular bag or sac, open at the upper extremity, and 
containing the branchial apparatus and viscera. In its general shape it is round, or 
more or less elongated, and cylindrical or depressed. To this body, distinguished as 
the posterior portion of the animal, the anterior or cephalic portion, consisting of the 
head and the arms or tentacula it sustains, is attached by one or more ligaments, 
some internal, others formed by the continuation of the skin of the body, and termed 
cervical or lateral ligaments, according to their position. The condition of these 
ligaments varies with the habits of the animal, and upon it generic characters are 
founded. They attain their greatest strength in the littoral Cephalopods, and in some 
genera afford a sufficient attachment between the head and the body; but in the 
Cephalopods more or less pelagic, in which the free and independent use of the organs 
of prehension, and a simultaneous active exercise of the bodily locomotive function, 
would be materially impeded by an extensive permanent attachment, the ligaments 
are considerably reduced; so much so, in fact, that they would afford a very imperfect 
* Myopside ; from pvw claudo, oxis visus. 
Oigopside ; from évyw aperio, ois visus. 
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