The Rev. P. Keith on the Structure of Animals. 91 



joined to the body without the intervention of any distinct 

 portion that can properly be called a neck; but about the 

 junction of the head and body we find on each side an ex- 

 ternal organ peculiar to fishes, namely, the gills, — their organ 

 of respiration. The body, which is covered with scales, is 

 rounded, and tapering from head to tail, as in the eel ; or a 

 little flattened in a vertical direction, as in the trout and 

 salmon ; or much flattened in a horizontal direction, as in the 

 sole and flounder, and in all flat fish : the limbs, whether an- 

 terior or posterior, are metamorphosed into fins to fit them for 

 the act of swimming in water. By the lateral flexion and ex- 

 tension of the caudal fins the body is impelled forwards with 

 great force, ascending or descending chiefly by means of the 

 compression or dilatation of the swim-bladder, an organ with 

 which most fishes are furnished ; but such as are destitute of 

 it, like soles and flounders, must be content to swim very near 

 the bottom. Some fishes have the capacity of leaping out of 

 the water ; and one, Trigla volitans^ — the flying fish, — has the 

 very singular property of being able to take a short flight even 

 in air. 



Class 4. In this class, the class of Reptiles, the general type 

 is in most cases very obvious, exhibiting a head, with a mouth 

 and eyes distinct; a visible neck; a body naked, as in the 

 frog ; or covered with shell, as in the tortoise; without a tail, 

 as in the toad ; or furnished with a tail, as in the lizard. The 

 limbs, anterior and posterior, are so excessively short as 

 scarcely to be able to raise the body above the level of the 

 ground ; and in the order Serpentes even limbs are wanting. 

 Many of the Reptilia are amphibious, and can live either on 

 land or in water; and most of them during the winter months 

 sink into a state of torpidity, from which they are aroused only 

 by the returning warmth of the succeeding spring. 



Division II. The Mollusca. — The next division of animals 

 in the descending scale is that of the Mollusca. They are distri- 

 buted into three classes, — the Cephalopoda^ in which the feet, 

 or organs of locomotion, are inserted in the head ; the Gaste- 

 ropoda, in which the foot, or organ of locomotion, is the ab- 

 domen ; and the Acephala, in which no distinct head is visible. 

 In each class there is an order that is inclosed in a shell, or 

 testaceous covering; and an order that is naked. 



Class 1. In the first class we find the Sepia, or cuttle-fish. 

 They are of the order of naked Mollusca, and in their general 

 aspect are but a shapeless mass. The head, however, is di- 

 stinctly visible, furnished with eyes and organs of hearing, as 

 well as with presumed organs of smell, from the fact of their 



N 2 



