130 PISCES. 



aquatic habits. Although there are in all classes of Vertebrates 

 forms which move and live in water, yet nowhere is the whole organi- 

 sation so completely adapted to an aquatic life as in Fishes. 



The body is in general spindle-shaped and more or less compressed, 

 but in details presents numerous modifications. There are cylindrical, 

 .snake-like fishes (Lampreys) as well as fishes with a spherical, balloon- 

 like form (Gymnodontct). Others are elongated and band-shaped, 

 and others again are very short, flat and unsymuietrical (Pleiironectidif). 

 Finally a dorsoventral flattening may lead to a flat discoidal form(]ays). 



Locomotion is effected mainly by lateral flexions of the vertebral 

 column, which are caused by the powerful body muscles. The effect 

 of these movements may be greatly increased by the unpaired dorsal 

 and ventral fins, which are capable of being elevated and depressed. 

 The two pairs of extremities the pectoral and pelvic fins appear, 

 on the contrary, to be used more as rudders to direct the course of 



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FIG. 583. Pereajluviatilis (regiie animal). 



the animal. The structure of the vertebral column, which is not 

 divided into many regions, corrresponds to the mode of locomotion. 

 The head is directly attached to the trunk, and is usually rigidly 

 connected with it. A moveable cervical region, which would be a 

 hindrance in swimming, is completely absent. The anterior part of 

 the body is rigid, but behind it becomes more flexible and passes 

 gradually into the the tail, the vertebrae of which permit of the most 

 complete movements on one another, and which on that account 

 constitutes the principal organ of locomotion. 



Fins. The system of unpaired fins is developed from a median 

 cutaneous fold of the embryo, extending over the back and tail as far 

 as the anus. Subsequently this fold becomes broken up into parts, the 

 definite unpaired fins. There are usually three such parts, constituting 

 the dorsal fin (pinna dor satis), the caudal fin (pinna caudalis), and anal 

 fin (pinna (mails) (fig. 583). These ridges of skin are supported as 

 a rule by firm rays the fin-rays ; in the Teleosteans either by hard, 



