CLASS I. FISHES 



Aquatic, cold-blooded vertebrates which have an integumentary 

 covering of dermal scales or plates, and which breathe by means of 

 pharyngeal gills. Median and paired fins are in most cases present, 

 the latter being homologous to the limbs of the higher vertebrates. 

 The heart has but one ventricle and one auricle, and contains only 

 venous blood. The head is joined with the trunk without articulation, 

 and there is no neck. No external ear-opening is present. With 

 a few exceptions all fishes lay eggs, which develop without an amnion. 



On the Identification of Fishes. — The body of most species of fishes 

 is more or less lenticular in shape and compressed laterally. In many 

 species, however, the anterior portion is cylindrical, as it is in the 

 eel, or more or less depressed (flattened), as in the catfish. The head 

 forms the anterior end of the body, the hinder margin of the opercles 

 marking the boundary between it and the trunk. The caudal or tail 

 fin forms the posterior end, the caudal peduncle being the slenderer 

 portion of the body from which it projects, the caudal fin and peduncle 

 being postanal in position. 



Measurements (Fig. i). — The length of a fish, as given in this book, 

 is the distance between the tip of the snout and the base of the caudal 

 fin, expressed in millimeters; the caudal fin is not included. The 

 measurements of the Jiead and of the depth {i.e., the greatest dorso- 

 ventral measurement) of the body are not given in millimeters or by 

 any other absolute scale, but are relative, being the number of times 

 the length of the body is greater than that of the head or the depth 

 of the body, respectively. Thus, the expression "head 3, depth 5" 

 means that the length of the body is 3 times the length of the head 

 and 5 times its own depth. 



Fins (Fig. i). — The fins of fishes are of two kinds, the median or 

 vertical fins and the paired fins. The median fins are the dorsal, caudal 

 and anal fins: they are the more primitive of the two kinds, and alone 

 occur in cyclostomate fishes. The dorsal fin lies in the middorsal plane 

 and in many fishes is divided, so that there are two or even three or 

 more of them. The caudal or tail fim is usually the largest and the 

 principal organ of locomotion; its hinder margin may be either forked, 

 concave, emarginate (slightly forked or notched), straight or rounded. 



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