Vlll EXTERNAL CHAEACTEES. 



among the elasmobranclis we perceive as a rule a single external gill opening 

 on either side, or even on the lower surface of the head, a different distribu- 

 tion is generally seen in the plagiostotnes which possess from five to seven, 

 and among the cyclostomes, which usually have seven as in lampreys, 

 although it may be single as in the hag ; in these latter, gill-covers as observed 

 in teleosteans, are absent. 



The hody or trunk commences behind the head, and amongst most bony 

 fishes is somewhat wedge-shaped, passing by imperceptible degrees into the 

 tail or caudal portion, the vent generally dividing the two regions, .but to 

 this rule there are many exceptions, for in certain genera the intestine opens 

 under the throat or near the caudal fin. In some slow-moving or bottom 

 fishes, the head as in the frog-fish, is depressed, while the entire body ruay 

 be compressed into a flattened disc as among the flat-fishes or pleuronectoids 

 which progress swimming on their sides. Or there may be lateral compression 

 conjoined to a short body and slow movements as in the dory or the sun-fish ; 

 or there may be lateral compression with a lengthening of the body as in the 

 band-fishes. The body is divisible into the back, the sides or lateral surfaces, 

 and the abdomen or the belly. 



The tail or that portion which is continuous with the trunk, is similarly 

 divisible into, the back, the sides and abdominal surfaces, while should a free 

 finless portion exist between the hind fin of the back and the tail fin this is 

 termed the peduncle or free portion of the tail. Among the Chondropterygii 

 the tail of some rays and skates appears as a slender appendage from the 

 body, while in some ground forms it seems to act mostly as a rudder. 



The slcin may be scaled or scaleless, or enclosed in armoured plates, while 

 should a row of pierced scales pass along the sides they are known as the 

 lateral-line (fig. 1, q), the number of scales on which are generally important 

 as assisting in discriminating the species; while the lateral transverse-row 

 (fig. 1, J)) is of similar use. . 



The number of fins, as well as their relative positions on the body, are 

 subject to great diversity ; the presence or absence of some, their situation, 

 and even their component parts furnishing characters often useful in classifi- 

 cation. If we examine the composition of the fins, we perceive among the'bony 

 or teleostean fishes that they consist of two distinct characters of rays, the 

 first being spinate or having spines destitute of any tranverse articulations, and 

 each ending in a hard or sharp point; the second being, softer, more especially 

 termed rays, and being simple or composed of numerous pieces articulated 

 one to another by transverse joints, and which rays may have their outer 

 extremities branched or else -simple and undivided.. But irrespective of the 

 foregoing three forms, modifications are seen as in the outer ray of the 

 pectoral of many siluroids, or in the anterior dorsal rays as in the barbel 

 (plate cxxxi), where one or more may take on a bony character, but still 



