CLASS PISCES. 



77 



The body is covered witli a closely-fitting armour of 

 scales developed in the skin. These scales (fig. 36, B) 

 are thin, flexible, horny plates, which overlap one another 

 like the tiles of a house, and which have their hinder 

 edges fringed with a comb-Kke row of spines. Running 

 from the back of the head to the root of the tail is a 

 peculiar row of scales (fig. 33, I), which constitute what is 

 called the "lateral line." Every scale in this line is per- 

 forated by a small hole, and communicates internally with 

 a peculiar system of canals. It used to be beueved that 

 the slime which covers the body was secreted by these 

 canals, and poured out by the lateral line of scales ; but it 

 is more probable that the slime or mucus which covers 

 the body is really the outer layer of the skin. 



The fish propels itself through the water by means of 

 the "fins," all of which have the common character of 

 being expansions of the skin in the form 

 of membrane, supported by numerous 

 "rays" or thin bony spines, much as an 

 umbrella is supported by its ribs. Some 

 of the "rays" (fig. 34, A) are imdivided 

 bony spines, and are termed "spinous 

 rays;" others are branched towards their 

 ends, and are divided by joints into num- 

 erous short pieces, when they acquire the 

 name of "soft rays" (fig. 34, B). Some 

 of the fins have only spinous rays ; some 

 have only soft rays ; and some possess 

 both kinds of rays combined with one 

 another. 



Whilst all the fins agree in consisting oiis'ray"of a "tin, 

 of membrane supported by bony "rays," sf^p^Je sphfe.^'^B! 

 they are nevertheless divided into two very "Soft ray" of a 

 distinct groups, quite different in their real 

 character. Some of the fins, namely, are placed iii pairs 

 on the sides of the body, and are really the lijtibs of the 

 animal. Others are unpaired — that is to say, they are 

 simply placed in the central line of the body, and have no 

 fellows. The "paired" fins are four in number, making 



Fip 



34- 



" Spin- 



