12 



THE STORY OF FISH LIFE. 



have probably arisen by a modification of these 

 balancing organs. Poised in the water the fish 

 is free to move in all directions. To adapt the 

 fish still more perfectly to its environment a 

 very peculiar organ, known as the air-bladder, 



has been developed, 

 at least in the 

 majority of living 

 forms, as well as in 

 the more modern 

 fossil types. This 

 air-bladder seems to 

 render the specific 

 gravity of the fish 

 the same as that 

 of the surrounding 



Fig. 1.— .4. Common Roach, showing Water. Modifications 



arrangement of fins and general which the air-bladder 

 shape of body. D. dorsal fin ; C. , . 



caudal or t:iil fin; A. anal fin ; PI. may Undergo Will be 



Pelvic or ventral fin ; P. pectoral f^,,^^ r.^ a Infprnacrp 



fin ; G. gill cover; L. lateral line ; lOUna on aiatei page. 

 «. transverse line. The significance FurtllCT adapta- 



of the transverse and lateral lines is .• , l' ^ 



explained further on. Only a few tlOU tO aquatlC neCClS 



scales have been indicated, In order ]-,r,(5 rpqnlfprl in a 



that the transverse and lateral lines ""^^ lebUltea in a 



may bo rendered more conspicuous. Complete investment 

 B. An outline figure of the Bur- r ^ji^vip . liowponinno: 



hot {Lota vulgaris), for comparison ^^ f^lime , noWCOpiOUS 



with fig. ^.,note the difference in this SCCretioU mav 



the shape and position of the fins i i rt \_ -ri 



and the absence of scales.— After t)e Oream-TlSnerS Will 



^^°th"- testify ! This slimy 



coat reduces friction to a minimum. Beneath 

 this slime we can generally descry a series of 

 overlapping plates — the scales ; these fit closely 

 one over the other so as to offer a perfectly 

 smooth surface. 



The fish, as we have just remarked, being 



