35° 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



revert to the primitive pointed (protocercal) condition (Fig. 146, A) 

 which occurs normally in fishes during early stages of development and 

 is found persisting in the adult in some of the more archaic groups of 

 fish to be dealt with later. 



A point to be noted specially is that the fins of fish — apart from 

 the caudal fin — are by no means fixed permanently in position but in 

 the course of evolution tend to undergo slow shifting so as in any 

 particular phase of evolution to be in the position in which they can be 

 used to best advantage, having regard to the habits and general form 

 of the body. An extreme illustration of this mobility of the fins during 

 evolution is afforded by the pelvic fin of such a fish as the Cod or 



Fig. 145. 

 Haddock {Gadus aeglefinus), a.f, Anal fins ; an, anus ; d, dorsal fins ; 



A Teleostean Fish- 

 1.1, lateral line ; olf.i and 2, olfactory openings ; 

 pl.f, pelvic fin. 



op, opercular opening ; p.f, pectoral fin ; 



Haddock which has become shifted so far forwards as to be " jugular " 

 in position — well in front of the pectoral fins (Fig. 145, pl.f). 



The skin of the Teleost is specialized for the diminution of " skin- 

 friction " — the friction between its surface and the surrounding medium 

 which constitutes the main obstacle to the passage of a solid body 

 through water. To minimize this the epidermis has scattered through 

 it innumerable gland-cells which secrete an exceedingly slippery mucus 

 and thus lubricate the surface of the body. 



Beneath the epidermis there are normally present scales of a more 

 highly evolved type than the placoid scales of the Elasmobranch. These 

 cycloid scales (Fig. 147) are very thin plates of peculiarly tough bone, 

 which overlap like slates on a roof and are consequently able to slide 

 over one another so as not to interfere with the flexibility of the body. 

 The growth of the fish is accompanied by increase in size of the individual 



