PISCES. 15& 



side of this aperture leading into the body-cavity. The remaining 

 external openings are all in the anterior part of the body. Upon 

 the under side of the snout the external nares or nostrils are seen 

 as large rounded openings, from each of which a groove, covered 

 by a fold of skin, leads back to the mouth. 



The oblique eyes, provided with upper and lower eyelids, are 

 placed on the sides of the head above the posterior corners of 

 the mouth. Just behind each of them is a small round hole, the 

 spiracle, opening out of the pharynx. It is of the same nature as 

 five oblique gill-slits which are seen further back immediately in 

 front of the pectoral fin. 



Two minute apertures on the top of the head communicate 

 with the auditory organs. A large number of regularly-arranged 

 pores can be seen upon the head, especially in its anterior part. 

 They are the openings of sensory tubes (jelly-tubes, mucous canals), 

 which lie under the skin. Another sensory structure underlies 

 the groove-like lateral line which runs along each side of the 

 body. 



The dogfish is of a whitish colour ventrally, grey with dark 

 brown spots dorsally and laterally. The fins are spotted as well 

 as the body. This colouration must make the animal extremely 

 inconspicuous when seen from above in its natural surroundings. 



The body is entirely covered by small sharp, placoid scales, 

 imbedded in the skin, but with projecting, backwardly directed 

 points. The scales near the mouth closely resemble the teeth. 



2. The skin consists of an epidermis composed of stratified epi- 

 thelium, and an underlying dermis made up of connective-tissue 

 traversed by blood-vessels, lymphatics, and nerves. 



The only important glands connected with the skin, if the jelly- 

 tubes are excluded, are the clasper-glands of the male, each of which 

 is a pouch underlying the skin between the pelvic fins and opening 

 backwards into the groove of the corresponding clasper. 



The small placoid scales are developed in the skin. Examined 

 with a powerful lens, or under a low power of the microscope, 

 each of them is seen to consist of a four-rayed basal plate, and of 

 a much larger spine attached to it. The end of the spine is leaf- 

 shaped and directed obliquely backwards with its flat side upper- 

 most. The basal plate is bone-like, the spine composed of hard 

 dentine, capped by exceedingly hard enamel secreted by the epi- 

 dermis. The rest of the scale is developed by the dermis. 



