170 ZOOLOGY. 



steering the propelling organ being the muscular tail and, 

 in a less degree, the trunk. 



The mouth is not terminal, as in the rabbit, frog, and 

 bony fishes it is ventral. The cloacal opening lies between 

 the pelvic^fina. In front of the mouth are the nares the 

 wid<r apertures of the olfactory organs. These have nothing 

 to do with respiration as in our other types, buTltrcTlsolely 

 concerned with smell. There are no internal nares, but an 

 external groove, partly covered in by a fold of skin, con- 

 nects each olfactory chamber with the mouth. The eyes 

 occupy the ordinary position, but have no movable lids. 

 Behind them, just where we might expect to find the 

 tympanic membrane, there is an opening on either side the 

 spiracle ; and at each side of the throat are five slit-like 

 openings, the gill-slits. These are concerned in respiration. 



2. Dermal Denticles. The skin of the dogfish is 

 closely set with pointed tooth-like spines, the d_ermal_den- 

 ticles (formerly called placoid scales), and these are con- 

 tinued over the ii^s^into_the_mquth as_ teeth. Each scale 

 consists of (1) a horizontal basal plate embedded in the 

 dermis, (2) a pointed spine arising from the upper side of 

 the plate and protruding through the epidermis. The plate 

 consists of true bone, the spine of dentine capped with 

 enamel, and there is a hole in the centre of the plate through 

 which blood-vessels pass into the cellular pulp-cavity in the 

 axis of the spine exactly the structure of a mammalian 

 tooth in all essentials (chap, vi., 12). The bases of these 

 scales are almost the only real bone-tissue in the dogfish ; 

 and perhaps, in addition to the teeth other relics of the 

 placoid scales are to be found, in the higher vertebrata, in 

 the membrane-bones. How placoid scales may have given 

 rise to these structures will be understood by considering 

 such a bone as the vomer of the frog. This bone lies on 

 the roof of the frog's mouth, and bears a number of little 

 teeth, and altogether there is a very strong resemblance in 

 it to a number of placoid scales the bony bases of which 

 have become confluent. In the salamander, behind the 

 teeth-bearing vomers comes a similar toothed parasphenoid 

 bone. The same bone occurs in a corresponding position in 



