GENERAL STUDY OF TYPICAL CHORD ATES 17 



rough feeling due to the spines. A whitish line,* the lateral line, extends along 

 each side of the body; it contains sensory cells whose function is the detection 

 of vibration in the water. Through the skin the zigzag muscle segments or 

 myotomes similar to those of Amphioxus can often be perceived. 



2. The head. — The head is triangular and somewhat flattened; its pointed 

 extremity is known as the rostrum. On the ventral side of the head is the narrow 

 crescentic mouth, bounded as in all vertebrates except cyclostomes by the upper 

 and lower jaws, both of which bear a number of teeth arranged in diagonal rows. 

 The head further bears the three pairs of sense organs characteristic of verte- 

 brates — the olfactory organs, the eyes, and the ears. The nostrils are a pair of 

 openings on the ventral side of the rostrum. A little flap of skin extends over 

 the center of each nostril, dividing the opening into two passages, by means of 

 which a current of water circulates through the olfactory sac, a rounded sac 

 into which each nostril leads. The oval eyes are situated on the sides of the head. 

 They have immovable upper and lower eyelids; in the smooth dogfish the lower 

 lid is extended into a thin membrane, the nictitating membrane, or third eyelid, 

 which can be drawn up over the eye. Behind each eye is a slight prominence, 

 which is best perceived by feeling with the finger; within this the ear is located. 

 There is no external ear, ear opening, or drum membrane, but the ears are con- 

 nected with the surface of the head by two canals, the endolymphatic ducts, 

 which open by a pair of small pores in the center of the dorsal surface of the head 

 just back of the level of the eyes. These may be difficult to find in some speci- 

 mens. The rostrum and adjacent parts of the head are punctured by many 

 pores, which are the openings of long mucous canals imbedded under the skin 

 and of uncertain function. 



3. Gill slits. — Just behind each eye is a circular opening, the first gill slit 

 or spiracle, through which water enters in the respiratory movements. A short 

 distance posterior to the spiracle is a row of five slits, the second to sixth gill 

 slits, through which water escapes in breathing. The slits communicate with 

 the cavity of the pharynx. 



4. Fins. — These are of two kinds, the unpaired or median fins, arising from 

 the median lines of the animal, and the paired or lateral fins, located on the ventral 

 side of the trunk near the median line. The unpaired fins consist of an anterior 

 and a posterior dorsal fin, arising from the median dorsal line, each, in the case 

 of the spiny dogfish, being provided with a spine in front; a ventral fin on the 

 ventral side of the tail, present in the smooth dogfish only; and the tail fin or 

 caudal fin surrounding the posterior end of the body. In the dogfish and other 

 elasmobranchs the caudal fin is asymmetrical with respect to the tail, which 

 bends upward in it, dividing the caudal fin into a narrow dorsal part and a broader 

 ventral portion; this type of tail is known as heterocercal. There are two pairs of 

 paired fins; they correspond to the limbs of land vertebrates. The anterior pair, 

 just behind the gill slits, is named the pectoral fins, the posterior pair at the junction 



