10 UNUTILIZED FISHES. 



idea that there is a complex cycle involving the whole series of 

 marine forms, from the minute micro-organic matter (animal and 

 vegetable) to the fishes. Scott (1899) gives a few notes on the food 

 of young fishes which frequent the British coast in the vicinitj' of 

 Piel. Linton (1901) in his Parasites of Fishes of the "Woods Hole 

 Region has a record of food found in the alimentary tract of the 

 fishes he examined for parasites. Scott (190:2) made a special study 

 of the food of small and immature fishes, and describes also the food 

 of several larger species. He shows that minute Crustacea form the 

 chief constituent in the food of the young of many species which 

 when adult live mostly on other organisms. 



These researches on the food of fishes, it will be observed, vary 

 greatly in character and purpose. In one case the object Avas to find 

 new species to add to a local f aunal list ; in others it was to find out 

 the primary food basis of the fishes; another author wished to 

 explain irregular and sudden migrations of fishes, and still another 

 studied the food in order to study the parasites of fishes. Much 

 therefore remains to be learned about a most important factor in 

 many scientific and economic questions. 



THE SMOOTH DOGFISH. 



CHARACTERS AND DISTRIBUTION. 



The smooth dogfish {Mustehis canis) has a slender, elongated body, 

 tapering from the dorsal fin back to the slender heterocercal tail. The 

 color is a uniform slaty gray on the back and sides and a light pearl 

 gray on the ventral surface. The skin is thickly beset with minute, 

 hard, .sharp tubercles — the placoid scales ; to the hand passing forward 

 over it the sensation is that of feeling a fine sandpaper. The head, 

 broad and depressed, terminates in a moderately sharp snout. The 

 mouth is small and situated on the ventral side of the head. It lies 

 transversely, somewhat crescentic in shape. The teeth are flat and 

 paA'ement like, well adapted for crushing rather than for seizing and 

 holding prey. A pair of nostrils open on the ventral side of the head, 

 one in front of each corner of the mouth. Two round small aper- 

 tures, the spiracles, open dorso-laterally, one behind each eye. There 

 is no operculum, the branchial clefts opening as five pairs of slits, 

 which lie in a vertical position on each side of the neck. There are 

 two dorsal fins and two sets of paired fins, the anterior pair, or pec- 

 torals, being situated ventro-laterally just behind the head, and the 

 posterior pair, or pelvic fins, being located on the ventral side just 

 anterior to the cloacal opening. In the male the inner sides of the 

 pelvic fins are modified into stiff fleshy rods called claspers. The 

 average length of the smooth dogfish is about 3 feet, but occasionally 



