274 SALT-WATER FISHES 



only, from the form and position of the claspers, infer dose 

 analogies to the habits of oviparous sharks and rays. Nor has 

 its egg-case been found ; but that of the Antarctic Callorhynchus 

 is a leaf-shaped structure with fringed edges, not unlike the 

 egg-case of rays, but lacking the horns at each corner- 

 Portions of such an egg-case were dredged by the Travailkur 

 at a depth of 615 metres in the Gulf of Gascony.* 



The Sturgeon (^Acipenser sturio) is a ganoid, and may be 

 regarded as furnishing in a measure a link between the sharks 

 and body fishes. Its place is, therefore, between the chimaera 

 and the bony fishes that compose the rest of this chapter. 



The sturgeons, which we associate particularly with the 

 rivers of Russia and other Continental countries, are all river- 

 fish, sluggish in habits, ground-feeders, spawning in fresh water 

 (their eggs furnishing the basis of true caviare), and returning 

 seawards in the cold months. On our own coasts, however, 

 the fish seems to be caught during summer as often as at other 

 times of the year. 



The scales of other fishes are replaced in the sturgeon by 

 five longitudinal rows of bony plates. The tail, like that of 

 sharks, has a long upper lobe and short lower one. The 

 snout is broad, pointed, and projecting, and four barbels grow 

 from it. The jaws are highly mobile, and can be protruded by 

 the fish when feeding in the soft mud. There are no teeth in 

 the mouth of the adult, but larval sturgeons have conical teeth, 

 which fall out during later development. The skeleton is 

 cartilaginous. There is a single gill-opening, with four gills 

 and a gill-cover (as in bony fishes), a spiracle (as in sharks), and 

 a large air-bladder. 



The sturgeon is brown on the back and lighter beneath. 

 It grows to a length of 1 8 ft. It visits our shores without 



* Exped. Scient. du Travailleur et Talisman (" Fishes," by Vaillant) 

 p. 80. 



