FISHES 



291 



Structure on the top of the head, armed with curved spines and 

 capable of being drawn back into a pit. The eyes are very- 

 large and the skin is smooth. Only four gill-slits are present on 

 each side, and they are covered over by a membranous flap or 

 gill-cover. There is no spiracle. Another species of Chimaera 

 is taken off the coast of Portugal, and the third is found on the 

 Pacific coast of North America. 



The Bottle - nosed Chimaera [Cal'lorhynchus antarcticus) of 

 the southern seas is more shark-like in form, and its name is 

 derived from the curious thickened form of the snout. A third 

 genus {Harriotta), recently discovered in very deep water both 

 in the Atlantic and Pacific, is comparatively small and is dis- 

 tinguished by its slender-pointed snout. 



Sub-class V.— ROUND-MOUTHS (Cyclostomata) 



The Lampreys and Hags which make up this sub-class are 

 so unlike other fishes in many respects that many zoologists 

 place them in a class of their own. They have a wide distribution 



Fig. i6g. — Lampem {Petromyzon Jluviatilis) 



in the temperate regions of both hemispheres, and the lampreys 

 occur in fresh as well as in salt water. There are four British 

 species, three of these being lampreys, any one of which will 

 serve to illustrate the chief characters of the sub-class. They 

 are the Sea-Lamprey {Petromyzon marinus), the River-Lamprey 

 or Lampern {P. fluviatilis) (fig. 169), and the Small Lamprey 

 {P. branchialis). The eel-like body is bordered by three narrow 

 unpaired fins, two dorsals and a caudal, but all traces of paired 

 fins are absent. Instead of possessing a slit-like mouth bounded 

 by jaws, as in all the backboned forms so far considered, a 

 rounded sucker-like concavity is present on the under surface of 

 the head, and the small mouth opens within this. The scientific 



