stomes are the lampreys (Fig. 32-39). Unlike 

 other vertebrates, these animals are parasitic. 

 The mouth lies at the center of a circular 

 sucker, which enables the animal to attach 

 itself to a passing fish. Also the mouth is 

 equipped with a sharp-toothed tongue. This 

 bores through the skin of the fish, allowing 

 the lamprey to suck out blood and other 

 body fluids. Then the parasite detaches it- 

 self and seeks another temporary host. Thus 

 large numbers of fish may be damaged or 

 even killed, if the lamprey population is 

 abundant. Recently, the lampreys of the 

 Great Lakes of North America have become a 

 serious menace to the fishing industry, espe- 

 cially with reference to the lake trout, a fish 

 of considerable economic value. Previously, 

 there were no lampreys in the Great Lakes, 

 but a marine species (Petromyzon marinus) 

 gained entrance via the Welland Canal, an 

 artificially constructed waterway. 



The Chondrichthyes. Today the cartilaginous 

 fish are represented mainly by the sharks 

 (Fig. 32-39), dogfishes, and rays (skates). How- 

 ever, the group can be traced back to Silurian 

 times when it included many forms long since 

 extinct. All are oceanic and, except for the 

 rays, which are bottom dwellers, all are fast- 

 swimming, voracious predators. One shark, 

 the whale shark, has the distinction of being 

 the largest true fish, reaching a length of 

 more than 50 feet. Some of the rays (Fig. 

 24-11) possess high-voltage electric organs 

 with which they stun their prey. 



The mouth of the cartilaginous fish dis- 

 plays definite jaws, hinged to the cranium 

 and derived from the gill arches; and usually 

 the mouth is located ventrally, some distance 

 behind the anterior end of the animal. The 

 entire skeleton, including the jaws, cranium, 

 vertebrae, and pieces supporting the paired 

 fins and gill arches, is composed of cartilage; 

 that is, no true bone (p. 289) is present. How- 

 ever, sometimes the cartilage may be par- 

 tially calcified. Remnants of the notochord 

 are found in and between the vertebrae. 



The skin of the typical cartilaginous fish 

 is clothed by a large number of small but 



The Animal Kingdom - 669 



uniquely formed, sharply pointed scales, 

 called placoid scales. These are minute rep- 

 licas of the larger teeth, many rows of which 

 are found in the anterior part of the mouth 

 cavity of the shark and other forms. Placoid 

 scales appear to be homologous to the teeth 

 of higher vertebrates generally, including 

 man. Externally each displays a layer of 

 dense hard material, which chemically and 

 histologically is indistinguishable from en- 

 amel (Fig. 16-14). Then there is a layer of 

 dentine, which encloses a pulp cavity, con- 

 taining a nerve branch, blood vessels, and 

 other soft tissues. Moreover, the fossil record, 

 which favors the preservation of teeth and 

 other hard parts, provides abundant evi- 

 dence that the teeth of higher vertebrates 

 have been evolved from the placoid scales of 

 lower forms. 



The cartilaginous fish (also called Elasmo- 

 branchi) possess another distinctive feature. 

 All have a complex spiral infolding of the 

 intestinal wall. This spiral valve serves to in- 

 crease the absorbing capacity of the digestive 

 tract. 



The Osteichthyes. These familiar bony fishes 

 inhabit salt and fresh waters in tremendous 

 abundance and variety (Fig. 32-39). 



Typically the skeleton is composed mainly 

 of true bone (p. 289), although in the em- 

 bryo, following the notochordal stage, the 

 pattern of the adult skeleton is first laid 

 down in cartilage, as in higher Vertebrata 

 generally. Well-developed hi?iged jaws sup- 

 port the anteriorly placed mouth; the cra- 

 nium in most forms is very completely ossi- 

 fied; the paired fins have a bony skeleton 

 and are suspended from the vertebral column 

 by girdles. Remnants of the notochord are 

 found in and between the segments of the 

 vertebral column. The scales are of several 

 kinds, but none of the bony fish has placoid 

 scales. 



Many bony fish have swim bladders, which 

 enable them to regulate their buoyancy in 

 accordance with the depth at which they 

 are swimming. In one group (the Dipnoi, or 

 lung fishes) the swim bladder retains a tubu- 



