156 MANUAL OF ANIMAL BIOLOGY 



mented, bony vertebral column. The external openings of the 

 gill slits are covered, on each side of the body, by a fold of tissue, 



the OPERCULUM. 



There are two subclasses recognized, namely, the Teleostomi 

 and the Dipnoi. The last named subclass contains only a few 

 species and these are comparatively rare, but are interesting be- 

 cause of the presence of either single or paired lungs. They are 

 often known, therefore, as the Lung-fishes. The presence of func- 

 tional lungs and certain other structural features as well as their 

 habits of life would seem to indicate a close relationship to the 

 Amphibia, the next higher class of Vertebrates. On the other 

 hand, certain features, such as the persistence of the notochord, 

 are regarded as more primitive in character. (W. f. 75.) 



The very much larger subclass, the Teleostomi, is subdivided 

 into four orders of which the last, the Teleostei, is the most impor- 

 tant and includes most of the common species of bony fishes. 

 Although the majority of species included show the typical spindle- 

 shaped body with which we are all familiar, there are, neverthe- 

 less, found many varieties which depart widely from this type. 

 As a matter of fact, the shape and general structural features are 

 typically found to be in close harmony with the habits of life. 

 The active swimming types of fishes, known to everyone, do not 

 have much variation in structure, but keep close to the laterally- 

 compressed, pointed spindle-shape which offers minimum resist- 

 ance as the animals are driven through the water by their mus- 

 cular swimming actions. (W. f. 71.) 



The less active species present a considerable number of inter- 

 esting structural variations. Thus, we may note the Sea-horse 

 (Hippocampus antiquorwri) which clings to objects with its pecul- 

 iar tail and thus maintains a vertical position in the water while 

 it'pokes around the sea weeds with its long snout. The shape of 

 the head of Hippocampus bears more than a passing resemblance 

 to that of- a horse. Or the group of Flat-fish, or Sole, which are 

 active and typically fish-shaped when young, but gradually be- 

 come flattened, not dorso-ventrally as in the Skates, but laterally 

 so that they lie on one side instead of on the ventral surface. 

 (W. fs. 72, 73.) 



An example of dorso-ventral flattening in a Teleost is found 

 in the large, sluggish Goose-fish (Lophius piscatorius) — one of 

 the most bizarre and almost repulsive looking species of fish to 



