CHAPTER X 

 PISCES 



The animals discussed in this and the following chapters all belong to 

 the phylum Chordata. This rather diverse group contains animals 

 that, at first glance, seem to be very slightly related; but it will be 

 found that they all possess, in a more or less typical condition, at some 

 period in their life-history, three structures or groups of structures that 

 are characteristic of the phylum. 



The first of these is the notochord, a longitudinal, dorsally placed, 

 cylindrical, skeletal axis, Fig. 80, nc, which, in the lower members of 

 the groups, persists throughout life, while in the higher forms it is 

 replaced, in the adult, by a cartilaginous or osseous back-bone. 



The second chordate character is the possession, either in the em- 

 bryonic state or throughout life, of a series of slit-like openings between 

 the pharynx and the exterior; these are the gill or visceral clefts, Fig. 80, 

 g. On the visceral bars, the tissue between the clefts, are found in 

 some forms, particularly the fishs, numerous branching, vascular 

 structures, the gills, by which the animal extracts oxygen from the 

 surrounding water. 



The third chordate character is a dorsally placed, hollow, nervcus 

 system, Fig. 80, s.c. 



The phylum chordata is usually divided into three or four sub-phyla 

 somewhat as follows: i. Enteropneusta, a few worm-like animals of 

 rather doubtful relationships. 2. Tunicata, sea-squirts and other 

 jelly-like animals that are more or less common in the sea. 3. Cephalo- 

 chorda, a small group of small, fishlike animals, called lancets, Fig. 80, 

 which exhibit in a very typical way the above-mentioned three chordate 

 characters and are hence extensively used to illustrate a typical chor- 

 date. 4. Vertebrata, a large and important group, including all of our 

 common domestic animals and others having a vertebral column or 

 back-bone. 



Of these four subphyla only the last has any appreciable economic 

 value except as zoological specimens or objects for study, so that the 

 first three need not be considered further. 



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