6 THE COMMERCIAL SEA PISHES OF 



are caused by fish ; how to form piscina ; the result of 

 transporting marine fish to fresh waters, or fresh-water 

 forms to the sea. 



In studying fishes in a practical manner, more especially 

 respecting such as are commercially useful to man, it must 

 be evident that the investigations of the pisciculturist 

 must be pushed into subjects which the casual observer 

 would hardly expect. The substances, whether animal or 

 vegetable, upon which fish subsist ; the reasons why certain 

 forms restrict themselves to distinct localities ; the causes 

 of their abundance, or the reason of their diminution ; and 

 this again leads to how inferior organisms are nourished, 

 and what extraneous causes have any effect upon them. 

 Not only, then, have we to investigate the food of fishes, 

 but the nature of their friends, and likewise of their 

 enemies. 



If we divide the animal kingdom into the vertebrate and 

 invertebrate forms, we are all aware that fishes, possessing 

 as they do a backbone or vertebral column, belong to the 

 highest of these subdivisions. This includes all animals 

 having a longitudinal axis or spine situated in a superior 

 or dorsal cavity, and one which contains the brain and 

 spinal chord, but is entirely separated from a lower cavity 

 or visceral tube, which represents the condition existing in 

 invertebrata. 



The classification of vertebrata now most usually adopted 

 is as follows : 



I. MAMMALIA. Young, born alive (viviparous), and 

 nourished by their mother's milk. 



II. SAUROPSIDA. As birds and reptilian forms, that are 

 oviparous or oviviparous ; are not mammals, and never 

 possess gills. 



III. ICHTHYOPSIDA. Such as batrachians and fish, also 



