VI EXTERNAL CHARACTERS. 



similar conditions emit light. Again we find forms seeking refuge in other 

 animals, or else as commensals using them as vehicles for conveying them 

 from one place to another, or fastening on their neighbours for their own 

 carnivorous purposes. Then there is the breeding of fishes whether natural 

 or artificial, hybridism and its effects, sterility and its causes ; diseases and 

 the mode of destruction of fish ; and lastly, fisheries, and how they are worked. 

 Fish are the lowest class of the vertebrate division of the animal kingdom, 

 destined to • pass their lives in a watery element, and having their bodies 

 very diversely modified in order to accommodate them to varied con- 

 ditions of existence. Some are fitted for salt waters, others for those that 

 are fresh, some for residing at great depths, others for shallows, some are 

 mere surface swimmers pursuing their finny food, while others, although 

 similarly surface swimmers, live on minute organisms, and are themselves 

 pursued by their more powerful neighbours. Some like clear water, others 

 prefer that which is muddy, the rapid stream, the whirling eddy, the placid 

 lake or pond, or the mountain torrent, each possess their fish life, modified 

 according to circumstances and frequently changing with the age of the 

 animal. 



EXTERNAL CHARACTERS. 



Eemarking generally on the external form of fishes we see a comparatively 

 large head attached directly to the trunk without the intervention of a neck, 

 while the body tapers off towards the 'posterior extremity. The regions 

 into which they are externally divided are those of the head, the body or 

 trunk, the tail and the fins. In such forms as the perch or carp and the 

 majority of species the head is separated from the body or trunk by the 

 gill-openings ; while the body and the tail have the vent as a division 

 between them. While external similarity in form is occasionally more 

 symptomatic of ah agreement in the diet on which they feed than on their 

 zoological relationship. 



In this class of animals modifications of form from what are seen in the 

 more typical perches 'or' carps are exceedingly numerous. The body -may 

 be laterally flattened, as in the sole (plate cvi) strongly depressed as in the 

 skate (plate clxvi), shortened as in the sun-fish (plate cxlviii), elongated and 

 rounded as in the eel (plate "cxlii), elongated and compressed as in the 

 scabbard-fish (plate li), shortened and rounded as in the globe fish (plate 

 cxlvii), or encased in square armour as in the box-fish Ostracion, or the pipe 

 fishes (plate cxliv) . The head may, when compared with the size of the body, be 

 enormously developed as in the frog-fish (plate xxix), or produced laterally 

 as in the hammer-headed shark (plate cliv), or the snout may be lengthened 



