269 



contents at once placed in spirit or other suitable preser- 

 vative for further microscopic investigation, as, in many 

 species, decomposition sets in with great rapidity, and all 

 trace of the more delicate food becomes rapidly destroyed. 



Having ascertained what minute forms or other descrip- 

 tions of life are employed for the nourishment of the fish, 

 the inquiry branches off into what it is that governs and 

 controls migration of these forms, as currents, soils, tempe- 

 rature, atmospheric vicissitudes, or some more minute food 

 upon which they themselves subsist ; what are their enemies 

 and their friends, and the conditions which favour the pre- 

 sence or absence of either class ? 



Fishes may for convenience' sake be divided into preda- 

 cious, omnivorous, and herbivorous species, which necessitate 

 an organisation suitable for each. Some are rapid swimmers 

 in order to overtake prey, or escape from their foes ; others, 

 which obtain their food by means of stratagem, are con- 

 structed for a slower mode of progression. But, for two 

 reasons, I must omit the consideration of form; first, 

 because such are familiar to my hearers, and have represen- 

 tatives among the collections shown in this Exhibition ; 

 and secondly, because such would unduly lengthen my 

 paper. The various descriptions of teeth in fishes are more 

 or less demonstrative of the great difference which must 

 exist as to the food they consume : some being prehensile, in 

 order to assist in capturing and retaining their prey ; others 

 are more rounded for the purpose of crushing the shells upon 

 which they mainly subsist In carp the teeth are restricted 

 to the throat, but the variations are too numerous to enter 

 upon in this place. 



The commencement of the alimentary canal, or the mouth, 

 is the common receptacle of water passing to the gills for 

 respiration and food transmitted to the stomach for nutri- 



