ORGANS OF SECRETION. 595 



animals keeps pace nearly with that of the entire organism, 

 the highest condition of all the normal secreting organs is 

 presented hy the vertebrated sub-kingdom ; and as the laws 

 are most uniform by which the secreting surface of glands 

 is gradually increased to the greatest extent, and the secreting 

 organ itself is confined to the smallest space, it is in this 

 division of the animal kingdom that most glands have per- 

 manently assumed their most compact and most conglo- 

 merate forms. The vertebrata, like the entomoid articulata, 

 being chiefly active air-breathing animals, with a high de- 

 velopment of the. organs of relation, they exhibit a cor- 

 responding high condition of all the secreting organs con- 

 nected with the functions of animal life. They have there- 

 fore the most numerous and the most varied forms of glands, 

 and the most diversified in function connected especially with 

 their cutaneous covering, their tegumentary apparatus, their 

 organs of the senses and of locomotion, besides the highest 

 condition of all those secreting organs more immediately con- 

 nected with individual nutrition and with generation. 



The fishes, like the uterine embryos of higher tribes, are, 

 from their aquatic habitat, still nearly in the condition of the 

 molluscous classes, with relation to their secreting organs, 

 and they exhibit chiefly those most essential to nutrition and 

 generation. From the moist condition of their food, and 

 their want of proper masticating teeth, from the shortness 

 and width of their oesophagus, and their swallowing their 

 food almost without division, from the aqueous constitution 

 of saliva, and the constant streams of water flowing through 

 their mouth for respiration, fishes require no salivary glands, 

 and they are more appropriately provided with numerous and 

 large buccal muciparous follicles, which lubricate and protect 

 the capacious cavity of the mouth and pharynx. Similar 

 mucous glands are developed along the parietes of the oeso- 

 phagus, and they abundantly perforate the mucous lining of 

 the stomach and intestine, as indeed in all the vertebrated 

 classes, and on most mucous surfaces throughout the animal 

 kingdom. As the aquatic respiration of the mollusca is not 

 effected through the mouth, this function does not interfere 

 with the great development of their salivary glands ; but in 

 fishes where the dense aqueous element is respired solely by 

 the mouth, this function is incompatible with the ordinary 



Q Q 2 



