EXCRETING ORGANS. 627 



granules, and opening near the anus, and which thus pre- 

 sents a close analogy to the renal organs of higher animals. 

 The muciparous gland of the turbinated testaceous gaster- 

 opods, pouring out so copious a secretion under the mantle, 

 near the anus, may perform a similar office, and also the 

 glandular sac opening near the anus in the doris and some 

 other naked species. The poison glands of scorpions and 

 insects, the glands for the deep-coloured excretions of certain 

 gasteropods, the anal ink-glands of cephalopods and the 

 muciparous glands of their oviducts, have likewise some 

 analogies to urinary organs. 



In the vertebrated classes, no organs are more constant 

 than the two essential urinary glands, the kidneys, and the 

 more accessory urinary bladder is one of the most variable 

 and inconstant. The urinary organs are generally of larger 

 size, and of a simpler internal structure, in those animals 

 which have a limited extent of respiration, so that their bulk 

 is commonly in the direct ratio of that of the biliary organs. 

 The urinary organs of vertebrata, like their genital organs, 

 are developed in the embryo from the cloacal end of the 

 alimentary canal, and in all the oviparous tribes, they con- 

 tinue in the adult state to communicate directly with that 

 cavity. The kidneys of fishes have a lengthened lobulated 

 form, extending along the sides of the vertebral column as 

 far forwards as the cranium, exterior to the peritoneum, and 

 behind the air- sac. In their elongated form, and in their 

 proximity to each other along the median plain, as well as in 

 their lobed structure, and in the parallelism of their compo- 

 nent tubuli uriniferi, they resemble the embryo-state of 

 these organs in mammalia. They extend forwards above 

 the heart and branchiae, and backwards into the pelvic cavity 

 behind the anus, bound to the sides of the bodies of the 

 vertebrae by the peritoneum, separated from each other by 

 the interposed vena cava, and by the two ureters which run 

 along their whole extent, as narrow tubes, without forming 

 a distinct enlargement or pelvis. The great size of these 

 emunctories of the aqueous part of the blood in fishes, may 

 have relation to their aquatic habitat, and the quantity of 

 fluids constantly taken with their food. The kidneys consist 

 of subdivided ureters, the tubuli of which are variously 

 disposed, and produce, by their subdivisions and convolu- 



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