790 SPECIAL PHYSIOLOGY. 



tinal orifice, generally into the branchial cavity. In the Lamellibranchiata, a 

 similar organ, but, in most cases, less distinct, is also found near the heart, 

 close to the lower end of the intestine, opening into the cavity of the mantle. 

 In the Mulluscmda, distinct renal organs have not yet been recognized. 

 Amongst the Annulosa, the Insecta, Myriapoda, and Arachnida, have excre- 

 tory organs believed to be renal, consisting of long tubes, often beginning by 

 clusters or tufts of vesicles ; they are sometimes few, as in Myriapoda, some- 

 times very numerous, as in the higher Insects. As in the Vertebrata, they 

 open into the lower part of the intestine, or even close to its orifice ; sometimes 

 the principal duct is dilated near its lower end, as if to form a urinary bladder. 

 The colored fluid discharged by the lepidoptera, on their emerging from the 

 chrysalis, proceeds from these vessels, and contains uric acid. No such renal 

 organs are found in the Crustacea, which are aquatic. In the Annuloida, in 

 which they are likewise absent, the water-vessels may have some excretory 

 function, and eliminate urinary products. 



In certain of the Codenterata, small clusters of cells projecting into the body 

 cavity, and containg guanin, are regarded as renal organs ; but in the simplest 

 forms of these animals, the excretion of the products of the decomposition of 

 albuminoid substances is probably accompanied by the external and internal 

 surfaces of their hollow bodies. 



Lastly, in the minute Protozoa, such products must also be eliminated by 

 the general surface. 



Special Secretions in Animals. 



Certain secretions or excretions, in animals, may perhaps be regarded, not 

 merely as serving a peculiar purpose in the economy, but also as fulfilling an 

 emunctory office, eliminating from the system substances which might be as 

 injurious to animal life as urea and uric acid. Amongst such, may be men- 

 tioned the castor of the beaver, the musk of the musk-deer, the peculiar secre- 

 tion of the civet-cat, and those of other Mammalia, also the venom of Serpents, 

 the acrid secretion of the skin of the toad, the ink of the cuttle-fish, which 

 yields the sepia color used by painters, the poisons of the stings of the bee and 

 the wasp, the sugar secreted by aphides, the odoriferous excretions of the bugs 

 and many other beetles, the poison in the tail of the scorpion and the mandibles 

 of the spider, the odoriferous exudations of the lumbrici, and even the threads 

 of the sea-nettles. Examples of special secretion are also met with in those 

 .glands which in many caterpillars supply the silk used in progression or for 

 the cocoons, in the spinneret glands of the Spiders, the cement gland of the 

 Cirrhopods, and the glandular structures which secrete the byssus of certain 

 Lamellibranchiata. 



However different and specialized may be the actions of the various glandu- 

 lar organs in the Animal Kingdom, which yield such widely different products, 

 they are all based on a common plan of structure and function. Even in the 

 highest animals, and in Man, their physiological relationship is evidenced by 

 an occasional tendency to a vicarious action, in which one gland or several 

 .glands take on the suspended function of another. 



THE SKIN AND ITS EXCRETIONS. 



By means of its sebaceous and sudoriferous glands, the skin secretes 

 and excretes fatty matter, and the perspiration or sweat. Besides 

 this, it exhales water from its surface, and throws off certain quan- 

 tities of carbonic acid gas. 



The sebaceous or oily matter, formed by the so-called sebaceous 



f lands (p. 361 ; Fig. 69, a), consists of a mixture of olein, saponi- 

 ed fat, cholesterin, a small quantity of an unnamed albuminoid sub- 

 stance, and a few epidermic cells. Its ashes abound in earthy phos- 



