II. Organs. 9, 10. Excretory, Urinary, and Reproductive Organs. 31 



urinary or excretory organs (kidneys), are present in most 

 animals. They are usually tubular or sac-sliaped glands opening on 

 to the surface of the body or into the hinder part of the alimentary 

 canal. The secretion (the urine) is either entirely liquid or it contains 

 hard, granular, or crystalline concretions. Sometimes a vesicle (a 

 urinary bladder) is formed, which serves as a reservoir for urine ; it 

 may either be a widened region of the canal of a single gland, or of 

 an excurrent canal common to several. In many animals, however, 

 are other organs also excretory in function; amongst some 

 of the Chsetopoda, the cells of a certain part of the alimentary canal 

 secrete hard concretions,* which are, undoubtedly, urea ; such occur 

 also in the hind gut of the Rotifers, and in some of the liver-cells of 

 the Gastropoda; also in the epithelium lining the body-cavity of 

 certain Chaetopods, and of Sharks, urea is formed, and is eliminated 

 in different ways. 



The waste products are, however, not always removed from the 

 body; in some, probably in many, cases, they are stored up in cells 

 which have no connection with the exterior. This occurs, e.g., in 

 certain Fly-larvee where a mass of cells containing excretory deposits 

 surrounds the heart ; in a Slug, whose true kidney is degenerate, cells 

 containing concretions of uric acid are scattered throughout the body 

 [See also Tunicata.) 



Tlie pigments so abundant in most animals, may also, in pai-t, represent waste 

 products, whicli have accumulated in cells in the way just mentioned. In other 

 cases large masses of pigment are regularly removed from the body : the pig. 

 ment present in the cuticle, particularly in the hair and feathers,t is got rid of 

 by ecdysis in many animals, by the shedding of hair in Mammals, by moxilting 

 in Bii-ds, 



10. Reproduction and Reproductive Organs. 



Eeproduction, the formation of new individuals, occurs in the 

 Animal Kingdom in two quite different ways, sexually and 

 asexually. Asexual reproduction will be first considered in its 

 two forms of fission and gemmation. 



In fission, a longitudinal or transverse furrow appears on the 

 individual concerned, and gradually deepens, until finally, the 

 organism divides into two approximately equal pieces, which grow 

 whilst the process is going on, or after it is complete, until each has 

 attained the size of the parent. Less frequently division occurs 

 without the preceding constriction, the animal breaking suddenly into 

 two pieces. Gemmation differs from fission in that only a small 

 part of the body of the original individual develops (by rapid growth) 

 into a new animal, so that it is possible to distinguish between the 



* Small bodies lying in the protoplasm, 



t The pigment present in the epidermis and hair of Mammalia is, at any rate for 

 the most part, not formed m situ, but is brought there by wandering cells which 

 migrate into the epidermis from the svibjacent connective tissiie. 



