40 ZOOLOGY SECT. 



the ovaries occur in one set of individuals the females and the 

 testes in another set the males, when the term unisexual or 

 dioecious is employed. Very frequently the male differs from the 

 female in other respects besides the nature of the reproductive 

 elements in size, colour, and the like ; when such differences are 

 strongly marked the animal is said to be sexually dimorphic. The 

 ova and sperms are usually conveyed to the exterior by canals 

 or ducts the ovarian ducts or oviducts, and the testicular ducts, 

 spermiducts, or vasa defer entia. In some instances the ova are 

 impregnated after being discharged from the oviducts, and the 

 development of the young takes place externally; in other cases 

 the impregnation takes place in the oviduct, and the young 

 become fully developed in the interior of a special enlargement 

 of the oviduct termed the uterus. In the former case the animal 

 is said to be oviparous, in the latter viviparous ; but there are^ 

 numerous intermediate gradations between these two extremes. 



6. THE REPRODUCTION OF ANIMALS. 



In a limited number of groups of animals reproduction takes 

 place by means of cells corresponding to ova developed in organs 

 similar to ovaries, but without impregnation by means of sperms. 

 This phenomenon is known as parthenogenesis (cf. p. 21). 



Besides the sexual process of reproduction by means of ova and 

 spermatozoa, there are in many classes of animals various asexual 

 modes of multiplication. One of these the process of simple 

 fission has been already noticed in connection with the reproduction 

 of Amoeba. The formation of spores is an asexual mode of multi- 

 plication which occurs only in the Protozoa, and will be described 

 in the account of that group. Multiplication by budding takes 

 place in a number of different classes of animals. In this form of 

 reproduction a process or bud (Fig. 27, bd) is given off from some 

 part of the parent animal ; this bud sooner or later assumes the 

 form of the complete animal, and may become detached from 

 the parent either before or after its development has been 

 completed or may remain in permanent vital connection with the 

 parent form. 



When the buds, after becoming fully developed, remain in vital 

 continuity with the parent, a sort of compound animal, consisting 

 of a greater or smaller number of connected units, is the result. 

 Such a compound organism is termed a colony, and the component 

 units are termed zooids. In some cases such a colony is produced 

 by a process which is more correctly termed incomplete fission 

 than budding. 



Alternation of generations ; heterogamy ; paedogenesis. 

 In the life-history of a considerable number of animals, a stage in 

 which reproduction takes place by a process of budding or fission 



