CHAP. XXXVI.] INVERTEBRATA. 53 1 



tion until a greater number of facts relating to these wonderful 

 processes should be discovered, rather than adopt a view which 

 future research may show to be erroneous. 



Sexual Organs. The generative organs are of two kinds, the 

 male and the female organs, the one secreting the " sperm cell/' 

 and the order the " germ cell." The generative apparatus consists 

 of two parts : a formative organ, in which the elements are pro- 

 duced, and which is essential ; and an efferent duct, by which the 

 products of secretion are carried off. 



The male and female organs may exist in one individual or in 

 separate individuals. The first condition is termed unisexual, and 

 the second bisexual generation. 



In some unisexual or hermaphrodite animals, self-impregnation 

 takes place, as is the case in the common tape-worm (Tsenia solium) ; 

 while, in other instances, concourse is necessary in order that the 

 ova should be exposed to the action of the spermatic fluid ; this is 

 the case with many of the mollusca, as the common snail, etc. In 

 these instances, each hermaphrodite animal impregnates its 

 neighbour. 



Besides the secretion of the formative organ, other secretions 

 are frequently poured into the efferent duct. The duct undergoes 

 great modifications in different parts of its course in various ani- 

 mals, according to the particular office it has to fulfil. We shall 

 now consider some of the most important characters which the 

 sexual organs exhibit in the different classes of animals. 



INVERTEBRATA. 



The Infusoria multiply by fission, and rarely by gemmation. No sexual 

 organs have yet been discovered in them, and ova are not met with in this 

 lowest class of the animal series. 



Fission may occur in the longitudinal direction, as in the Vorticella : or 

 transversely, as in Stentor and some others ; or in both directions, as in Bur- 

 saria, Paramtecium, and others. The Vorticellse are also propagated by the 

 formation of buds. When division is about to take place, the cell within the 

 body, known as the nucleus, is seen to divide into two ; each half containing, 

 therefore, a newly-formed nucleus. 



The Polyps multiply by gemmation and by the formation of ova, very 

 rarely by fission. In gemmation, the young polyp may be ultimately set 

 free, as in the Hydra ; or it may remain attached to the stem or common 

 body or polypidon, as in the majority of the members of this class. Some 

 polyps are hermaphrodite, while in many the sexes are distinct. At the time 

 when the common hydra is about to propagate by ova, the male and female 

 organs are both developed as excrescences upon the outer surface of the body. 

 Others, again, are sexless, and give rise to the development of medusa-buds, 

 or split up into discs, as already described in p. 528. 



