234 ANIMAL BIOLOGY 



In many of the nearest allies of Hydra, it will be recalled that 

 the buds remain permanently attached so that eventually a large 

 colony of organically connected polyps is developed. Moreover, 

 this condition leads to a physiological division of labor between 

 the various polyps which may become more or less changed in 

 structure so that, for instance, feeding, protective, and repro- 

 ductive individuals are established, and thereby the Hydroid 

 colony exhibits polymorphism. Our present interest is confined 

 to the reproductive polyps, which in many of the Hydroids are so 

 modified that they are dependent upon the colony as a whole for 

 all the necessities of life and are merely bodies which form asexually 

 by budding other individuals known as medusae. The medusae 

 liberate their sexual products in the water where fertilization 

 occurs, and the zygote gives rise to a free-swimming embryo which 

 soon becomes attached to some submerged object and develops 

 into a hydroid colony. (Figs. 37, 57.) 



Thus the common Hydroids, such as Obelia, exhibit two dis- 

 tinct phases, or generations, in their life history — the fixed, poly- 

 morphic colony of polyps which is produced sexually but is itself 

 asexual; and the free-swimming medusae which are produced 

 asexually but are themselves sexual. The asexual and sexual gen- 

 erations alternate with each other in regular sequence, so that an 



ALTERNATION OF GENERATIONS OCCUTS. 



Alternation of asexual and sexual methods of reproduction, 

 attended by more or less difference in structure of the individuals 

 of the generations, is fairly widespread among the Invertebrate 

 groups, particularly in forms which have adopted a parasitic mode 

 of life as, for example, the Liver Fluke. Frequently the life his- 

 tories are exceedingly complicated: several asexual, sexual, and 

 parthenogenetic generations succeeding one another in response 

 to the special conditions imposed by adaptation to a life within 

 another animal or series of animals. (Fig. 251.) 



It is clear from such life histories that the conception of special 

 germ cells early set aside, as it were, from the somatic cells must 

 not be taken too literally. The same point is emphasized by the 

 power exhibited by plants and animals in restoring parts lost by 

 mutilations of one kind or another. Among many plants, pieces 

 of the root, stem, or, in special cases, of the leaf may give rise to 

 individuals complete in every respect. Until the middle of the 

 eighteenth century this was considered a property peculiar to 



