176 -PRINCIPLES OF ANIMAL BIOLOGY 



plan, while the feeding polyps, sensory polyps, and tentacles are con- 

 structed on the polyp plan. In a few species the gonophores may 

 separate from the colony, as do the medusae in typical hydroids, but 

 usually they remain attached. 



The Portuguese man-of-war Physalia (Fig. 146) differs from the 

 generalized form described above in possessing a float which sits high 

 above the water and serves as a sail. It has no swimming bells or bracts. 



Origin of Colony Formation. — Among the metazoa the formation of 

 colonies, the integral union of individuals of the same species, occurs 

 only in those groups which employ an asexual mode of reproduction such 

 as budding or fission. Animals which employ the sexual method of 

 reproduction alone do not form colonies. Colony formation, especially 

 when it involves polymorphism and division of labor, may have made 

 for greater efficiency in the performance of certain functions, but it should 

 not be considered that efficiency is a goal toward which species have 

 striven. It seems rather to have been an accident made possible by the 

 existence of an asexual method of reproduction and to have been due 

 to a failure of the mechanism by which budding or fission is normally 

 completed. 



Limits of Asexual Reproduction. — ^Asexual reproduction occurs only 

 among the lower forms of life. It never occurs among vertebrate animals, 

 and there are a number of great groups of invertebrate animals which 

 never employ it. Even in those groups in which it occurs, there are 

 many species which never use it. Nevertheless, asexual reproduction is 

 very widespread. Because its mechanism is less complicated than that 

 of sexual reproduction and because it is employed chiefly by animals of 

 simple structure, it is regarded as the primitive method of reproduction. 

 Animals must have reproduced asexually for ages before even the simplest 

 arrangement for reproductive cooperation of two cells or individuals 

 arose. 



References 



Hegner, R. W. The Germ Cell Cycle in Animals. The Macmillan Company. 



(Especially (!liaps. I and 11.) 

 LocY, W. A. liiology and Its Makers. Henry Holt & Company, Inc. (Abiogenesis, 



pp. 277-293.) 

 LoEB, J. Artifi('ial Parthenogenesis and Fertilization. Chicago University Press. 



(Chap. I, history of attempts to initiate development artificially.) 

 MiNCiiiN, E. A. Introduction to the Study of the Protozoa. E. J. Arnold & Son, 



Ltd. (Chap. VII, fission; Chap. VIII, conjugation and sex.) 

 Thomson, J. Akthur. The Study of Animal Life. John Murray. (Chap. XIV.) 

 Wilson, E. B. The Cell in Development and Heredity. 3d Ed. The Macmillan 



Company. (Chap. III.) 



