102 



PRIXCIf'LES OF AXIMAL BIOLOGY 



have an individual sheath. Whereas in the colonial ccclenterates there 

 is a common digestive cav^t}', in the tunicates each individual has its 

 own mouth opening and digestive cavity which discharges through a 

 common cloacal chamber. Such a condition is well shown in Botrj'llus. 

 In Pyrosoma (Fig. 70) the oral apertures open on the outer surface of 

 the cylinder in which the individuals are embedded and the anal (cloacal) 

 opening on the inner surface. 



Among the metazoa the formation of colonies fas distinct 

 from societies such as exist among ants, wasps, and bees) oc- 

 curs 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 or aggregates. Unlike the Protozoa, among 

 which colonies are sometimes formed by rearrangement of 

 cells following fission, among the metazoa colonies are never 

 formed by a rearrangement of the individuals. 



Fig. 6^. Fig. 69. 



Fig. 68. — Tapeworm, Hymenolepis murina, from the intestine of the rat. At the 

 upp)er end of the figure is the scolex or so-called head with suckers and hooks. The seg- 

 ments of the body are equivalent to individuals. (From Hesse and Doflcin.) 



Fig. 69. — Boiryllus gouldii, portion of a colony attached to a piece of seaweed. 

 Indi\-iduals are here arranged in the form of five rosettes, each rosette about a common 

 cloacal aperture. 



Origin of Colony Formation. — The colonies described above are 

 organic colonies since each is an intimate union of individuals of the same 

 species. Such colonies may consist of like individuals or there may be 

 differentiation, pohinorphism, and di\'i.sion of labor. Differentiation of 

 individuals and di\'ision of labor exhibit their beginnings in the Protozoa 

 but find their greatest development in the cells of the metazoan indi\'idual 

 and the indi^'iduals of the metazoan colonies. Colony formation occurs 

 chiefly among the less highly organized groups of animals and it is most 



