CELL AGGREGATION AND DIVISION OF LABOR 



103 



frequent among attached forms. The formation of colonies may be of 

 advantage to the species in that it permits members of the species to 

 occupy as sohd masses positions which are favorable to their life proc- 

 esses. This is true of the corals which by reason of their method of 

 building solid skeletal structures as an activity of the whole colony are 

 enabled to thrive in situations where single polyps could not do well. 

 Colony formation, especially when it involves polymorphism and division 



jca 



tent 



cloaca 



atr.ap 



Fig. 70. — The colonial ascidian Pyrosoma. A, external view of the entire colony, 

 diagrammatic, ca, colonial aperture; m, mouth of individual; tp, test process. (From 

 Mctcalf and Hopkins, after Rilter.) B, lonsitudinal section of a colony; atr.ap, atrial 

 aperture; or. ap, oral aperture of individual; ph, pharynx; proc, process of test; stol, stolon 

 from which buds arise; tent, tentacles. (After Hcrdmann in Challenger Reports.) 



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. Colony formation seems rather to 

 have been more or less of an accident made possible because of the pres- 

 ence of an asexual reproductive method. It seems not improbable that 

 its origin was due in some groups to a failure of the mechanism by which 

 budding or fission is normally completed. 



