PRINCIPLES OF ANIMAL BIOLOGY 31 



(see demonstration). The medusae detach themselves later from the 

 blastostyle, emerge from the gonangium through an opening at the tip 

 which in younger gonangia is plugged up by the broad end of the blasto- 

 style, and live a free swimming existence. Examine one of the free medusae 

 in a demonstration. Note the manubrium in the center. It is homologous 

 with the hypostome and contains the mouth. Four radial canals extend out 

 from the manubrium as far as the circular canal along the margin. The 

 reproductive organs are usually borne on the radial canals. The convex 

 side of the medusas is called the ex-umbrella and the concave side the sub- 

 umbrella. The medusae reproduce by eggs and spermatozoa. The 

 structure of a medusa may be better appreciated from an examination 

 of larger medusae belonging to other species such as Gpnionemus or 

 Poly orch''s. 



The tree-like branch you have examined is not an entire colony. In 

 a demonstration, note that numerous such branches may be connected 

 by a horizontal creeping portion, the hydrorhiza, from which the branches 

 arise. 



Obelia illustrates a simple form of polymorphism, in that it comprises 

 three kinds of individuals in differing form: (1) The hydranth or nutri- 

 tive individual; (2) The blastostyle which produces (3) Medusae, the 

 dispersing members of the species. 



Draw a branch showing a hydranth and a gonangium in detail. Draw 

 also a medusa. 



9. Physalia, the "Portuguese Man-of-war," will be on demonstration. 

 It is a very complex polymorphic colony. The various individuals cannot 

 be made out without more careful examination than the demonstration 

 will permit. 



C. SUMMARY 



What is the simplest form of aggregation into which cells may enter? 

 What animals show this simple collectivism? Are these aggregations 

 colonies, err individuals? 



Arrange the animals you have studied in order of ascending complexity. 

 What is the first increase in complexity beyond the simplest condition 

 mentioned above? Point out what are the further increases in complexity, 

 step by step, through the rest of the series. 



Is there division of labor in any of these types of aggregation? If so, 

 which ones? Is division of labor lacking in any of them? Is there any 

 cooperation, not involving division of labor, in any of them? If so, 

 where? 



Is there any parallelism between aggregations of cells and aggrega- 

 tions of individuals? If so, in what respects? 



Do you see any value in the construction of a scale of complexity 

 such as the foregoing? If so, indicate its use. 



