178 ELEMENTARY ZOOLOGY. 



food, others may swallow and digest it, and still others may become 

 reproductive bodies by which new colonies are formed. (See Figs. 

 48, 49.) This is seen in its most extreme form in the Portuguese 

 Man-of-war (Fig. 49). This degree of differentiation of individuals 

 could not come about but for the fact that all may get food from 

 the common digestive tract. 



The life of the polyps goes on much as in Hydra, the growth and 

 budding continuing in a purely non-sexual way. But, unlike Hydra, 



ab. o. 



o 

 b 1 



FIG. 50. Diagrams of jelly-fish. A, viewed from the oral surface; B, a section 

 through the center of the animal and along the line a-a 1 in A. The shaded portion is 

 the gastro-vascular cavity, lined with entoderm. The outer boundary is ectoderm 

 and the unshaded part is mesoderm. o-ab.o., oral aboral axis; a-a 1 , b-b l , radial sub- 

 ordinate axes. 



Questions on the Figure. Work out the details of the symmetry 

 here. What does it mean to say that the animal is radially sym- 

 metrical? By comparing the two figures and other figures in the 

 reference texts, trace the course and division of the gastro-vascular 

 cavity. Identify the mouth, the gullet, the radial canals, the 

 circular canal, tentacles, the sexual bodies. 



many of these polyps have no power of producing eggs and sperm. 

 Under certain conditions, however, they produce buds very different 

 in appearance and internal structure from the usual polyps. They 

 are shorter and more fleshy (Fig. 48, 6). These are known as 

 medusa or bells. 



193. Medusa Stage. These new buds do not bud when mature, 

 as their parents did, but break loose and become free-swimming 

 animals. They have a sort of bell or umbrella form, with the mouth 



