The Development of a Primitive Animal 41 



polyp and medusa ) . One can ask why it is that in an Obelia colony a bud 

 on the stem of a feeding polyp gives rise to another polyp, while a bud 

 on the cylinder of a reproductive polyp gives rise to a medusa. Are the 

 constituent cells different, or is it merely that they inhabit different areas 

 of the colony and so are subject to different environmental conditions? 

 Actually the same sort of question can be asked about our own bodies; 

 namely, why is it that the cells in one part of a human embryo develop 

 into brain cells and in another part develop into liver cells? These ques- 

 tions will be taken up in detail in the chapter on cellular differentiation. 

 The second level of polymorphism is exemplified in Obelia by the 

 specialization of the polyps. Another genus, Hydractinia, displays an even 

 greater range of differences in polyp structure, as Fig. 21 shows. Hy- 

 dractinia colonies live on the surface of a specific type of snail shell but 

 only when inhabited by hermit crabs. Three types of polyp develop: 



1. Feeding polyps. These correspond in essential features to the 

 feeding polyps of Obelia. 



2. Defensive polyps. These are concentrated at the lip of the snail 

 shell. They lack a mouth and their tentacles are reduced to small knobs 

 arranged in two circular rows. The stems have considerable musculature 

 and can whip about fiercely, thereby permitting the cnidoblasts on the 

 tentacle knobs to discharge a concentrated volley upon contact with the 

 foe. 



3. Reproductive polyps. In Hydractinia, the medusa stage is absent. 

 Instead, the reproductive polyps produce sex cells directly. There are two 



Fig. 21. Polymorphism in Hydracfinia. 



Feeding 

 polyp 



Female reproclucti> 

 polyp bearing eggs 



Spiral polyp 



