76 



ANIMAL BIOLOGY 



coiled, nematocyst-bearing threads that can be protruded in pro- 

 fusion through both the mouth and numerous pores in the body 

 wall, and so are efficient offensive and defensive weapons. Further- 

 more, the tentacles of Metridium are not only well supplied with 

 nematocysts, but also with a coat of cilia that plays a crucial part 

 in the ingestion of food. Once the prey has been paralyzed by the 



Gullet 



leading into enteron 



Tentacles 



Lip about mouth 



v End of a 



^-mesenterial 

 filament 



Primary mesentery 

 Reproductive organ 



A ' Edge of mesentery 



Fig. 41. — Sea Anemone, Metridium marginatum. A, View of polyp with 

 one quadrant removed; B, Diagram of transverse section showing mesenteries. 



discharge of the nematocysts, the tentacles bend inward and over 

 so that ciliary action sweeps the food slowly but surely toward the 

 mouth. Here other cilia, aided by muscular contractions, carry it 

 on down into the enteron. 



Metridium typically reproduces sexually, though asexual repro- 

 duction by budding may also occur. The reproductive organs are 

 within the enteric cavity where, in the case of females, the eggs 

 are fertilized, and the embryo develops into a ciliated larva before 

 making its exit to settle down and assume the adult characters. 



Metridium, then, may serve to illustrate the general type of 



