486 EVOLUTION OF ANIMALS Part V 



dozens in the tide pools along our north Atlantic coast (Fig. 23.1). Metridium 

 has a cylindrical body topped with a crown of hollow tentacles arranged in 

 circlets around its slit-shaped mouth. When full grown and expanded it is 

 about 4 inches high and its oral disk may be three inches wide. Its skin (epi- 

 dermis) is soft and slimy but tough even to sharp scissors. 



Partial septa extend vertically from the body wall; some are attached to the 

 gullet, others extend only part of the way toward it (Fig. 23.16). Their free 

 edges are thickened by digestive filaments containing the nutritive cells. Some 

 of these secrete digestive fluids into the gastrovascular cavity; others engulf 

 particles of food and digest them within food vacuoles in the cells. The struc- 

 ture of hydra, jellyfish, sea anemone, and coral is fundamentally similar 

 (Fig, 23.11). Stinging cells are active and abundant on the tentacles and on 

 the stinging threads (acontia) borne near the bases of the digestive filaments. 

 These threads may be shot through pores in the body wall or out of the mouth 

 and extended three or four inches into the water, paralyzing animals which the 

 tentacles cannot reach. 



Ovaries and testes are in separate individuals and the young develop from 

 fertilized eggs. Anemones also reproduce asexually by longitudinal division 

 and by pedal laceration, the pinching off of fragments of the basal disk. In an 

 aquarium, the base of the anemone may be spread against the glass side and 



Gullet 



Ostta, holes in the partitions through which 

 water passes from one chamber to another. 



Hollow tentacle 



Circular muscle 

 Partition 



Longitudinal muscle on the 

 partition 



Reproductive organ on edges of 

 partitions. 



Stinging filaments are shot out 

 through body wall. 



Here the filaments overlay the 

 partition like threads laid on gloss 



Cut into body Creeping basal disk 



Gostro-vascuiar covity 



Fig. 23.16. Sea anemone with a side of the body cut away to show the partial 



partitions in the coelenteron. 



