STRUCTURE OF A SEA ANEMONE. 149 



Fourth Type (/Ccelentera. A Sea Anemone, such as 

 Tealia crassicornis. 



Class, SCYPHOZOA. Sub-Class, Anthozoa or Actinozoa. 



Most sea anemones live fixed to the rocks about low- 

 water mark. Some, e.g., Tealia crassicornis, are often half 

 buried in sand ; a few are unattached. The sedentary forms 

 are able to shift their positions by short stages. They feed 

 on small animals, — molluscs, crustaceans, worms, which are 

 caught and stung by the tentacles, but many must depend 

 largely on minute organisms, while others may be seen trying 

 to engulf molluscs decidedly too large for them. A few 

 anemones, without pigment or with little, have symbiotic 

 Algse in their endoderm cells ; the bright pigments of many 

 others seem to help in respiration. Besides the normal 

 se.xual reproduction (in which the young are sometimes 

 developed within the parent), some sea anemones exhibit a 

 power of asexual multiplication by detaching portions from 

 near the base, and fission occurs in a few forms. 



External Appearance. 



The cylindrical body is fixed by a broad base ; it bears 

 circles of hollow tentacles around the oral disc ; the mouth 

 is usually a longitudinal slit. The tentacles are contracted 

 when the animal is irritated, and the whole body can be 

 much reduced in size. Just below the margin of the oral 

 disc there is a powerful sphincter muscle, this contracts, and 

 pulls together the body wall over the mouth and retracted 

 tentacles. Water may pass out gently or otherwise by a pore 

 at the tip of each tentacle, and long white threads, richly 

 covered with stinging cells, are often ejected through the 

 walls of the body. In certain states, especially if dying, the 

 sea anemone protrudes its gullet, and turns itself partially 

 inside out. 



General Structure of the Body. 



The Anthozoon polype differs markedly from the Hydroid 

 — not only because an invagination from the oral disc in- 

 wards has formed a gullet tube, which hangs down freely 



