134 



COLLEGE ZOOLOGY 



secrete a calcareous skeleton, known as coral. Two types are 

 described in the following pages: (i) the sea-anemone, and 

 (2) the coral polyp. 



a. A Sea-Anemone — Metridium 



Metridium marginatum (Fig. 83) is a sea-anemone which 

 fastens itself to the piles of wharves and to solid objects in tide- 

 pools along the North Atlantic coast. It is a cylindrical ani- 

 mal with a crown of hollow tentacles arranged in a number of 



circlets about the slit-like 



mouth. The tentacles as 



well as the body can be ex- 



JjjgzkSsiS panded and contracted, and 



|jf^|SS|^ the animal's position may be 



changed by a sort of creeping 

 movement of its basal disc. 

 The skin is soft but tough 

 and contains no skeletal struc- 

 tures. The tentacles capture 

 small organisms by means of 

 nematocysts, and carry the 

 food thus obtained into the 

 mouth. The beating of the 

 cilia which cover the tentacles 

 and part of the mouth and gullet is necessary to force the food 

 into the gastrovascular cavity. At each end of the gullet, or 

 slomodcBum (Fig. 84, 4), is a ciliated groove called the sipho- 

 noglyphe (Fig. 84, j). Usually only one or two siphonoglyphes 

 are present, but sometimes three occur in a single specimen. 

 A continual stream of water is carried into the body cavity 

 through these siphonoglyphes, thus maintaining a constant 

 supply of oxygenated water. 



If a sea-anemone is dissected as shown in Figure 84, the 

 central or gastrovascular (ccelenteric) cavity will be found to 

 consist of six radial chambers; these lie between the gullet or 



FlG. 83. — A sea-anemone. (From 

 Weysse, after Emerton.) 



