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 mar^inatum (Fig. 8.0 is a sea-anemone w ; hich 

 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 

 moutty t The tentacles as 

 well as the body ca^be ex- 

 panded and contracted, and 

 the animal's position may be 

 changed by a sort of creeping 

 movement of its lasal disc. 

 The skin is soft but tough 

 and contains no skeletal struc- 

 tures. The tentacles capture 

 small organisms by means of 

 np.mntnr.ysts f and carry the 

 food thus obtained into the 

 mouth. The beating of the 

 ell 'a which cover the tentacles 

 and part of_the mouth and wClt is necessary to force the food 

 into the eastro^scylar cavity. At each end of the gullet, or 

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

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

 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 qastrovascular (ccelen'eric) cavity will be found to 

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



FIG. 83. A sea-anemone. (From 

 Weysse, after Emerton.) 



