STRUCTURE OF ALCYONIAN POLYPES. 



517 



FIG. 620. ALCYONIAN POLYPE. 



remain the same. On comparing the external appearance of one 

 of them with that of a Hydra or Sea-Anemone, we perceive that, 

 whilst the mouth is surrounded by tentacula as in them, there is 

 a marked difference in the form and number of these prolonga- 

 tions. Instead of long delicate filaments like those of the Hydra- 

 form Polypes, or short cylindrical tubes like those of most of 

 the Actinice, we find six or eight 

 broad leaf-like expansions, dis- 

 posed with great regularity 

 around the mouth, so as very 

 much to resemble a Star-fish 

 (Asterias). Hence the term 

 Asteroida has been proposed as a 

 designation for the group ; and 

 it represents sufficiently well its 

 chief external character. These 

 tentacula are unprovided with 

 cilia ; but a number of little pro- 

 jections may be seen along their 



margins, which probably increase their prehensile power. 

 The mouth leads into a stomach, which resembles that of the 

 Actinia^ in being suspended in the midst of the general cavity of 

 the body, by partitions radiating from its walls. Instead of 

 being closed at its lower extremity, however, it opens like that of 

 the Sertularians, into the canals which ramify through the fleshy 

 mass, and which thus connect all the Polypes into one system. 

 This opening is surrounded by a circular muscle, or sphincter, by 

 the action of which it may be expanded or entirely closed. The 

 chambers which surround the stomach correspond in number 

 with the tentacula ; and these are hollow, opening below into the 

 chambers (as in the Actinia), whilst they have a small orifice 

 at their extremity. The chambers are continuous at their lower 

 part with the ramifying canals just mentioned ; and the mem- 

 branous folds which support the stomach do not cease at its 

 lower extremity, but are prolonged downwards as plaits of the 

 lining of these canals, until they gradually disappear. 



