16 COMPARATIVE ANATOMY. 



Polypi. Composed of a sac divided into chambers by 

 partitions arranged to the periphery (integument), as radii 

 to a central point the stomach. Such partitions may be 

 more, though never less than six in number. "The upper 

 margin of body is fringed by hollow tentacles, each of which 

 opens into one of the chambers." They may be naked and 

 present indefinite numbers of partitions and tentacles, as in 

 A ctinia (sea-anemone); or possess a definite yet large number 

 of tentacles often accompanied with depositions of calcareous 

 matter within walls between partitions, as in Madrepora 

 (coral); or the tentacles may be restricted to six, with a tend- 

 ency to formation of calcareous skeleton, as in Alcyonium. 



Acalephse. This is a numerous and diversified group, com- 

 posed for the most part of individuals, which, when mature, 

 have a dome or umbrella-like form. The organs and systems 

 of vessels are arranged in a stellate manner to a central point. 

 Divided into Ctenophorse, Discophorse, and Hydroida. 



Acalephs move by means of minute flappers arranged in 

 vertical comb-like rows on outer side of the dome-like struc- 

 ture, as in Ctenophorse; or through the movements of the 

 edges or general contractile power of the Discophorse and 

 Hydroida. 



In the last two groups, remarkable features are seen in the 

 development of the young, the larval condition, as a rule, 

 remaining fixed and dissimilar in form to the adult. (See 

 p. 124.) 



Echinodermata. This is the highest group of radiates, 

 composed of stellate globular or oblong forms, rarely fixed, 

 and possessing, with the exception of one group, a system 

 of foot-like processes (ambulacra), which may be inflated 

 with water through ambulacral vessels, communicating by 

 means of a peculiar canal (madreporic body)* with the ex- 

 terior. The spaces between the ambulacral pores are the 

 ambulacral spaces. 



The majority of Echinodermata are more or less enclosed 



* So called from the resemblance its sieve-like opening bears to a polyp 

 chamber of Madrepora. 



