ACTINOZOA. 



85 



the body-cavity, where they serve to maintain a circulation of 

 the contained fluids. The only digestive apparatus consists 

 of a tubular or sac-like stomach, which opens inferiorly 

 directly into the body-cavity (Fig. 13, a), and communicates 



Fig. 25. — A. Transverse section of an Actinosoon. a Digestive sac; & Outer-wall of the 

 body or ectoderm ; b' Endoderm ; m Mesenteries, connecting the stomach with the 

 body- walla, and dividing the space between the two into a number of vertical chambers. 

 B. Transverse section of the body of a Hydrozoon, showing the single tube formed by 

 the walls of the body. 



with the outer world through the mouth. A nervous systeta 

 has not been shown to exist in any of the Actinozoa except 

 the Ctenophora, and in none are there any traces of a circula- 

 tory system. Distinct reproductive organs are always present, 

 and true sexual reproduction occurs in all the members of the 

 class. In a great many forms, however, of the Actinozoa we 

 have composite organisms or colonies, produced by a process 

 of " continuous " gemmation or fission, the zo<3ids thus origi- 

 nated remaining attached to one another. In these cases — 

 as in most of the corals — the separate beings or zoOids thus 

 produced are termed " polypes," the term " polypite " being 

 restricted to the Sydrozoa. In the simple Actinozoa, how- 

 ever, such as the sea-anemones, the term " polype " is applied 

 to the entire organism, as consisting of no more than a single 

 alimentary region. It follows from this, that the entire body 

 of any Actinozoon may be composed of a single polype, or 

 of several such produced by budding or cleavage, and united 

 to one another by a common connecting structure or coenosarc. 

 Most of the Actinozoa are permanently fixed, like the corals ; 

 some, like the sea-anemones, possess a limited amount of 

 locomotive power ; and one order, the Ctenophora, is com- 

 posed of highly-active free-swimming organisms. Some of 

 them are unprovided with hard structures or supports of any 

 kind, as the sea-anemones and Ctenophora ; but a great many 



