IIO MANUAL OF ZOOLOGY. 



interior of the somatic cavity, where they serve to promote a 

 circulation of the digestive fluids contained therein. The sole 

 digestive apparatus in the Actinozoa consists of a tubular 

 stomach-sac, which communicates freely with the outer world 

 by means of the mouth, and opens inferiorly directly into the 

 general body-cavity. In most, the " perivisceral space" be- 

 tween the body-walls and the digestive sac is subdivided into 

 compartments by a series of vertical lamellae, which are 

 called the " mesenteries " (fig. 28, b). Upon the faces of these 



m of Actinozoa. Diagrammatic vertical section of Actinia. 

 -sac ; b Mesentery ; c Craspedum ; d Tentacle. 



are borne the reproductive organs in the form of band-like 

 ovaria or spermaria. 



Thread-cells, often of very complicated structure, are almost 

 universally present, some of the Ctenophora having been asserted 

 to be without them ; and some of the Actinozoa are able to 

 sting very severely. 



A nervous system has not yet been proved to exist in any of 

 the Actinozoa, except in the Ctenophora, and in none are there 

 any traces of a vascular system. 



Distinct reproductive organs . occur in all the Actinozoa, but 

 these are internal, and are never in the form of external pro- 

 cesses as in the Hydrozoa. Sexual reproduction occurs in all 

 the members of the class, but in many forms gemmation or 

 fission constitutes an equally common mode of increase. 

 Some Actinozoa, therefore, such as the common sea-anemones, 

 are simple organisms, whilst others, such as the reef-building 

 corals, are composite, the act of gemmation or fission giving 

 rise to colonies composed of numerous zooids united by a 

 ccenosarc. In these cases the separate zooids are termed 

 "polypes," the term "polypite" being restricted to the Hy- 

 drozoa. In the simple Actinozoa, however, the term "polype" 



