126 



MANUAL OF ZOOLOGY. 



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. 40, m). Upon the faces of these are 



Fig. 40. A, Actinia mesembryanthemum, one of the Sea-anemones (after Johnston) ; 

 B, Section of the same, showing the mouth (a), the stomach ($), and the body- 

 cavity (c). 



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

 or spermaria. There are no differentiated respiratory organs 

 as a rule. Some forms, however, which live half-buried in 

 sand or mud, have lobed and crimped organs attached to or 

 near the tentacles, which have been supposed to act as 

 breathing-organs ; whilst structures supposed to be gills are 

 developed in some Zoanthids on either side of the primary 

 mesenteries. 



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. Some Actinice, are said 

 to have short optic nerves distributed to the pigment-masses at 

 the bases of the tentacles; and these masses possess crystalline 

 lenses. 



Distinct reproductive organs occur in all the Actinozoa^ but 



