152 MANUAL OF ZOOLOGY. 



especially in the 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 



Fig. 68 A, Transverse section of a HydrozoSn, showing the body-cavity in the form 

 of a single tube enclosed by the body-walls. B, Transverse section of an Actinozoon : 

 j Digestive sac ; m One of the primary mesenteries, dividing the body-cavity into 

 vertical compartments. Between the six primary mesenteries are seen the secondary 

 and tertiary mesenteries, which fall short of the walls of the stomach, a Ectoderm ; 

 b Endoderm. 



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 " peri- 

 visceral space " between the body-walls and the digestive sac 

 is subdivided into compartments by a series of vertical lamellae, 

 which are called the " mesenteries " (fig. 68, m). Upon the 

 faces of these are 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 few forms having been asserted to be 

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

 severely. 



A nervous system has not yet been proved to exist in the 

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

 there any traces of a vascular system. Some Actinia are said 

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

 the bases of the tentacles, and these masses are clearly of a 

 sensory nature ; whilst the same animals are affirmed to have 

 a general nervous system as well. 



