162 



PHYLUM CCELENTERA. 



ridget, r reproductive cells, almost always eithej* ova or 

 spermatozoa, rarely both ; and (d) in some cases offensive 

 threads or acontia. The mesenteric filaments seem to be 

 closely applied to the food, and perhaps secrete digestive 

 juice. Intracellular digestion also occurs. Sea-anemones 

 have no sense organs; the sapphire beads, which are so 

 well seen at the bases of the outermost tentacles of the 

 common Actinia mesembryanthemum^ are batteries of 

 stinging cells. The nervous system is uncentralised, and 



consists of superficial sen- 

 sory cells connected with a 

 plexus of sub - epithelial 

 ganglion cells. 



The layers of the body. 



The ectoderm which clothes the 

 exterior is continued down the 

 inside of the gullet. The endo- 

 derm lines the whole of the 

 internal cavity, including mes- 

 enteries and tentacles. The 

 mesogloea is a supporting plate 

 between these two layers, and 

 forms a basis for their cells. 

 FIG. 80. Section through sea- Tne ectoderm consists of 

 anemone (across arrow in Figure ciliated, sensory, stinging, and 

 79). After Andres. glandular cells, and also of sub- 



A,B, Directive septa; ,./., mesenteric epithelial muscle ' and ganglion 

 filaments; ^., genital organs; m.l. t cells based on the mesoglcea, but 

 longitudinal muscles; s., primary sep- mainly restricted to the circum- 

 tum ; s'., secondary septum ; s"., tertiary ora l region. 



septum. Ine arrow enters between two mv j j I i 



primary septa (an intra-septal cavity) The endoderm consists mainly 



and passes out between two tertiary of flagellate cells, with muscle 

 se P ta - fibres at their roots. These form 



the chief muscle bands of the 



wall, the mesenteries, and the gullet. Nor are glandular and even 

 sensory cells wanting in the endoderm. 



The mesenteries. In sea-anemones and nearly related Anthozoa, 

 twelve primary mesenteries are first formed. These are grouped in 

 pairs, and the cavity between the members of a pair is called intra- 

 septal, in contrast to the inter-septal cavities between adjacent pairs. 

 In these inter-septal chambers other mesenteries afterwards appear in 

 pairs. Two pairs of mesenteries, however, differ from all the rest those, 

 namely, which are attached to the two corners of the mouth and to the 

 corresponding grooves of the gullet. These two pairs of mesenteries 

 are called " directive," and they divide the animal into bilaterally sym- 

 metrical halves. Anatomically, a pair of directive mesenteries differs 

 from the other paired mesenteries, because the retractor muscles, which 



