156 



PII YL UM C CELENTERA . 



mf. 



ml. 



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

 sive threads or acontia. The rnesenteric 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 un- 

 centralised, and consists of superficial sensory cells con- 

 nected 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 

 mesoglcea is a supporting plate 

 between these two layers, and 

 forms a basis for their cells. 



The ectoderm consists of 

 ciliated, sensory, stinging, and 

 glandular cells, and also of sub- 

 epithelial muscle and ganglion 

 cells based on the mesogloea, but 

 mainly restricted to the circum- 

 oral region. 



The endoderm consists mainly 

 of flagellate cells, with muscle 

 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 



FIG. 73. --Section through sea- 

 anemone (across arrow in Figure 

 72). After Andres. 



A ,_ />', directive septa ; nt.f., mesenteric 

 filaments; g-., genital organs; m.l., 

 longitudinal muscles ; s., primary 

 septum ; s'., secondary septum ; s"., ter- 

 tiary septum. The arrow enters between 

 two primary septa (an intra-septal 

 cavity), and passes out between two 

 tertiary septa. 



