194 



PHYLUM CCELENTERA 



Stinging cells. The nervous system Is iincentralised, and 

 consists of superficial sensory 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 endoderm lines the 

 whole of the internal cavity, including mesenteries and tentacles. The 

 mesogloea 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 circumoral region. 



The endoderm consists mainly of flagellate cells, with muscle fibres 

 at their roots. These form the chief muscle bands of the wall, the 



s s 



Z A 



Fig. ioi. — Z, Diagrammatic section of Zoantharian ; A, of 



Alcyonarian. — After Chun. 



The line S-S in Z is through the siphonoglyphs (a), the line 

 T-T passes through two inter-septal spaces. The retractor 

 muscles are represented by dark thickenings on the mesen- 

 teries — all on one (the ventral) side in the Alcyonarian. The 

 line S-S in A represents the axis of symmetry. 



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

 extend in a vertical ridge along them, are turned away from one another, 

 and run on the inter-septal surfaces, whereas in the other mesenteries 



