124 CCELENTERATA. 



of the mesenteries. They originate at the aboral end and 

 are inserted in the peristome. On contraction they shorten 

 the animal. There are also diagonal or parietal muscles 

 across the lower corners, connected with the suction of the 

 base, and thin radial muscles on the mesenteries. 



The mesenteries in a large Actinia are very numerous, 

 but in the young form there are only six pairs. Of these 

 the two pairs opposite the siphonoglyphs are called the 

 directive mesenteries and can be recognised by having the 

 muscles on their outer surfaces. The muscles on the other 

 four pairs are opposite each other on the inner surfaces of 

 each pair. The cavities within the pairs of mesenteries are 

 termed intra-septal^ those between the pairs are known as 

 inter-septal. 



All six pairs join the gullet. The subsequent mesenteries 

 grow from the outer wall in pairs towards the centre. They 

 always have opposite muscles, never join the gullet, and 

 arise only in the inter-septal cavities. They are known as 

 secondaries^ tertiaries, quaternaries, and so on, and continue 

 to grow and increase in number throughout life. 



The cellular structure of the anemone is somewhat in 

 advance of that of Hydra, The ectoderm contains nemato- 

 Hi t 1 cysts, sensory cells and unicellular glands. 



Scattered nerve-cells have also been described. 

 The mesogloea is a thicker layer than in Hydra and passes 

 along the mesenteries. The endoderm contains, as in 

 Hydra, flagellate and amoeboid cells and also glandular 

 and possibly sensory cells. 



Actinia reproduces both sexually and asexually. Buds 



are periodically produced and shed. Our type is somewhat 



Reproduction exceptional in being viviparous /..., the 



young are developed m the radial cavities 



and leave the parent by the mouth. Most of the group 



have a free larval development with a planula larva. 



The important point to notice in Actinia is the advance 

 in complexity upon Hydra. The perfect axial symmetry 

 of Hydra is replaced by a symmetry intermediate between 

 this and piano-symmetry, namely bi-plano-symmetry. We 

 can distinguish two ends with siphonoglyphs and two sides, 

 but we cannot distinguish between the two ends. Some 

 allies have only one siphonoglyph and are piano-symmetric. 



