124 C@LENTERATA. 
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 Actimza 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 swzfaces. The muscles on the other 
four pairs are opposite each other on the inner suzfaces of 
each pair. The cavities within the pairs of mesenteries are 
termed zztra-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, guaternaries, 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- 
cysts, sensory cells and unicellular glands, 
Scattered nerve-cells have also been described. 
The mesoglcea is a thicker layer than in Hydra and passes 
along the mesenteries. The endoderm contains, as in 
ffydra, flagellate and ameeboid cells and also glandular 
and possibly sensory célls. 
Actinia reproduces both sexually and asexually. Buds 
are periodically produced and shed. Our type is somewhat 
exceptional in being viviparous, ze. the 
young are developed in the radial cavities 
and leave the parent by the mouth. Most of the group 
have a free larval development with a p/anw/a 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 plano-symmetry, namely bi-plano-symmetry. We 
can distinguish two ends with siphonoglyphs and two sides, 
but we cannot distinguish Jetween the two ends. Some 
allies have only one siphonoglyph and are plano-symmetric. 
Histology. 
Reproduction. 
