COZLENTERATA 63 
processes, and the manubrium is short. The coelenteron is 
ciliated. In Rhizostoma the four oral processes are divided into 
eight, and have fused together in such a way as to occlude the 
mouth. This is compensated for by the presence of fine canals 
which open at one end into the gastric cavity, and at the other 
on the frilled edges of these processes. The food is absorbed 
through these canals. 
The gelatinous layer of the mesogloea in Awelia is 
traversed in all directions by fibrous cells and by wandering 
amoeboid cells. The muscles are in some cases formed of 
transversely-striated processes of epithelial cells; in the oral 
processes of some medusae, however, distinct nucleated muscle 
cells occur. 
The sense organs of Aurelia are modified tentacles, which 
bear endodermal otocysts and ectodermal pigment spots or 
eyes. An aboral and an oral pit, both lined by specialised 
epithelium on the surface of the disk, are regarded as olfactory. 
The coelenteron is prolonged into the modified tentacles. 
No regular nerve ring exists in Scyphomedusae, with the 
exception of one genus, Charybdaea ; but nerve fibres occur in the 
region of the sense organs, and are connected with scattered 
ganglion cells. 
The ova and spermatozoa arise from endodermal cells, and 
escape through the mouth. Four large pits may be excavated 
in the sub-umbrella ectoderm, and these form sub-genital 
pouches, the skin of which is thin, and forms only a slight mem- 
brane between the sea-water and the genital cells (Fig. 39, IIL.). 
This arrangement may promote the respiration of these parts. 
In some Scyphomedusae, eg. Pelagia, the medusa rises 
directly from the ovum without the intervention of a Scyphi- 
stoma. 
Cuiass C. ACTINOZOA. 
CHARACTERISTICS.—Single or colonial Coelenterata. The mouth 
leads into an ectodermic invagination, the stomodaeum or oeso- 
phagus ; this is attached to the walls of the body by a series of 
radial mesenteries, so that the coelenteron is divided into a 
central portion and a series of radiating intermesenteric 
chambers. The generative cells are endodermic, and le im 
