COELENTERA TA 63 



processes, and the manubrmm is short. The coelenteron is 

 ciliated. In BMzostoma 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 Aurelia 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 

 epithehum 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, III.). 

 This arrangement may promote the respiration of these parts. 



In some Scyphomedusae, e.g. Pelagia, the medusa rises 

 directly from the ovum without the intervention of a Scyphi- 

 stoma. 



Class C. ACTINOZOA. 



Chaeacteeistics. — Single or colonial Coehnterata. The mouth 

 leads into an eetodermic invagination, the stomodaeum or oeso- 

 phagus ; this is attached to the walls of the body hy 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 lie in 



