178 



PHYLUM COELENTERA 



There is no fundamental physiological difference between the two 

 processes ; a food-vacuole is as certainly a " dead space " as a cavity 

 lined by cells is. In some flagellate Protozoa digestion takes place 

 in a single vacuole of very large size ; these may be regarded as 

 physiologically transitional types. 



Some of the endoderm cells have muscular roots like those of the 

 ectoderm. They lie on the inner side of the middle lamina, in a trans- 

 verse or circular direction. A few cells near the mouth and base are 

 described as glandular, and the presence of a few stinging cells has 

 been recorded, though some suggest that the last are discharged ecto- 

 dermic nematocysts which have been swallowed. 



The middle lamina, representing the mesogloea, is a thin homo- 

 geneous plate, bearing on its outer and inner surfaces the muscular 

 roots of ectodermic and endodermic cells (Fig. 88, D). 



It is historically interesting to notice the important step which was 

 made when, in 1849, Huxley definitely compared the outer and inner 

 layers of the Coelentera with the epiblast and hypoblast which embry- 

 ologists were beginning to demonstrate in the development of higher 

 animals. Not long afterwards, Allman applied to the two layers of 

 hydroids the terms ectoderm and endoderm ; and these are now used 

 embryologically. 



The division of labour among the cells of Hydra is not very strict, 

 but already the essential characteristics of ectoderm and endoderm are 

 evident. We use ectoderm and epiblast, endoderm and hypoblast, 

 as synonymous. 



The reproductive organs. — [a) From nests of repeatedly dividing 

 interstitial cells, several (1-20) simple male organs or testes are formed. 

 Each consists merely of a clump of male elements or spermatozoa, 

 bounded by the distended ectoderm. Through this the spermatozoa 

 are extruded at intervals, and one may fertilise the ovum of the Hydra. 

 In other words, self-fertilisation, which is very rare among animals, 

 may occur. The spermatozoon is a motile cell, with a minute cylin- 

 drical " head " consisting of nucleus, a more minute middle-piece, and 

 a long thread-like vibratile tail (Fig. 89, i). 



(ft) Usually there is but one female organ or ovary, but in H. fusca 

 as many as eight have sometimes been observed. The ovary arises, like 



