TYPES OF CCELENTERA HYDRA. 



147 



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 homogene- 

 ous plate, bearing on its outer and inner surfaces the muscular roots of 

 ectodermic and endodermic cells (Fig. 69, 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 Ccelentera 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. 



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 may summarise these as follows, comparing them with 

 the characteristics of epiblast and hypoblast in higher animals : 



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. 70, l). 



(b) 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 

 the testes, from a nest of interstitial cells, in the centre of which, distinct 

 from the start, the single ovum lies. In rare cases in H. viridis, 

 H. fusca, and H. grisea there are two ova ; in H. dioscia there may be 

 several. 



Development. The ovum of Hydra is the successful central cell 

 in the ovary. It is at first amoeboid, and becomes more and more 

 rich at the expense of its neighbours. Their remains (perhaps nuclei) 



