TYPES OF C@&LENTERA—AVDRA. 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 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. 
Tie 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 :— 
OutTER LAYER. 
MrippLe Layer. 
INNER Laver. 
In Hydra the ectoderm 
forms— , 
Covering cells, stinging 
cells, nerve cells, muscle 
cells, etc. 
None in Hydra, apart 
from the middle lamella. 
In Hydra the endoderm 
forms— 
Digestive cells lining 
the food canal, and also 
muscle cells, etc. 
The embryonic epiblast 
of higher animals grows 
into epidermis, nervous 
system, and essential parts 
The mesoblast of higher 
animals becomes muscu- 
lar, connective, and skele- 
tal tissue. 
The embryonic hypo- 
blast of higher animals 
always lines the digestive 
part of the food canal. 
of sense organs. 
The reproductive organs.—(a) From nests of repeatedly dividing 
interstitial cells, several (I-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 Aydra, 
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, 1). 
(4) Usually there is but one female organ or ovary, but in A. 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 A. viridis, 
H. fusca, and H. grisea there are two ova; in . diecéa there may be 
several. 
Development.—The ovum of Hydra is the successful central cell 
in the ovary. It is at first amceboid, and becomes more and more 
rich at the expense of its neighbours. Their remains (perhaps nuclei) 
