56 ZOOLOGY. 
It will be seen, to anticipate somewhat, that the Hydra, 
exactly as in the vertebrates, including man, arises from an 
egg developed from a true ovary, which, after fertilization, 
passes through a morula stage; that the germ consists at 
first of two germinal layers, while from the outer layer, as 
probably in the vertebrates, an intermediate or nervo-mus- 
cular layer is formed, which Allman thinks is the homologue 
of the middle germ-lamella of the vertebrates (mesoderm) 
supposed to have originally split off from the ectoderm. 
In all the other Hydroids the sexes are separate, and we 
for the first time in the animal kingdom meet with two 
sorts of individuals—i.e., males and females. 
Fig. 87.—Colony of Hydractinia echinata on a shell t ted 1 i 
natural size.—From Brehm’s Thierleben. Se enenas AY 36 ETE CTE 
The simplest form next to Hydra is Hydractinia, in 
which the individual is differentiated into three sets of 
z00ids—i.e., a, hydra-like, sterile or nutritive zooids ; dand 
ce, the reproductive zooids, one male and the other female, 
both being much alike externally, having below the short 
rudimentary tentacles several spherical sacs, which pro- 
duce either male or female meduse. These medusa-buds 
(gonophores) are in structure like the free medusx of Co- 
ryne. The marine Hydroids, then, are usually sexually dis- 
tinct, growing by colonies, which are either male or female. 
