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. 37. Colony of Hydraetwia echinata on a shell tenanted by a hermit crab, 

 natural size. From Brehm's Thierleben. 



The simplest form next to Hydra is Hydractinia, in 

 which the individual is differentiated into three sets of 

 zooids i.e., a, hydra-like, sterile or nutritive zooids ; I and 

 c, 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 medusae. These medusa-buds 

 (gonophores) are in structure like the free medusa? of Co- 

 ry ne. The marine Hydroids, then, are usually sexually dis- 

 tinct, growing by colonies, which are either male or female. 



