66 



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, n-hile fi'om the outer layer, as 

 probably in the vertebrates, an intermediate or nervo-mus- 

 cular layer is formed, which AUman thinks is thehomologue 

 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 sejmrate, and we 

 for the first time in the animal kingdom meet with two 

 sorts of individuals — i.e., males and females. 



« JH;F:~^°'^2?' °L^l^'?^"^, oMnaia on a sheU tenanted by a hermit crab, 

 natnral size.— From Brefim's Thierleben. uciuiu, ..lao. 



The simplest form next to Hydra is Hydractinia, in 

 which the individual is differentiated into three sets of 

 zooids—i.e., a, hydi-a-like, sterile or nutritive zooids ; J and 

 c, the reproductive zooids, one male and the other female, 

 both being much alike external! v. having below the short 

 rudimentary tentacles several spherical sacs, which pro- 

 <3uce either male or female medusae. These medusa-buds 

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

 rijne. The marine Hydroids, then, are usuallv sexuallv dis- 

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



