CCELENTERATA : CORYNIDA. 



119 



the two primitive layers of the germ of the Vertebrata. The "gastrula " 

 stage appears to be one very generally p_assed^through by all animals 

 higher than the Protozoa, and by the Spongeir~amongst the latter, but 

 there is a difference as to the manner in which the central cavity is formed. 



Fig- 43-. Diagram of sporosacs supported upon a gonoblastidion (or blastostyle). a 

 Chitinous investment (periderm) of the colony; b Ectoderm; c Endoderm ; p Poly- 

 pite ; g Gonoblastidion, or columniform zooid, carrying sporosacs (s s) with ova in 

 their interior. (Altered from Allman.) 



In some cases it is formed by the hollowing out of the original sphere, and 

 the formation of an opening (the primitive mouth) at one end, as seems to 

 be generally the case in the Ccelenterata ; or, in other cases, it may be 

 produced by an invagination or inversion of the primitive vesicle in such a 

 manner as to form a central chamber, with a single aperture opening on 

 the exterior. By fixation of the "gastrula " at its hinder extremity to some 

 foreign object, and by the formation of tentacles round the mouth-opening 

 at the other extremity, a hydraform polypite is at once produced, which (if 

 not belonging to one of the simple forms) proceeds to develop the com- 

 posite adult by a process of gemmation. In this process in the Corynida 

 (as also in the Sertularida and Campanularida) the new polypites are 

 developed at or near the distal end of the hydrosoma, the distal polypites 

 being thus the youngest ; whereas the reverse of this obtains amongst the 

 Oceanic Hydrozoa. 



The subject of the reproduction of the Corynida having 

 been treated at some length, so as to apply to the remaining 

 Hydroida, we shall now give a brief description of the leading 

 types of structure exhibited by the order. 



