88 SLTtVEY OF THE REPRODUCTIVE PROCESS 



Coelenterate or Hydraform polypes, and one which distinctly 

 presents the rudiments of the MoUuscan type. In parti- 

 cular there are always distinct muscular fibres, a well- 

 defined alimentary canal with two openings and proper 

 walls, and traces even of a nervous system, in a single 

 ganglion situated on one side of the oral aperture. 



Their reproduction presents some interesting peculiarities, 

 both in the protomorphic and gamomorphic stages. The 

 early development of the germ has been followed out by 

 Professor Allman, particularly in some of the freshwater 

 species. By his account we have first, as the immediate 

 result of the development of the ovum, " a ciliated sac-like 

 embryo [germ], resembling in form and habit an infusorial 

 animalcule. As development proceeds, we find the ciliated 

 embryo, while still confined within the coverings of the egg, 

 presenting in some part of its surface an opening which 

 leads into the central cavity; and through this opening an 

 unciliated hernia-like sac is protruded by a process of eva- 

 gination. In the interior of the protrusible portion . 



. a polypide [polype] is developed. The gemmation 

 of the first polypide is immediately followed by that of 

 another close beside it, so that the young polyzoon has now 

 the appearance of a transparent closed sac, filled with fluid, 

 the posterior part ciliated, the anterior part destitute of 

 cilia, and partially or entirely pushed back into the posterior 

 by a process of invagination, while the sac carries mthin it 

 two young polypides, which are suspended from the inner 

 surface of the unciliated portion."* 



The prominent peculiarity here is the formation from the 

 original germ of two gemmae, which are really the embryos 

 of the first pair of polypes. 



The polypiform, which is also the orthomorphic or typical 

 phase of development, once acquired, the tendency to gem- 



* Allman's Freshwater Polyzoa (Ray Soc), p. 41, 33, and 34. 



