404 Original Articles. [July, 



the process of gemmation, and before the termination of the organic 

 connection between the nascent young and the parent, a composite 

 being is formed, and we have thus in a transient stage of the life- 

 history of the Hydra, a representation of that which is the per- 

 manent condition of most of the Hydroid Zoophytes. At a certain 

 point of development, the budding Hydras detach themselves, and enter 

 upon independent existence. 



Let us examine the bud in its earliest condition, when it appears 

 as a slight excrescence on the side of the body. It is a mere bulging 

 of the double membrane, which forms the wall of the central cavity, 

 a little cul-de-sac projecting from its surface, and freely communi- 

 cating with the interior. This minute, sac- like extension of the two 

 layers is gradually moulded into a perfect Hydra. 



But besides the multiplication of this species by means of a 

 gemmiparous process, strictly analogous to the phenomena of vegetable 

 growth, we find also a true sexual reproduction. At certain seasons 

 only, and within certain definite zones on the surface of the body, 

 other buds are produced, which instead of taking the line of de- 

 velopment just described, become laboratories wherein the generative 

 elements are matured. These are the analogue of the flower-bud in 

 the plant. 



In essential structure they seem to be identical with the early 

 Btage of the Hydra-bud. They consist, like it, of a small sac, 

 formed by an outgrowth of the body- wall — a cavity communicating 

 with the stomach of the Hydra, and bounded by the double mem- 

 brane. In the reproductive bud, however, growth is soon arrested, 

 and it remains a small tubercle on the surface of the body. The 

 generative products — ova or the male element — are developed within 

 it, between the two membranes, which they push asunder as they increase 

 in bulk. 



Here, then, we have the simplest form of the reproductive system 

 found amongst the Hydroida. But the point to which we would draw 

 special attention is this : we have here the essential element of that 

 system, under all the modifications which it presents throughout the 

 Order. Whatever may be superadded, whatever accessory parts may 

 be introduced, the ova and spermatozoa are produced universally 

 within the walls of a body, which is essentially identical in struc- 

 ture and mode of growth with the simple reproductive pouch of the 

 Hydra. The so-called Medusa is only such a pouch, somewhat 

 modified, advanced a degree or two in development with a view to 

 free existence, floated by means of a contractile bell, and supplied 

 with a few rudimentary organs of sense. 



A series of transition-forms, in which there are now no important 

 gaps, links the simplest condition of the sexual zooid* with the most 

 complex ; the closed and stationary egg-bag at one extremity of the 

 scale, with the free mercurial Medusoid at the other, which not only 

 produces, but also distributes the ova. 



* This term is used to denote the several distinct elements, which make up 

 the sum total of the individual life-series, e.g. the polypites, and the reproductive 

 bodies, whether fixed or free. 



