410 Original Articles. [July, 



It is our business now briefly to trace the connection between the 

 two — to sbow that the one extremity of the scale is united to the 

 other by a series of intermediary forms, and that there is the same 

 essential element in all. We have to link the fixed bud that vegetates 

 and liberates its germs, and then withers away on the spot, where it 

 came into being, to the winged and freighted seed-vessel, floating far 

 and wide through the waters. 



Let us pass then at once to the free Gono-zooid (the quondam 

 Medusa), and examine its structure. We have a representation of it, 

 while still attached to the Hydroid stock (Plate i. fig. 3a) ; and 

 after liberation (Plate i. fig. 2). In its former state, it is connected 

 by a pedicle with the body of the polypite, which springs from the 

 midst of a cluster of buds in various stages of development. We 

 might naturally take it to be a permanent portion of the colony, a 

 bud of peculiar structure, and destined for some special function, 

 but not more remarkable amongst the polypites than the flower-bud 

 amongst the leaves on the plant. Such, however, is not its history. 

 After a while it dissolves its connection with the polypite, and thence- 

 forth leads an independent existence. By a reference to Plate i. 

 fig. 2, the reader will obtain an idea of the general form and structure 

 of the free Medusiform zooid. A somewhat globose and transparent 

 disc floats gracefully in the water, and within the cavity a cylindrical 

 body hangs from the central point of the dome — a rose-coloured pen- 

 dant in a crystal sphere. The disc or umbrella is truncate below and 

 partially closed by a filmy membrane or veil, in the centre of which 

 there is an oblong aperture. From its margin depend four extensile 

 and beaded tentacles, which take their origin in as many bulbs, on each 

 of which a rose-coloured ocellus glitters. From the base of the pen- 

 dant body four tubular canals pass through the substance of the disc 

 to these bulbs, where they join a circular vessel that runs round the 

 margin, and connects them all. The disc, which is the striking feature 

 of the structure, is a float and swimming bell, and by means of its 

 rhythmical contractions the zooid is jerked backwards through 

 the water. The pendant body is furnished with an orifice at its free 

 extremity (the mouth), which admits to a central cavity, and this 

 communicates at the base with the four canals, and through them 

 with the circular, marginal vessel. We have here the nutritive and 

 circulatory systems. The tentacles are fishing-lines, and the ocelli, 

 it may be, rudimentary light-perceiving organs. 



In time ova or spermatozoa, as the case may be, are developed 

 behveen the two membranes which constitute the walls of the pendant 

 body (Plate ii. fig. 12a)* and when these are matured and dis- 



* This figure represents the pendant portion of a Gono-zooid apart from the 

 swimming-bell, with the ova developed in the walls. It is proper to mention here 

 that in certain families the generative elements are not produced in this situation, 

 but in small sacs, which bud from the radiating canals. These canals, however, we 

 must remember, are but prolongations of the central cavity, so that the reproductive 

 bodies are still in connection with (essentially) the same portion of the structure. 

 The difference is that in this case instead of lying between the two layers which 

 compose the wall of the canal, the generative j>roducts are contained in sacs, which 

 bud from them, and which are similar in structure to those of the Hydra. 



