18 THE OCEANIC HYDROZOA. 



opens at its extremity, acquires the structure, and takes on all the functions of a polypite. At 

 the same time, tentacular organs and lithocysts may be developed from the margins of the 

 calyx. Eventually, the ova or spermatozoa appear either in the walls of the polypite, or in 

 sjiecial spermaria or ovaria developed in the parietes of the canals of the calyx of this inde- 

 pendent niedusiform zooid, which is occasionally a more complex organism than that from 

 which it sprang. 



This is still more remarkably the case in most of the LucernariadtB, whose reproductive 

 zooids surpass the " Hydrte tubse," whence they proceed, many thousandfold in size and mass, 

 and no less remarkably, in complexity of organization. Known under the same general title 

 of " Medusas " as the foregoing, and in many ways analogous to them, they are, nevertheless, 

 widely different in structure and origin. Their swimming apparatus is an umbrella, and neither 

 in structure nor in mode of development a gonocalyx, and they are developed, not by budding 

 from a limited area of one side of a coenosarc or its polypite, but by the transverse constric- 

 tion of a polypite into superimposed segments, each of which becomes lobed at its margins, 

 and assumes the form of an umbrella with a central polypite. 



The arrangement of the tentacles, again, is quite different from that which obtains in the 

 other Medusa, and the lithocysts differ in being covered by a hood, and in not having each 

 mineral particles which they contain, spheroidal and enveloped in a distinct vesicle. 



In CyancBa, the polypite is simple and single, and the spermaria or ovaria are saccu 

 lated portions of the distal (or under) wall of the umbrella, inclosed in a peculiar chamber 

 formed by the outgrowth, into a thick ridge, of the ectoderm round each reproductive organ, 

 so as to leave only a small, circular aperture, leading into a chamber, from whose roof 

 the plaited membrane containing the reproductive elements depends. 



In this genus, moreover, the edges of the produced angles of the lips of the polypite, which 

 hang down like four great arms, are so folded as to form little cups, into which the fertilised 

 ova are received, there to undergo their first changes. 



In the Blnzosiomida, a complex, tree-like mass, whose branches, the stomatodendra, end 

 in, and are covered with, minute polypites interspersed with clavate tentacula, is suspended 

 from the middle of the umbrella in a very singular way. The main trunks of the dependent 

 polypiferous tree, in fact, unite above into a thick, flat, quadrate disc, the si/ndeiidriiim, which 

 is suspended by four stout pillars, the dendroslylcs, one springing from each angle, to four 

 corresponding points on the under surface of the umbrella, equidistant from its centre. Under 

 the middle of the umbrella, therefore, there is a chamber whose floor is formed by the 

 quadrate disc, while its roof is constituted by the under wall of the central cavity of the 

 umbrella, and its sides are open. The reproductive elements are developed within radiating, 

 folded diverticula of the roof of this genital cavity.^ 



In passing from Hydra to BJiisosfoma, we thus see the reproductive organs acquiring a 

 greater and greater relative mass, when compared with the organism from which they spring, 

 and, as it were, grouping round themselves and subordinating to their own perfection 

 a greater and greater number of morphological elements. First, they are parts of the 

 body wall, indistinguishable in form from the rest ; then they are distinct sacs ; then they are 



^ I have described at length and figured, tlie structure here indicated, in my " Memoir on the 

 Anatomy of the Medusse,'' ' Phil. Trans./ 1819. 



