RADIATA. 



697 



■RUDS FROM 

 'e stnges 



than it previously possessed ; and a constriction or indentation is seen around this cylinder, just below tne nng 

 tliut surrounds the mouth and gives origin to the tentacula (Fig. 8, a.) Similar constrictionb are soon repeated 



around tlie lower parts of the cylinder, so as to give to the whole 

 somewhat the appearance of a rouleau nf coins {h). Still, how- 

 ever, a sort of fleshy bulb, — somewhat in the form of the original 

 polype, is left at the base. The number of the circles is indefi- 

 nite, and all are not formed at once ; new constrictions appear- 

 ing below, after the upper portions have been detached. As 

 many as twenty-seven disks have thus been progres^sively sepa- 

 rated in one animal. The constrictions then gradually deepen, 

 so as almost to divide the cylinder into a pile of distinct saucer- 

 like bodies ; the divisions being most complete above, and the 

 upper disks often presenting a considerable increase in their 

 .diameter. As the disks thus become more distinct from each 

 other, and of enlarged dimensions, their edges are no longer 

 plain but lobed {c) ; and the lobes soon present the clefts and 

 ocelli characteristic of the detached Medusas. Up to this period 

 the tentacula of the original polype surmounted the highest of 

 the disks ; and a general contraction and relaxation of the whole 

 cylinder, causing the intervals between the disks to be diminished 

 or increased, might be occasionally seen to take place. But before 

 the detachment of the topmost disk, the circle of tentacula by 

 which it was originally surrounded disappears, — in what precise 

 manner has not been ascertained ; and meanwhile a new circle 

 of tentacula is developed upon the summit of the bulb that re- 

 mains below the pile of disks. At last, a sort of convulsive 

 movement takes place in the topmost and largest disk, which 

 becomes detached and swims freely away ; and the same series 

 of changes takes place from above doTrawards, until the 

 whole pile of disks is detached and converted into free-swim- 

 ming Medusae. (At d is shown the lower part of the compound 

 structure, the disks of which have nearly separated from each 

 other.) But tJie original polypoid hody still remains, and may return 

 to its polypoid life and gemmiparous production, becoming the progenitor of a new colony of hydrte, each one 

 of which may develope in its turn a pile of medusa-disks. This last fact, which we owe to the patient and long- 

 continued observations of Sir J. G. Dalyell, is of fundamental importance ; as p-oving that the curious process 

 now described is not, as maintained by some, a subdimsion of the polypoid body into medusa-disks ; but that it is 

 a gemmiparous production of Medusa-buds from the polypoid body, of the same kind as that of which examples 

 will hereafter be described under the head of ITydraform Polypes ; save that the buds are here developed between 

 the body and the tentacular circle, instead of being protruded, as in tlie latter case, from the sides of the body. 



The disks thus detached, although Medusan in their character, are far from possessing the form or structure 

 they are ultimately to present. This is attained during the progress of their growth, by a dilferunce in the rate 

 of development of different parts, rather than by 

 an entire metamoi-phosis. The segments or lobes .^ -^, <■ 

 of the margin increase very little in size, vi'hUst the <jf['^ 

 intervals between them gradually fill up; tubular 

 prolongations of the stomach extend themselves 

 over the disk, and its border becomes furnished 

 with longpendentprehensile tentacles. The mouth, 

 which even in the youngest detached animal allows 

 of being greatly extended and protruded, is quad- 

 rangular, and presents four extensible angles. 

 These angles grow more rapidly than the four- 

 sided oral tube or proboscis ; so that, in the more 

 advanced animals, the mouth appears as if it had 

 split during the growth into four lobes ; and the 

 minute serratures which appear on the edges of 

 these are the commencement of the lobes and 

 fringes wliich are observed on the tentacula of the 

 adult animal. The reproductive organs are at last evolved, the sexes being kept distinct ; and by their agency 

 ova are produced, from which the animalcular embryo is developed as before into a polypoid body. 



The propagation of the Med.us<x is not only effected by ova, but also in some instances by gemmation ■ another 

 indication of their close alliance to Zoophytes. This has been observed by Sars in Cytais octopur.ctuta [Li~-ia rf 

 Professor E. Forbes, op. cit.), and by Professor E. Forbes in two species of Sarsia. In the former case the 

 gemmae are produced from the external wall of the stomach ; in Sarsia gemmifera, they grow from the lower'part 

 of the peduncle, or proboscis-like prolongation of the mouth ; and in Sarsia prolif era they spring from the bases 

 of the tentacula that hang from the margin of the disk. 



n^. 



NT OF RIedusa— Disk. 



f, successicestage* 



