Order 4. 



PHYSOGRADA. 



685 



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

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



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

 somewhat the appearance of a rouleau of coins (b). 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 iudefi. 

 nite, and all are not fonned at once ; new constrictions appear. 

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

 many as twenty-seven disks have thus been progressively 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 Medusae. 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 larg 'St disk, which 

 becomes detached and swims freely away ; and the same scries 

 of changes takes place from above downwards, 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 the original polypoid body still remaUis- and may return 

 to its polypoid life and gemmiparous production, becoming the progenitor of a new colony of hydrie, 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. DalyeU, is of fundamental importance ; as p-oving that the curious process 

 now described is not, as maintained by some, a subdioision 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 Ilydraform Polypes ; save that the buds are here developed between 

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



The disks thus detached, although iledusan 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 difference in the rate 

 of development of different parts, rather than by 

 an entire metamorphosis. The segments or lobes ^jMi-'Z, 



Fio. 8. — Productiok 

 Polype BoniRs: o. 



MEnrsA— Buds from 

 rf, successive stages 



c'^<=6W, 



'y^ 



of the margin increase very little in size, whilst the 

 intervals between them gradually fill up ; tubular 

 prolongations of the stomach extend themselves 

 over the disk, and its border becomes furnished 

 with long pendent prehensUe 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 which are observed on the tentacula of the ^'°- '-D^velopmf.nt of MEDus..-D.sK.i»,4.c,rf,.,/, ,ucce«ive,u«„ 



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 Medusce 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 octopunctata (Lizzia of 

 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 gemmifcra, they grow from the lower part 

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

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



