SPONGES AND ZOOPHYTES. 



133 



FlG 360 



somewhat of the form of the original polype, being still left at the at- 



tached extremity. The number of circles is indefinite, and all are not 



formed at once, new constrictions appearing below, after the upper por^ 



tions have been detached; as many as 30 or even 40 have thus been pro- 



duced in one specimen. The constrictions then gradually deepen, so as 



to divide the cylinder into a pile of saucer-like bodies; the division being 



most complete above, and 



the upper disks usually pre- 



senting some increase in 



diameter; and whilst this is 



taking place, the edges of 



the disks become divided 



into lobes (B), each lobe soon 



presenting the cleft with 



the supposed rudimentary 



eye at the bottom of it, 



which is to be plainly seen 



in the detached Medusas 



Development of Chrysaora from Hydra tuba: A, de- 

 tached individual viewed sideways, and enlarged, showing 

 the proboscis a, and b the bifid lobes: B, individual seen 

 from above, showing the bifid lobes of the margin, and the 

 quadrilateral mouth; c, one of the bifid lobes still more en- 



(?) at the bottom of 

 as seen swimming in 



(Fig. 360, c). Up to this 

 period, the tentacles of the 

 original polype surmount 

 the highest of the disks; but 

 before the detachment of 

 the topmost disk, this circle 

 disappears, and a new one 

 is developed at the summit 

 of the Inilb which remains 

 at the base of the pile (c, c). 

 At last the topmost and 

 largest disk begins to ex- 



hibit a Sort Of Convulsive farged showjng the rudimen taiy eye (5 

 Struggle; it becomes detach- the cleft; D, group of young Medusae, a; 

 ed, and SWimS freely away; the water, of the natural size. 



and the same series of changes takes-place from above downwards, until 

 the whole pile of disks is detached and converted into free-swimming 

 Medusae. But the original polypoid body still remains, and may return 

 to its original polype-like mode of gemmation (D, e) ; becoming the pro- 

 genitor of a new colony, every member of which may in its turn bud-off 

 a pile of Medusa-disks. 



524. The bodies thus detached have all the essential characters of the 

 adult MeduscB. Each consists of an umbrella-like disk, divided at its 

 edge into a variable number of lobes, usually eight; and of a stomach, 

 which occupies a considerable proportion of the disk, and projects down- 

 wards in the form of a proboscis, in the centre of which is the quadrangu- 

 lar mouth (Fig. 360, A, B). As the animal advances towards maturity, 

 the intervals between the segments of the border of the disk gradually 

 fill-up, so that the divisions are obliterated; tubular prolongations of the 

 stomach extend themselves over the disk; and from its border there 

 sprout forth tendril-like filaments, which hang down like a fringe around 

 its margin. From the four angles of the mouth, which, even in the 

 youngest detached animal, admits of being greatly extended and pro- 

 truded, prolongations are put forth, which form the four large tentacles 

 of the adult. The young Medusas are very voracious, and grow rapidly, 

 so as to attain a very large size. The Cyanece and Chrysaorce, which are 



