POLYPOID DEVELOPMENT OF MEDUSA. 



469 



to be plainly seen in the detached Medusae (Fig. 227, c). Up to 

 this period, the tentacula 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 bulb which remains at the base of the pile (c, c). 

 At last, the topmost and largest disk begins to exhibit a sort of 

 convulsive struggle; it becomes detached and swims freely 

 away ; and the same series of changes takes place from above 

 downwards, until the whole pile of disks is detached and con- 

 verted into free swimming Medusae. But the original polypoid 

 body still remains ; and may return to its polype-like and original 

 mode of gemmation (D, <?), becoming the progenitor of a new 

 colony of Strobilae, every one of which may in its turn bud off a 

 pile of Medusa disks. 



307. The bodies thus detached have all the essential characters 

 of the adult Medusce. 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 propor- 

 tion of the disk, and projects downwards in the form of a pro- 

 boscis, in the centre of which is the quadrangular mouth (Fig. 

 227, A, B). As the animal advances towards maturity, the in- 



Development of Medusce from the detached gemmae of Strobila : A, individual viewed sideways, 

 and enlarged, showing the proboscis, a, and b the bifid lobes; B, individual seen from above, show- 

 ing the bifid lobes of the margin, and the quadrilateral mouth ; c, one of the bifid lobes still more 

 enlarged, showing the ocellus (?) at the bottom of the cleft; D, group of young Medusae as seen swim- 

 ming in the water, of the natural size. 



tervals between the segments of the border of the disk gradually 

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

 tions of the stomach extend themselves over the disk; and 

 from its borders there sprout forth tendril-like filaments, which 



