96 ALTERNATION OF GENERATION. PART m. 



again repeat this singular process. However, the 

 young medusae are not yet perfect. As they increase in 

 size the divisions on the edge of the cup fill up; a pro- 

 boscis-shaped stomach, with its four coloured cells and 

 its square mouth, is developed from the centre of the 

 sub-umbrella; the radiating canals extend from the cen- 

 tral cavity, the encircling canal and fringe form round 

 the umbrella-shaped cups, and the result is a highly 

 organized Thaumantia pilosella, in whose life-history a 

 simple hydra forms a singular stage. 



Thus hydrae produce medusae whose offspring are 

 hydras, and perfect medusas produce hydras whose off- 

 spring are perfect medusas. However, the law of the 

 alternation of generation is by no means peculiar to 

 the Thaumantias. Many species of medusae are subject 

 to it, as the Turris neglecta, a beautiful little medusa 

 not larger than a hempseed, common on the British 

 coasts. It has a white muscular pellucid umbrella, a 

 large proboscis of a rich orange colour at its upper part : 

 in the orange-coloured flesh of it there are ovaries con- 

 taining rose-coloured eggs, which are hatched within 

 them, and come into the water as ciliated gemmules, 

 which, after swimming about for a time, become fixed 

 and are developed into small hydras of a rich purple 

 colour with sixty-four tentacles. From these hydras 

 others bud off indefinitely till the time comes when one 

 of them becomes lengthened, constricted, divided into 

 cups which drop off, and finally become a brood of the 

 Turris neglecta. 



The naked-eyed medusas are extremely numerous. 

 There are six orders of them and many genera, chiefly 

 distinguished by the position and nature of their ovaries 

 and the number of canals which radiate through their 

 swimming organs. Both of the medusas that have been 

 described have four radiating canals ; yet they belong to 

 different orders, for the ovaries of the Thaumantia are 

 in the edge of the umbrella, while those of the Turris 



