4 S 4 CCELENTERATES. 



this disc lies the similarly shaped pneumatophore, which is also traversed by 

 concentrically arranged canals opening outwards. The polyps hang on the lower 

 side of the cartilaginous disc, a large nutritive polyp occupying the centre, surrounded 

 by concentric circles of smaller nutritive polyps. As in the Auronectidce, these 

 polyps carry at their bases genital clusters, but no capturing filaments. The 

 tentacles are arranged round the margin of the disc, and are very short. The 

 genus Velella, one species of which is frequently found in the Mediterranean, has 

 an irregular oval disc, surmounted by a sloping comb, which acts as a sail. These 

 animals, which are of a deep indigo colour, are often found in swarms. 



Order Hydromedusae. 



Having considered the complicated colonies of swimming-polyps constituting 

 a Siphonophore, the individuals of which have each been simplified for the 

 performance of a limited number of duties, we turn to the solitary swimming- 

 bells, each one of which forms an individual competent to perform all the many 

 functions required in its struggle for existence. There are hosts of these bells, of 

 almost all sizes, some being large and beautiful, but dangerous to touch, while 

 others are quite minute creatures, which have to be examined under the microscope. 

 In regard to these swimming medusae, it has been already mentioned that they 

 were primitively individuals broken loose for a free-swimming life in the open 

 sea from a stock attached to the ground at the bottom. The eggs of some of these 

 forms have now given up passing through the attached stage, and hatch out at 

 once as young medusae. Now, examination has shown that this host must be 

 divided into two groups, having remarkable differences, the one being called the 

 Hydromedusae, and the other the Scyphomedusae. The two came from two 

 different kinds of attached stocks, and consequently, as free-swimming animals, in 

 spite of their general resemblance to one another as jelly-fish, each has organs 

 which the other wants. Taking the Hydromedusae first, as closest to the Siphono- 

 phora, we describe a few in detail, in order to give a clearer idea of the alternation 

 of generations. 



Among the Hydromedusae there are the following different life -histories. 

 Beginning with the highest, we have — (1) jelly-fish alone, the eggs of which have 

 given up forming stocks, but hatch out jelly-fish ; (2) jelly-fish, the eggs of which 

 still form stocks, some individuals of which swim away as jelly-fish ; (3) stocks in 

 which the sexual individuals do not swim away as jelly-fish. We need not here 

 describe any of the medusae in detail, since the much larger jelly-fish of the 

 Scyphomedusae will claim our attention presently, but two remarkable forms, which 

 have taken to creeping on the ground, deserve attention. In Dalmatia, on seaweed, 

 a delicate, pale object can often be discerned with a magnify ing-glass creeping 

 laboriously about on its long arms. If detached from the seaweed, it falls to the 

 bottom, as it is unable to swim. In each point of its structure this animal is a 

 medusa, related to the genus Eleutheria, or Cladonema, but still further removed 

 from the ordinary medusa in one respect, since the Cladonema alternately swims 

 and creeps. This creeping medusa (Clavatella prolifera) has six arms, the tips of 

 which are provided with true suckers. On these it walks, as on stilts, while from 



