4&4 
CCELENTERA TES. 
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 Auronectidoe, 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 Hydromedus^e. 
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 medusas 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 magnifying-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 
