3458 CCELENTERA TES 



that all Siphonophora under certain circumstances and at certain seasons appear at 

 the surface. Many pass through their larval development at a great depth, and 

 the young Physophora larvae found at the surface in the spring descend to greater 

 depths at the commencement of summer, and only return, when their metamor- 

 phoses are complete, to develop into sexually mature animals. In the Physophondce 

 we had the different individuals in a long series. In the Auronedid.ee we found 

 them arranged in a compact mass; and, lastly, in the Velellidce, the body is flattened 

 out to a disc, which is traversed by a system of canals. On this disc lies the 

 similarly shaped pneumatophore, which is also traversed by concentrically arranged 

 canals opening outward. 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, sur- 

 mounted by a sloping comb, which acts as a sail. These animals, which are of a 

 deep indigo color, are often found in swarms. 



Order HYDROMEDUS^ 



Having considered the complicated colonies of swimming polyps constituting a 

 Siphonophore, the individuals of which have each been simplified for the perform- 

 ance 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 Hydromedusse, and 

 the other the Scyphomedusas. 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 jellyfish, each has organs which the other wants. 

 Taking the Hydromedusae first, as closest to the Siphonophora, we describe a few 

 in detail, in order to give a clearer idea of the alternation of generations. 



Among the Hydromedusse there are the following different life histories: Begin- 

 ning with the highest, we have (i) jellyfish alone, the eggs of which have given 

 up forming stocks, but hatch out jellyfish; (2) jelly fish \ the eggs of which still form 

 stocks, some individuals of which swim away as jellyfish; (3) stocks in which the 

 sexual individuals do not swim away as jellyfish. We need not here describe any 

 of the medusae in detail, since the much larger jellyfish of the Scyphomedusae will 



