JELLY-FISriES. 89 



These forms, represented on our southern coasts by Millepo)-a alcicornis, form a 

 coral which, however, is composed of calcareous fibres and is traversed in all directions 

 by canals. The little cups occupied by the polyps are shallow, but as one ]iolyp dies 

 another succeeds it, forming a jiartition separating the new cuj) from the old, so that 

 in time the pits of tlie coral become deep but are divided up by a series of transverse 

 ]iartitions. A similar structure exists in some of the true corals. 



Among the fossil forms referred to, the hydroids, the Graptolites of tlie Silurian 

 pci'iod are most prominent. In these forms we hayn an approximation in general 

 aj)iiearance to the Sertularian. These, so far as we are able to discover their anatomv, 

 consisted of a narrow tube bearing, on one or both sides, a series of hollow toetli, 

 through which the tube communicates with the exterior. It 

 is supposed that each of tliese teeth was occupied by a zooid 

 similar to that found in Sertularia or Plmnularia. Haeckel 

 has described, from the lithographic stone of Solenhofeu fi'-- -•-•- i'"iii ■" ■ i m c.iai.i^- 

 (Jurassic age), two fossil medusre, which he refers to the 



Trachymedusro, and from later beds portions of the incrusting iiydrorliiza of ILjdrac- 

 tinia have been found, and true Sertularians occur in the pleistocene of England. 

 Other forms referred with more or less doubt to this group occur in the Cambrian, 

 carboniferous, etc. 



Samukl F. Ci.auke. 



Order II. — DISCOPHORA. 



Among the Meduste A\hich attain the greatest size and jirobably are the most 

 commonly observed are those called the Discophora, " sea-nettles," " sea-blubs," or 

 " jelly-fishes." The members of this group are very characteristic, and are named 

 from tlie disk-shaped outline of their bodies. Although the group of Discojihora 

 is not a large one, there being barely a half dozen genera found in our waters, it 

 presents some of the most interesting features of all the known Medusje. 



The bodies of all the jelly-fishes are very gelatinous, composed for the most part 

 of water, and when taken from their native element speedily melt away, leaving a 

 thin film behind. Although these animals are not the only ones which have gelatinous 

 bodies, they excel all in the amount of fluid in their tissues, and are consequently 

 among the most transparent of animals. 



The habits of the Discophora are very curious. Many swim at or near the surface, 

 sometimes protruding their bodies a little out of the water. Some arc confined to the 

 dee]) seas and are drifted only by accident into shallow waters. A rough sea sends 

 many below the reach of the agitated waters, and in a calm they i-ise to the surface, 

 to come into the range of the sun's rays. Whatever their purpose is in tliis latter 

 habit, we may trace to it the name of "sun-fish," which they bear in some localities. 

 Most of these animals in their adult condition are free swimming, while many arc 

 attached to the ground in their younger stages of growth in a way which will be 

 treated of in our account of the develo]nnent. C'assioj}ea is attached in the aduil 

 state in the following jteculiar manner. This genuc is very common along the Florida 

 reefs where, instead of swimming about in the water, it lies at the bottom on tlie coral 

 sand, lazily fla]i])ing its disk in a monotonous manner. It is not firmly anchored to the 

 bottom, yet rarely changes its position any considerable distance. 



