[31] MEDUSA FROM THE GULF STREAM. 957 



velum — the peronia being a projection extending from the teutacnlar 

 base to the margin of the velarium — is not so evident.* The position 

 and general appearance of the peronisB in Solmaris recall the same 

 structure in Cunina discoides, Few. If we regard the tentacles as 

 in reality ending at the margin of the umbrella, and not at the free 

 margin of the velarium in Solmaris, it seems probable that the walls of 

 the velarium are homologous with the marginal lappets of the Acras- 

 peda. That the velarium is, in fact, formed by a consolidation of mar- 

 ginal lappets on their edges along the lines of the peronise cannot be 

 demonstrated, but it is certainly indicated by the anatomy of Solmaris. 



An interesting habit of the genus Linerges, a medusa which has other 

 hydroid affinities, may be mentioned in considering the homology last 

 spoken of. Liner ges when at rest carries its marginal lappet folded in- 

 ward at right angles to the almost vertical walls of the bell. They seem 

 in a measure to perform a like function as the velum of the hydroid 

 gonophore in partially closing the opening into the bell cavity. Sup- 

 pose in Linerges the edges of the marginal lappets thus folded should 

 be united. We should then have a structure homologous to the return 

 of a free hydroid gonophore. 



In some other genera, also, as in the younger forms known as Ephyrse, 

 and in certain adults of the family of Ephyridae, where almost the whole 

 movement of propulsion is produced by the vibration and repeated 

 strokes on the water of the margina lappets, we have a like infolding 

 of these bodies. The probability that an JEphyra-like medusa is the 

 ancestral form of the Acraspeda, and the fact that in it motion is accom- 

 plished mainly by the movements of the marginal lappets, leads one to 

 expect that in some TrachymedussB we may look for a like function in 

 an homologous organ. Although in Solmaris, both the umbrella and the 

 velarium probably work together in the propulsion of the medusa in many 

 allied genera, Cunina and others, the bell walls are sometimes rigid and 

 the velarium and velum are the sole means of propulsion. 



HYDROIDA. 



The Craspedote medussB are represented in the collections by a very 

 small number of genera and species.t This scarcity is not wholly due 

 to their small size. Naturally enough, as we have seen, the gonophores 

 of "free hydroids," Trachymedusse, would be well represented, but the 



* Have we not a similar condition in Turris tpiscopalis, where a spur from the ten- 

 tacle extends along the side of the bell ? 



t There is little doubt that many of the hydroid medusaj recorded from Charleston, 

 S. C, Beaufort, N. C, Newport, R. I., and elsewhere on our eastern coast, are bron<j;ht 

 there by the Gulf Stream. Many others from the same localities belong to a strictly 

 littoral fauna. The difficulty of distinguishing the former from the latter has led me, 

 at present, to eliminate both, and to iuclude only those recorded from the Gulf Stream 

 region. When a more complete account of the Gulf Stream medusa) is prepared it 

 will x)robably embrace a large number of genera of Acalephs, common in the bays 

 and harbors of the eastern coast of the United States. 



