TRACHOMEDUSAE 29 



appear in development before there is any trace of a sub-umbrella 

 cavity, and this has been interpreted to be a transitory but 

 definite Hydroid stage. It may be supposed that the elimination 

 of the hydrosome stage in these Coelenterates may be associated 

 with their adaptation to a life in the ocean far from the coast. 



During the growth of the Medusa from the younger to the 

 adult stages several changes probably occur of a not unimportant 

 character, and it may prove that several genera now placed in 

 the same or even different families are stages in the development 

 of the same species. In the development of Liriantha ajpjpen- 

 diculata, 1 for example, four interradial tentacles appear in the 

 first stage which disappear and are replaced by four radial 

 tentacles in the second stage. 



As with many other groups of free-swimming marine animals 

 the Trachomedusae have a very wide geographical distribution, 

 and some genera may prove to be almost cosmopolitan, but the 

 majority of the species appear to be characteristic of the warmer 

 regions of the high seas. Sometimes they are found at the 

 surface, but more usually they swim at a depth of a few fathoms 

 to a hundred or more from the surface. The Pectyllidae appear 

 to be confined to the bottom of the sea at great depths, 



The principal families of the Trachomedusae are : 



Fam. Olindiidae. This family appears to be structurally 

 and in development most closely related to the Leptomedusae, 

 and is indeed regarded by Goto' 2 as closely related to the Eucopidae 

 in that order. They have two sets of tentacles, velar and ex- 

 umbrellar ; the statocysts are numerous, two on each side of the 

 exumbrellar tentacles. Eadial canals four or six. Manubrium 

 well developed and quadrate, with distinct lips. There is an 

 adhesive disc on each exumbrellar tentacle. 



Genera: Olindias, Olindioides, Gonionema (Fig. 139), and 

 lie I i calyx. 



As in other families of Medusae the distribution of the genera 

 is very wide. Olindias millleri occurs in the Mediterranean, 

 Olindioides formosa off the coast of Japan, Gonionema murhachii 

 is found in abundance in the eel pond at Wood's Holl, United 

 States of America, and Halicalyx off Florida. 



Two genera may be referred to in this place, although their 



1 E. T. Browne, Proc. Zool. Soc. 1S96, p. 495. 

 2 Mark Anniversary Volume, New York, 1903, p. 1. 



