Medusae of the Philippines and of Torres Straits. 169 



layer being the stomach; and thus the name gastrula is applied to 

 this stage. Jellyfishes are essentially in the gastrula stage even when 

 adult. Yet so extraordinary are the foldings and outgrowths that have 

 arisen in their two body-layers during the vast time they have existed 

 upon the earth that, ultimately simple as they are, no class of the 

 animal kingdom exhibits a more surprising variety of forms than do the 

 Jellyfishes and their close allies the Siphonophorse. 



It is interesting to observe that the large Jellyfishes, scyphomedusse, 

 which have gastric cirri and no marginal diaphragm or velum, are 

 probably only very remotely related to the small Jellyfishes, the hydro- 

 medusae, which have a velum and lack gastric cirri. Indeed we have 

 good reason to believe that the jellyfish-shape and peculiar locomotion 

 through pulsation have been derived independently in the two groups. 

 The scyphomedusse are probably allied to the actinians or sea-anemones, 

 while the hydromedusse have probably been derived from hydroids. 

 In fact a jellyfish-like shape and pulsating body have been acquired 

 independently in widely different kinds of animals, such as Pelagothuria, 

 a holothurian which bears a wonderfully close resemblance to a jelly- 

 fish and swims actively through the water in the tropical Pacific; and 

 in Craspedotella, a minute unicellular marine animal, which would 

 certainly have been mistaken for a jellyfish had it not been of micro- 

 scopic size. 



Indeed there is reason to lead us to believe that the bell of the 

 Narcomedusse is a mere outgrowth from the sides of the pyrif orm larva, 

 and has thus been acquired in a manner quite different from that of 

 the other hydromedusae. Thus the umbrella-like bodies of Jellyfishes 

 have probably been acquired in at least three different waj^s within the 

 group itself. 



