Meduse 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. Yetso 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 Siphonophore. 
It is interesting to observe that the large jellyfishes, seyphomedusz, 
which have gastric cirri and no marginal diaphragm or velum, are 
probably only very remotely related to the small jellyfishes, the hydro- 
meduse, 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. 
Thescyphomeduse are probably allied to the actinians or sea-anemones, 
while the hydromeduse 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 
Narcomeduse is a mere outgrowth from the sides of the pyriform larva, 
and has thus been acquired in a manner quite different from that of 
the other hydromeduse. Thus the umbrella-like bodies of jellyfishes 
have probably been acquired in at least three different ways within the 
group itself. 
