152 PHYLUM C@LENTERA. 
a fixed asexual hydroid colony, the planula settles down, 
loses its cilia, buds out tentacles, and develops into a new 
hydroid. ; 
In many hydroid colonies, as has been already noticed, 
the sexual members are not set free, but remain as buds 
attached to the parent. These fixed “gonophores” show 
many stages of degeneration ; some, notably in the floating 
colonies of Siphonophora, differ little structurally from true 
medusoids, while others, as in Wydractinia, are simply small 
closed sacs enclosing the genital products (Fig. 87). 
Third Type of CELENTERA.—The common Jelly-fish 
—Aurelia aurita. Class SCVPHOMEDUSZ 
This Medusa is almost cosmopolitan, and in the summer 
months occurs abundantly around the British coasts. It 
swims by pulsating its disc, and also drifts along at rest 
without any pulsations. They often occur in great shoals, 
and hundreds may be seen stranded on a small area of flat 
sandy beach. The glassy disc usually measures about four 
inches in diameter, but may be twice as large. The jelly- 
fish feeds on small animals, such as copepod crustaceans, 
which are entangled and stung to death by the long lips. 
External appearance.—The animal consists of a gela- 
tinous disc, slightly convex on its upper (ex-umbrellar) 
surface, and bearing on the centre of the other (sub- 
umbrellar) surface a four-cornered mouth, with four long 
much-frilled lips. The circumference of the disc is fringed 
by numerous short hollow tentacles, by little lappets, and 
by a continuation of the sub-umbrella forming a delicate 
flap or velarium. Conspicuously bright are the four re- 
productive organs, which lie towards the under surface. 
Nor is it difficult to see the numerous canals which 
radiate from the central stomach across the disc, the eight 
marginal sense organs, and the muscle strands on the lower 
surface (Fig. 73). 
The three layers.—The ectoderm which covers the 
external surface bears stinging cells, sensory and nerve cells, 
and muscle cells. The ectoderm seems also to be invagin- 
ated to form the gullet or stomodeum. The endoderm 
lines the digestive cavity, is continued out into its radiating 
