44 HYDROIDS AND JELLY-FISH 



a primitive eye. The mouth leads into a digestive cavity (enteron) 

 which occupies the whole of the interior of the manubrium, and 

 from its base sends off four very fine tubes (radial canals), which, 

 passing at equal distances from each other through the mass of 

 the umbrella to its margin, open into a circular tube or canal 

 which runs parallel with and close to the edge of the umbrella. 

 By means of its fringe of tentacles, which are well armed with 

 thread or stinging cells, the medusa captures its microscopic 

 prey, and the food, swallowed by the mouth, is digested in the 

 manubrium and distributed throughout the entire medusa by 

 the system of canals. 



Placed at equal distances on the under surface of the umbrella, 

 and in immediate relation with the radial canals, are four oval 

 bodies, each containing a mass of cells which will develop into 

 ova or sperms, for each medusa bears organs of one sex only, and 

 therefore, according to whether it is a female or a male medusa, 

 will produce only ova or sperms. When these sexual cells are 

 ripe the sperms of the male medusa are shed into the water and 

 carried by the currents to the females, where they fuse with the 

 ova. The fertilised egg develops into a little oval creature called a 

 planula, covered with cilia, by means of which it swims about in 

 the sea. After a little time the tiny planula gives up its active, 

 free-swimming life, and, settling down on a piece of submerged 

 timber, rock, or seaweed, fixes itself by one end and becomes con- 

 verted into a little polyp with a waving circle of tentacles. It 

 captures food, grows, sends out lateral buds, and soon be- 

 comes converted into a complex, branching Obelia colony, the 

 polyp members of which are destitute of fully developed sex 

 cells. 



In Obelia we have an instance of what is termed an alternation 

 of generations a branching colony of polyps, certain of which 

 produce asexually formed medusae, which in turn become detached, 

 develop male and female organs, and produce sexually a ciliated 

 offspring, which becomes the founder of a colony of sexless polyps 

 multiplying by a process of budding. This interesting pheno- 

 menon is very common among the Hydrozoa, being particularly 

 marked in cases like Obelia and allied species, where the asexually 

 formed generative polyp or medusa becomes detached and swims 

 away. But these free-swimming generative medusae do not occur 



