226 THE OCEAN WORLD. 



plant, which throws out its slender stems in all directions, covering all 

 the neighbouring ground. 



The young Medusa lives some time under this form. Then one of 

 the polyps becomes enlarged and its form cylindrical. This cylin- 

 der is divided into from ten to fourteen superposed rings. These 

 rings, at first smooth, form themselves into festoons, and separate into 

 bifurcated thongs; the intermediate lines become channeled. The 

 animal now resembles a pile of plates, cut round the edges. In a 

 short time each ring is stirred at the free edge of its fringe : this 

 becomes contractile. The rings are individualised. Finally, these 

 annular creatures, obscure in their lives, isolate themselves. When 

 detached, they begin to swim : from that time they have only to 

 perfect and modify their form. From being flat, they become concave 

 on the one side and convex on the other. The digestive cavity the 

 gastro-vascular canals become more decided ; the mouth opens, the 

 tentacles are elongated, the floating marginal cirri become more and 

 more numerous ; and now, after all these metamorphoses, the Medusa 

 appears : it perfectly resembles the mother. 



We have already said that recent researches have led to a separation 

 of a class of animals from the Sertularia, and to their being united 

 with the Medusae. Of these creatures we formerly only knew one of 

 the forms, namely, the polyp form ; or, rather, the first stage of it. 

 During their earliest days they possess a polyp, furnished with ten- 

 tacles, and a bell-shaped body. During their medusoid age, they present 

 a central stomach, with four canals in the form of a cross, and four 

 to eight tentacles with cirri. The animals constitute the Tubularidse, 

 comprehending many genera ; among others the Tubularia and Cam- 

 panularia, in studying which Van Beneden of Lou vain discovered most 

 interesting facts connected with the subject of alternate generation. 



The class of zoophytes ranged among the Tubularia have the power 

 of secreting an inverting tube of a horny nature, in which the fleshy 

 body can move up and down, expanding its tentacles over the top. 

 Others of them give forth buds, each of which takes the form of a 

 polyp, and these, being permanent, give it a shrub-like or branched 



