710 SCIENTIFIC RECREATIONS. 



colouring, and the creature undulates through the water in a remarkable 

 manner. 



The luminosity of the medusae is clearly perceived, the so-called phospho- 

 rescence being due chiefly to the minute jelly-fish which abound near the 

 surface of the sea. It appears impossible, for most, at any rate, if not all, 



Fig. 834. Sea cucumber. 



these medusae to sink beneath the surface, for they can be found in hundreds 

 cast ashore, melting away into film. We might imagine that they would be 

 provided with some means of sinking themselves, but being apparently only 

 air and water, it is necessary for them to remain upon the surface to exist 

 at all. 



The term Acalepha, by which they are known, means " stinging " fish 

 or sea-nettles, the Greek word meaning nettle. 



The ACTINOZOA comprise corals and the popular sea anemones (actinidae). 

 They resemble the hydrozoa in possessing tentacles, and also the two inner 



and outer tissues of the body. But they differ 

 from the hydrozoa in their interior arrangement in 

 the possession of a kind of stomach between the 

 " body cavity " 2nd the mouth which the hydrozoa 

 do not posses^. The appearance of the sea 

 anemone is well known. It fixes itself by the 

 flat base and hangs out its tentacles to obtain food. 

 When we touch an anemone with a stick we 

 perceive how it contracts itself, but there is no 

 nervous system nor any respiration. The repro- 

 duction of its species is carried on within, not as 

 in other animals, like the hydra, by exterior budding. 



The corals belong to the same class as the sea anemones, and are 

 called zoanthidae. We have already in previous portions of this volume 

 mentioned the " coral " building polypes, but we may again describe them 

 here. We have the black coral or aniipathida, which live in masses and are 



