ii 4 MANUAL OF ZOOLOGY SECT. 



A few inhabit the deep sea, and have been dredged from 

 as great a depth as 2000 fathoms. Nearly all are free- 

 swimming in the adult state ; some, however, live on coral- 

 reefs or mud-banks, and are found resting, in an inverted 

 position, on the ex-umbrella ; and a few, such as Lucernaria, 

 are able to attach themselves at will by a peduncle. Many 

 are semi-transparent and glassy, but often with brilliantly 

 coloured gonads, tentacles, or radial canals. In many cases 

 the umbrella, oral arms, etc., are highly coloured, and some 

 species are phosphorescent. They are all carnivorous, and, 

 although mostly living on smaller organisms, are able, in the 

 case of the larger species, to capture and digest crustaceans 

 and fishes of considerable size. 



3. THE ACTINOZOA 



The simplest and most familiar of the Actinozoa are the 

 Sea-anemones, which are to be found attached to rocks, 

 seaweeds, shells, etc., on the sea-shore. When expanded a 

 sea-anemone has the form of a cylindrical column attached 

 to a rock or other support by a broad base. The distal or 

 free surface of the column, termed the disc or peris to me, 

 bears in the middle an elongated, slit-like aperture the 

 mouth. Springing from the disc and encircling the mouth 

 are numerous cylindrical tentacles, disposed in circlets, their 

 total number being some multiple of five. 



Obviously the sea-anemone is a polype, formed on the 

 same general lines as a polype of the Hydrozoa. But 

 certain important differences from the Hydrozoan polype 

 become manifest when we examine the internal structure 

 (Fig. 54). The mouth does not lead at once into a spacious 

 undivided enteric cavity, but into a short tube (gitl), having 

 the form of a flattened cylinder, which hangs downward 



