34 ECHINODEEMATA. 



The class Ca3lenterata contains the Corals {Aetinozoa), Dead- 

 men's Fingers (Octactinia), Sea - anemones (Sexactinia), the 

 polypoid and medusoid Hydrozoa, and the Ctenophora. A 

 polyp is a simple tubular body fixed at the posterior end and 

 pierced by an oral opening at the free end, the mouth being 

 surrounded by a circle or several circles of tentacles. Polyps 

 may reproduce by male and female cells — spermatozoa and 

 ova — or by budding. All colonial forms are produced by the 

 latter process. A medusa or jellyfish is free- swimming, and 

 consists of a flattened or arched gelatinous disc, that we so 

 often see floating on the top of the sea. From underneath 

 this disc there hangs down a stalk, the manubrium, at the 

 free end of which opens the mouth. Tentacles may be de- 

 veloped around the mouth and edge of the disc. Here in the 

 medusa we find that the distinctly defined mouth leads into a 

 canal that runs up the stalk and enters a cavity in the disc, the 

 stomach, from which canals run out to the edge of the disc, 

 where they form a circular canal surrounding it. 



A medusa may be compared to a flattened polyp. In the 

 hydroid polyp stock reproduction takes place by budding, so 

 that the individual colony increases ; but every now and then a 

 modified bud forms — a medusoid bud — in the place of a polyp. 

 This bud breaks off and floats away as a medusa, which becomes 

 sexually mature, producing ova : these ova hatch into free- 

 swimming larvae that settle upon some rock or stone, when each 

 larva turns to a polyp which creates a colony by repeated 

 gemmation. Thus we get an alternation of a fixed asexual and 

 a free sexual generation. 



EOHINODERMS. 



Starfish, Sea-urchins, and Sea-cucumbers are united into one 

 class known as the Echinodennata. All these animals are 

 marine. They are characterised by their radial symmetry. 

 They have generally a hard calcareous cxoskeleton, which may 



