36i 



CHAPTER XXII. 



5 UB - KINGD 031 IV.— E CHINODERMA TA. 



General Characters of the Echinodermata. 



The sub - kingdom of the Echinodermata comprises the animals 

 usually known as Sea-urchins, Star-fishes, Brittle-stars, Sea-cucum- 

 bers, Sea-lilies, &c., and may be briefly denned as follows : — 



Simple marine organisms, which are mostly bilaterally symmetrical 

 when young, but which in the adult condition have this bilateral sym- 

 metry more or less extensively masked by a radial (usually pentamerous) 

 arrangement of their parts. An alimentary canal is present, with or 

 without a distinct anus, separate from the proper body-cavity. A 

 system of water-vessels^ often communicating directly with the exterior, 

 and generally connected with protrusible tubes ("feet"), is present. 

 The nervous system is radiate, consisting of an oesophageal ring and 

 radiating branches. The integument is characteristically hardened by 

 the deposition in it of carbo?iate of lime in the form of plates, granules, 

 or spicules. 



The adult Echinoderms always exhibit a more or less conspicuous 

 radiate structure, the symmetry of the body being essentially penta- 

 merous ; but the larval forms are usually distinctly bilateral, and bi- 

 lateral symmetry is in general more or less clearly recognisable in the 

 fully grown animal. The radial symmetry of the body of the adult 

 is exhibited not only in the external skeleton, but also in many of 

 the internal organs, and especially in the nervous and ambulacral 

 systems. The alimentary canal, unlike that of the Ccelenterates, is 

 always completely shut off from the general cavity of the body, a 

 mouth being always present, and an anal aperture usually existing as 

 well. When a mouth and anus are present, these openings may be 

 at opposite poles or on the same side of the body, the mouth being 

 usually central in position, while the anus may be completely ex- 

 centric. In the free-living Echinoderms, the mouth is usually either 

 terminal and placed in front, or is situated in the centre of the in- 



