RADIATA. 153 



CHAPTER XII. 



RADIATA. 



DESCENDING in the scale of animal organization, we 

 come next to the subkingdom RADIATA, or that in 

 which the parts are arranged in a radiate manner 

 around a centre. Of this there are three classes, the 

 ECHINODER'MATA (e^tvo?, hedgehog, Sepfia, skin), con- 

 taining the Sea-urchins (Echinus), Starfishes, &c., in 

 which the skin is furnished with hard calcareous 

 projecting spines or curiously formed imbedded cal- 

 careous corpuscles, forming a rudimentary skeleton ; 

 the ACALEPH.E (a/caXr}^, a nettle), or Sea-nettles; 

 and the POL'YPI (77-0X1)5, many, TTOU?, foot), to which 

 we shall confine our notice. It may be remarked 

 that the last two classes have recently been united to 

 form the single class CCELENTERATA (icolKov, hollow, 

 evrepovj intestine). 



POLYPI. These animals are mostly marine. They 

 are either single (PL XI. fig. 5), or compound (PL XI. 

 fig. 15), i.e. the bodies are united ; in the latter 

 case the bodies being usually situated in horny cells 

 upon a branched polypidom. But in many of them, 

 which do not occur in this country, there is an in- 

 ternal solid calcareous skeleton, of which coral is an 

 example. The animal bodies are soft, and furnished 

 at the front end with a crown of tentacles (fig. 15 a) 

 these are contractile, and serve to enable the animals 

 to catch their prey. The horny, branched, and plant- 

 like polypidoms are often found on the seashore, and 

 are popularly confounded with sea-weeds. 



Hydra vulgdris (PL XI. fig. 5) is a fresh-water 

 species, which is commonly met with among collec- 



