456 



CHARACTERS OF INVERTEBRATE ANIMALS 



star-fish. The stomach is not capable of being protruded as in 

 one of the ordinary Stars. 



CLASS III. SEA-URCHINS (ECHINOIDEA) 



Among the animals commonly brought up by the dredge in 

 British seas, and sometimes thrown up on the beach by storms 

 is the Edible Sea- Urchin (Echinus esculentus) (fig. 280), which, 



A. 



AMBU 

 AR.EA 



AMBULACRAL 



AREA 



ADREPORITE 

 NTESTINAL 



Fig. 280. Edible Sea-Urchin (Echinus esculentus] 



B, c, D, Enlarged to various scales. A, Diagram of upper surface, with most of spines removed. B, Plates of 

 small part of an ambulacral and an interambulacral area, with spines removed. Pores for tube-feet and knobs for 

 spines are shown, c, Diagrammatic section through spine and knob, to show mode of attachment. D, Pedicellariae. 

 E, Diagram of side-dissection, most of gut having been removed, w.v. is an abbreviation for " water vascular". 



though it differs very much in appearance from a star-fish, is really 

 constructed on much the same type. The spheroidal body sug- 

 gests in appearance a curled-up hedgehog, whence, as previously 

 explained, the name " sea-urchin ", the appearance being due to 

 the presence of an enormous number of spines, movably jointed 

 to the underlying calcareous plates which, united firmly at their 



