48 Invertebrata. 



found in most species. They display a radiating sym- 

 metry, but either in the larval or in the adult condition 

 there are traces of a bilateral disposition of parts. A 

 body cavity is always present, containing within it the 

 digestive organs, and the stomach cavity never com- 

 municates with this surrounding space in the perfect 

 or adult animal, as it does in the Coelenterates. The 

 Echinoderms possess a nervous system of radiating 



FIG. 27. 



Section of the purple Sea Urchin (Strongylocentrotits Ihndus) : a, anus ; 

 % oesophagus ; f. intestine ; s, one of the rods of the tooth apparatus ; 

 nt. muscles of the jaws , fl, vessels of the sucking feet.; po, extremity of 

 tne water vessel ; c a, ocular plate ; v, ovary. 



threads ; united by a ring of nerve-matter around the 

 mouth, and some of them exhibit pigment masses, 

 which are supposed to be simple eyes containing the 

 ends of nerve fibrils embedded in them. Calcareous 

 matter is deposited in the skin, either in the form of 

 spicules, or of plates which, by being jointed together, 

 build up a shell or outer ' test ' for the body. 



When one of our common Echinoderms is put into 

 a vessel of sea-water, locomotion can be seen to take 



