344 



ECHINODERMATA 



[CH. 



over the rest of the body ; they are lined by ciliated cells which 

 keep up a constant current of fresh water pouring into them and 

 thus they fulfil the same function as the " dermal branchiae" of 

 star-fishes. 



Class III. ECHINOIDEA. 



The general appearance of the dried skeleton of a Sea-urchin 

 or Echinoid is familiar to most people, but many would fail to 



FIG. 159. Strongylocentrus drobacJiiensis x 1. Aboral surface. 

 1. Expanded tube-feet. 2. Spines. 



From Agassiz. 



recognise any resemblance to a star-fish in the slightly flattened 

 sphere covered with spines. If, however, we are fortunate enough 

 to see one living, we at once perceive that along five meridians, the 

 sphere is beset with beautiful semi-transparent tube-feet, ending in 

 suckers, exactly like those of the star-fish. In fact, the Sea-urchin 



