THE BIOLOGY OF THE SEA-SHORE 



being exceedingly long and forming a whip-like process which 

 sweeps through the water. 



Structures with a similar function to those of avicularia 

 are characteristic of many Echinoderms. These are the 



Fig. 7. — Pedicellariae of sea-urchin 

 (Echinus), (i) Tridactyle (after Chad- 

 wick) ; (2) Gemmiform (after Chadwick) ; 

 (3) Ophicephalous (after Chadwick) ; (4) 

 Trifoliate (after Uexkull). 



pedicellariae (see Fig. 7), which may be regarded as modified 

 spines. They occur scattered over the surface of the starfish 

 and sea-urchin, but reach their greatest development in the 

 latter group. In Echinus esculmtus there are four varieties 

 of pedicellarias : *' tridactyle," " gemmiform," " trifoliate," 



