256 



ECHINODERMS OF THE BRITISH ISLES 



in each arc. The tube-feet are always situated near the outer 

 edge of the plate, and there, accordingly, the corresponding 

 pore series are found on the naked test. Forms with three pairs 

 of pores in each compound plate are designated " oligojwrous " , 

 those with more than three pairs of pores in each compound plate 



Fio. 140. — Ambulacral plates of various Echinoids, 

 (From Danmarlc's Fauna.) 



x6. 



1. Simple ambulacral plates of £«eutorjs (nftt/Zoic^es. 2. Compound, oligoporous plates 

 of Echinus elegnns. 3. Compound, polyporous plates of Strong ylocentrotus drobachienns . 

 po. Pores of the tube-feet ; pr. Primary plates ; prt, Primary tubercles ; st, secondary 

 tubercles. 



''polyporous^' (Fig. 140). This is a character of very great 

 systematic importance. In the living animal the test is covered 

 with a thin, ciliated skin. It is thus really an inner skeleton, like 

 the skeleton of the vertebrates, not an outer test that encloses 



Fig. 141. — Apical system of Echinoids. x 6. 



(From Danmark'' s Fauna.) 



1. Of a regular form, Strongylocentrotus drobachie.nsis. 2. Of an irrestular form, 

 Spatangus pnrpureus. a. Anal plate ; af. Anal area ; an. Anal opening ; g. Genital plate ; 

 gp. Genital pore ; m, Madreporite ; o, Ocular plate ; op, Ocular pore. 



the body of the animal, as is the case with the shell of the 

 molluscs. 



In the middle on the upper side is situated a small, distinctly 

 limited area, the apical system (Fig. 141), consisting of a circle 

 of 10 plates, 5 situated radially, off the ambulacra, designated 

 the ocular plates or oculars, and 5 situated interradially, off the 

 interambulacra, designated the genital plates, because the 

 openings of the genital organs are situated in them. In the 



