2G0 ECHINODERMS OF THE BRITISH ISLES 



pedicellarige are exceedingly varied in structure. Usually they 

 are found of four different types, namely the globiferous pedi- 

 cellaria?, which usually carry poison glands on the valves, the 

 trideniale, ophicephalous , and triphyllous pedicellarise (Fig. 143). 

 In some forms (Cidarids) the ophicephalous and triphyllous 

 pedicellarire, in others (Saleniids, Arbaciids) the globiferous, are 

 lacking ; in others (Spatangoids), again, a fifth form, the rostrate 

 pedicellarise, are found. These peculiar, small organs are of very 

 great svstematic importance and will be mentioned more in detail 

 under the various groups. 



The sph(^ridia (Fig. 145) are quite small, glassy, spherical or 

 oval bodies, attached in the middle of the ambulacra or near the 



tube-feet, from the edge of the peri- 

 stome some w^ay upwards on the test. 

 They are lacking in the Cidarids ; in 

 ^"^ " ^ the Clypeastroids they are concealed 



.^^ , in grooves in the test. They are 

 ''' ^-^Z^/ supposed to have the function of static 



or of taste organs. For classification 

 they are of very small importance. 

 ^ .^. ^ /•• The tube-feet in the regular Echi- 



noids end in a sucking disk, containing 



Fig. 145.— Lower part of am- ^ beautiful calcareous skeleton com- 

 bulacral area of Stroncpiloc. ., „ . , . . . 



drohachiensis, showing in posed of various parts. As a rule the 



the mid-Hne between the foot contains more or less numerous 



s^esTf'sph?4idiar'some'^ spicuUs of various forms, irregular 



what enlarged. (After rods, or C-shaped bodies (" bihamate " 



^anta') ^'''"' ^"'''^'''^'^''' spicules) (Fig. 144, 4). In the Spatan- 

 goids the tube-feet are very diversified, 

 penicillate, leaf-shaped, or quite small and simple ; in the Ch^pe- 

 astroids they are usually very small and numerous, spreading also 

 over the interambulacral areas. Especially in the regular Echinoids 

 and, parth% the Spatangoids the tube-feet are exceedingly ex- 

 tensible. To each tube-foot, excepting the small, simple ones in 

 Clypeastroids and Spatangoids, correspond two pores in the test. 

 The gills are bush-shaj)ed outgrowths from the peristomial 

 membrane, 10 in number, one pair at each interambulacrum. 

 On the test is seen a cut in the peristomial edge, more or less deep, 

 corresponding to each gill (Fig. 139, 1). In the Cidarids and the 

 irregular Echinoids gills are lacking. 



The mouth is in the regular Echinoids and the Clypeastroids 

 provided with 5 strong teeth, which are fastened in a very compli- 

 cated apparatus, the so-called "Lantern of Aristotle" (Fig. 146), 



