THE ECHINOIDEA 



319 



We have seen that the upper parts of the ambulacra of Clypeaster, etc., 

 are modified into petals. In one section of the Atelostomata there is an 

 analogous expansion of the ambulacra round the peristome into " floscelles." 



711 

 VI 



FIG. XXXVIII. 



The types of ispatanjiuiil Sit-ma. 1, merldosternoas ; 3, 4, ainphisternou.s ; _', intermediate. 



1, Ei-ld iint-tii'ii* KI: a t"t if ; '-, Tujvxter ticni'di '.mum ; '.i, <'i.(s*!.<!nlu3 pacijicux ; 4, Sjuj.t(in<jus purpureus. 

 in. mouth ; , anus. 



The peristomial interambtilacral plates are raised into projecting ridges 

 known as " bourrelets " ; while the ambulacra are expanded into leaf-shaped 

 areas known as " phyllodes." The pore-pairs of the phyllodes are much 

 larger than those of the rest of the ambulacrum. The five bourrelets 

 and five phyllodes together form the ttoscelle, which is typical of the 

 Cassidulidae. 



Among the Atelostomata large spines like those of Cidaris are never 

 found. The spines are very 

 numerous and generally small, 

 forming a fur over the test. 

 In some cases specially mudi- 

 fied spines occur crowded 

 together along bands (Figs. 

 XXXIX.-XLI.), forming "fas- 

 cicles." There are five dif- 

 ferent varieties : (1) The 

 " peripetalous," which encloses 

 the petaloid portions of the 

 ambulacra ; (2) the " subanal :: 

 (a/ in Fig. XXXVIII. 4), which 

 encloses a space, or "plastron," below the anus; (3) the "marginal," along 

 the border of the test ; (4) the " internal," which crosses the petaloid 

 portions of the ambulacra; and (5) the two "lateral," which run from 

 the peripetalous to below the anus. All five kinds of fascicles never 

 occur together in the same Echinoid. 



KK;. XXXIX. 

 Simple fascicle on Agctssizia. 



