340 



COMPARATIVE ANATOMY 



CHAP. 



adjoining apical system, tliese plates being seijarated by meridional (perpendicular)' 

 sutures. 



The plates of the Ecliiaovh'a are most frequently pentagonal. In the two ]ier- 



pendicular rows of an ambulacrum or 

 c\,\yy an interamliulacruni the consecutive 



^ plates usually alternate in such a way 



that the suture between the two row.s 

 forms a zigzag line. The sutures 

 '•< x>-^Ti^ / i ^^^ between the plates, which lie one 



,Y~^S^i}Z^\'^C- \ below the other in a row, usually run 



\' ''*• '--^ \\r~? ' horizontally (Fig. 232, p. 291). 



D 



P<^5^v^ ^ . r "VnWT^' ) (*) "^^ Pores perforating the Plates 



^TIXk^CI^^^ .^^^^^yAvVY °^ *^^ Ambulacra! System. 



■ - ^^li/Ttv'- / As a rule, in the Echinoidea, the 



$f^Sj^~r^ jl<\kl/> pores occur in pairs. These double 



' (^ jy \5iir§5S2l""^/-C\ ' ' pores occur only on the ambulacral 



TVjII/iy^jri, -> i^lates. One double pore belongs to 



/■~\^^-;^:j^^^ each ambulacral foot.^ From the 



. . , ^ ^ J. . . . amiiuUa, under the test (at its inner 



Pig. 301.— Apical system and adjommg pen- ■j\ + + i- i 



some of Melonitas multipora, Norw. (after Meek side), two canals run out, which, 

 and Worthen). For lettering see p. 317. running separately through the plate, 



unite at the base of the tube-foot to 

 form a single canal, which runs through the foot and ends blindly at its tip. 

 Originally, there was only one pair of pores on each ambulacral plate. Where two 

 or more pairs occur on one plate, the plate can bo proved to be composed of just as 

 many fused plates as there are pairs of pores. Primary plates are such as reach 

 from the lateral edge of a two-rowed ambulacrum as far as the median suture 

 between the two rows of ambulacral plates. Half plates are such as do not reach 

 the suture, and included plates such as do not reach the edge of the ambulacrum. 

 Isolated plates reach neither the edge nor the median suture of the ambulacrum. 



Besides the double pores there are, in the GlypcnstniUhi, and Spatangoida, single 

 pores as well, to which small tentacles belong. The arrangement of these pores 

 varies, and they are often not confined to the ambulacra, but are also found on the 

 iuterradii, especially on the oral surface. Occasionally they are scattered, often in 

 grooves, the so-called ambulacral grooves, which radiate out from the peristome, 

 and may stretch more or less far towards the ambitus or even beyond it, and may 

 be more or less branched. 



(c) The Symmetry of the EcMnoid Shell. 



The test of the regular Echinoids {Cidaroida, Diadematoida, and most 

 PalccecMiioidea), viewed superficially, appears to be strictly radiate. The anal area 

 lies at the apical, and the oral area at the diametrically opposite oral pole. All the 

 ambulacra and interambulacra appear similar one to the other, and the ambitus, 

 with few exceptions, is circular or regularly pentagonal with rounded corners. In 

 the Holectypoida also the test, as a rule, appears radial, with regard both to the 

 circular (or regularly pentagonal) form of the ambitus and to the similar develop- 

 ment of the ambulacra and interambulacra. The peristome occupies its place at the 

 centre of the oral surface. Notwithstanding this, the longitudinal axis and the 



^ For the different forms and arrangements of these feet or tentacles, cf. section on the 

 ambulacral system, p. 416 et seq. 



