340 



COMPARATIVE ANATOMY 



CHAP. 



adjoining apical system, these plates being separated by meridional (perpendicular) 



sutures. 



The plates of the Eckinoidea are most frequently pentagonal. In the two per- 

 pendicular rows of an ambulacrum or 

 an interambulacrum the consecutive 

 plates usually alternate in such a way 

 that the suture between the two rows 

 forms a zigzag line. The sutures 

 between the plates, which lie one 

 below the other in a row, usually run 

 horizontally (Fig. 232, p. 291). 



(6) The Pores perforating the Plates 

 of the Ambulacral System. 



As a rule, in the Echinoidea, the 

 pores occur in pairs. These double 

 pores occur only on the ambulacral 

 plates. One double pore belongs to 

 each ambulacral foot. 1 From the 

 ampulla, under the test (at its inner 

 side), two canals run out, which, 

 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 be 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 Clypeastroida 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 

 interradii, 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. 



FIG. 301. Apical system and adjoining peri- 

 some of Melonites multipora, Norw. (after Meek 

 and Worthen). For lettering see p. 31V. 



(c) The Symmetry of the Echinoid Shell. 



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

 Palceechinoidea), 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 Holedypoida 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 



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

 ambulacral system, p. 416 et seq. 



