ECHINOIDEA. 555 



are each perforated by a pore which transmits the azygos tentacle and 

 the nerve to the eye speck. The pore is double in many Palaeo-echinoidea. 

 From their relation respectively to the genital apertures and the eye specks 

 the basals and radials are generally termed genitals and oculars. The 

 plates of the ambulacra abut against the radials, of the interambulacra 

 against the basals, and new plates in both these series are added only at 

 the margin of the apical system, between it and the plates already present. 

 And as new plates are added successively, the older plates recede towards 

 the peristome. Consequently they extend in meridional lines from the 

 apex to the margin of that area. One ambulacrum is anterior ; one inter- 

 ambulacrum posterior, and easily recognisable by the presence of the anus 

 in it in exocyclic forms, i. e. Clypeastroidea and Petalosticha. In these 

 forms a plane of bilateral symmetry is established through the two 

 meridians named. The ambulacra, one on either side the anterior ambu- 

 lacrum, constitute with it the trivium : the remaining two, one on either 

 side the posterior interambulacrum, make up the bivium. It has been 

 found that the plates in the ambulacra immediately bordering the peri- 

 stome show certain constant peculiarities in the trivium and bivium, and it 

 is consequently possible to recognise the corresponding tracts in the endo- 

 cyclic forms, i. e. Desmosticha, which are characterised by their regular and 

 radial symmetry 1 . Bearing this plane of bilateral symmetry in mind, and 

 imagining the Echinoid as lying with the peristome downwards, there are 

 two interambulacra on the left and right sides, and the right anterior basal 

 is somewhat enlarged and porous forming the madreporite. In some 

 Clypeastroidea the madreporite extends into the centre of the apical system 

 and in the majority all the basals and radials are porous and the sutures 

 between them fused. In this case the genital apertures are frequently 

 displaced outside the apical system in the interambulacra. In many 

 Spatangidae the madreporite extends backwards into the posterior inter- 

 ambulacrum, thus dividing the apical system, and in this case the left 



1 If a Spatangus be placed mouth upwards the reverse of the natural position and the ambu- 

 lacrum in the bivium to the left of the observer be numbered I and that to the right V, and the remain- 

 ing ambulacra II, III, IV, from left to right ; and if the two plates in each ambulacrum bordering the 

 peristome be lettered a and b, following the same direction from left to right, then it will be found 

 that the plates I a, II a, III b, IV a, V b are large and are pierced by a double pore, while the plates 

 I b, II b, III a, IV b, V a are small and pierced by one pore. The basal plate of the interradius 

 between II and III is the madreporite. Taking this same plate and interradius as a guide, Loven 

 has shown that if the ambulacra and plates in any other Echinoid be marked in the same manner, 

 the same sequence of large and small plates with more and fewer pores may be noted. Even in the 

 Cidaridae, where the pores are alike on all plates, a difference of size, &c. is noticeable. It may be 

 added that the larger plates in the bivium are placed symmetrically with reference to the odd inter- 

 ambulacrum. See Loven, A. N. 39, 1873, or Etudes sur les Echinoides, Kongl. Svenska Vetenskap- 

 Akademiens Handlingar. xi. No. 7. Stockholm, 1874; also Agassiz, Challenger Reports, iii. 1881, 

 for a criticism. 



The ambulacra II and III are the two that form the bivium in the Holothurioid, I, IV, V the 

 three that form trivium. 



