344 



GOMPARA TIVE ANATOMY 



CHAP. 



As a further example, let us take the test of a young Toxopncustrs drcebachiciisis, 

 4 mm. in diameter (Fig. 303). If we examine it we shall find that, of the ten 

 ambulacral plates bordering the peristome, five, belonging to different ambulacra, 

 are of greater size (consisting each of three primary plates), and show three double 

 pores, while the five others are smaller (consisting of but two primary plates) and are 

 perforated by only two double pores, ff e can place the test in only one position, viz. 

 that given in the figure, in which the formula la, Ila, I1I6, IVa, VJ, and lb, lib, Ilia, 

 IVJ, Va holds good. In this we see that a median plane, corresponding with that of 

 the iircijular Echinoidea, can be established also for the regular Echinoidea. The 

 accuracy of this law can be proved by investigating the position of the madreporite. 

 In the above'case this actually lies in the right anterior basal plate between the radii 

 II and III. 



Loven's law also ajiplies to other plates besides the ambulacral marginal plates of 

 the peristome. 



It may be remarked in passing here that the system of marking above described 

 can be used for naming all the plates of the Eohinoid test ; in this way we have 

 the ambulacra I-Y, the ambulacral rows of plates la, Vi, 11a, lib, Ilia, III6, IVa, 

 IV6, Va, and V6, and in the apical system the radials I-V. If we mark the inter- 

 radii (interanibulacra) 1 -5, starting from the one lying to the left of ambulacrum I, 

 .ind proceeding in the direction of the hands of a watch (viewing the test orally), 

 we get the interambulacral rows of plates la, 16, 2a, 2b, 3a, 3J, 4a, 46, 5a, 56, and 

 the basals 1-5. The madreporite lies in basal 2. The consecutive plates, counting 



along each row of ambu- 



3 



lacral and interambulacral 

 plates, start from the edge 

 of the oral disc. 



The arrangement of 

 plates revealed by Loven's 

 law, taken together with 

 the special position of the 

 madreporite, and with the 

 excentric position of the 

 anus in the anal area of 

 the regular Eohinoids, show 

 us that, strictly speaking, 

 no Echinoid is either radi- 

 ally, or bilaterally, sym- 

 metrical. 



{d) The Relation of the 

 Ambulacral and In- 

 terambulacral Plates 

 to the Peristome. 



Three cases must be 

 distinguished. 



1. The plates, both of 

 the ambulacraand of the interambulacra, arecontinued in a modified form over the edge 

 of the peristome, and on the peristome itself, towards the mouth (Cidaroida, Fig. 304). 

 2. Only the ambulacral plates are continued on to the oral integument (Biade- 

 matoida), forming either several concentric rings of plates (Slreptosomatd, Ecldno- 

 thuridce), or as five pairs of plates lying isolated in the integument, the so-called 

 buccal plates {Stereosomata). 



Fig. 304. —Peristome and neighbouring parts of tlie test 

 ot Cidaris hystrix, Lamk. (after Lov^n). 



