344 



COMPARATIVE ANATOMY ' 



CHAP. 



As a further example, let us take the test of a young To,ro2mcnstes drcebachiejisis, 

 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 primaiy plates) and are 

 j)erforated by only two double pores. We can place the test in only one ])osition, viz. 

 that given in the figure, in which the formula la, lla, lllb, IVa, \b, and Ih, lib, Ilia, 

 lYb, \a holds good. In this we see that a median plane, corresponding with that of 

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

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

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

 II and III. 



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

 the peristome. 



It may be remarked in i)assing here that the system of marking above described 

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

 the ambulacra I-V, the ambulacral rows of plates la, Ih, lla, lib. Ilia, lllh, IVa, 

 IV&, Va, and \b, and in the ajiical system the radials I-V. If we mark the inter- 

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

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

 we get the interanibulacral rows of plates la, 16, 2a, 2b, 8a, Sb, 4a, 4&, 5a, 5b, and 

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



along each row of ambu- 

 lacral and interanibulacral 

 plates, start from the edge 

 of the oral disc. 



The arrangement of 

 l^lates 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 Echinoids, show 

 us that, strictly speaking, 

 no Echinoid is either radi- 

 ally, or bilaterally, sym- 

 metrical. 



((/) The Relation of the 

 Ambulacral and In- 

 terambulacral Plates 

 to the Peristome. 



Three cases must be 

 distinguished. 



1. Tlie plates, both of 

 the arabulacraand 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 (Diade- 

 matuida), forming either several concentric rings of plates {Streptosomata, Echino- 

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

 buccal plates [Stereosomata). 



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

 of Cidaris hystrix, Lamk. (alter Lov^n). 



