ECHINOIDEA. 221 



are, with regard to their size or pores, symmetrical with each other, while 

 the corresponding plates of the trivium are not symmetrical. To parti- 

 cularize further let us take the case of Spatangoids (Fig. 163) : if the shell 

 be viewed from the oral pole and the trivium be directed forward and the 

 bivium backward and the radii be numbered according to the plan used above 

 for Echinoids, viz. in such a way that the right bivial radius (to the left of 

 the observer in this view which is ventral) be numbered I and the left 

 bivial radius V, the other radii being numbered II, III, and IV, and if 

 further the two rows of plates (Fig. 163) in each radius be called a and b, 

 in such a way that row a in radius I is the row next the posterior interradius, 

 and so on all the way round, so that in radius V the row next the posterior 

 interradius is row b, then it is found that the ambulacral marginal plates 

 I a, II a, III b, IV a, V b possess two pores and are larger than the others, 

 viz. I b, II b, III a, IV b, V a, which also only possess one pore. Further 

 it will be observed that the ambulacral marginal plates of the bivial radii, 

 viz. I and V, are symmetrical with each other, while the corresponding 

 plates in the lateral radii of the trivium, II and IV, are asymmetrical. This 

 law, namely that the marginal ambulacral plates of radii I and V, are 

 symmetrical with each other, while those of radii II and IV are asym- 

 metrical holds for most Echinoidea. 



In the regular forms, e.g. Strongylocentrotus droebachiensis (Fig. 164), 

 the same law is followed, the difference in the marginal ambulacral plates 

 consisting in their size and in the number of primary plates of which they 

 are composed. 



In Clypeastroids Loven's law is followed only with regard to the size 

 of the plates, and not with regard to the pores, and in some genera its 

 application is very difficult to make out. In the Pourtalesiidae it does not 

 hold at all for the majority of the species, the constitution of the peristome 

 in this family being different from that of other Echinoids. If the law 

 has the importance which Loven attributes to it, it is most remarkable 

 that in the same family it should hold for some species and not for 

 others. 



Pedicellariae * are always present. They have stalks contain- 

 ing a calcareous rod, and three, rarely four, calcified blades 

 (Fig. 166). The bases of the blades are broad and usually 

 concave on the inner side, the concavity being traversed by a 

 vertically directed crest, the a-pophysis (Fig. 167, a). The 

 blades are articulated together basally and not with a special 

 calcareous piece. There is a special musculature for moving 

 the blades, and the stalks are movable on the shell. Glands are 

 frequently present on the stalk or on the outer sides of the 

 blades. The pedicellariae are covered with a ciliated epithe- 

 lium. On the inner sides of the blades patches of this epithe- 

 lium may be modified as special organs of sense (Fig. 165), the 



* Mortensen, op. cit., p. 4. Von Uexkiill cited on p. 224. Prouho, 

 Arch. Zool. Exp. et gen (2), 5, 1887, p. 213. C. Stewart, Journ. R. Mic. Soc. 

 3, 1880, p. 909. 



