342 



COMPARATIVE ANATOMY 



CHAP. 



moving from left to right may be marked a, the second //, and these letters may 

 further indicate the rows to which these plates belong. In this way we can 

 name each of the ten ambulacral plates bordering the i)eristome. Examining 

 these ten plates carefully, we see that those indicated by la, Ila, III?', IVffi, Yh 



'^'^^ 



Fio. 302.— Kleinia luzonica (Gray). Apical system, spread out (after Loven). fa, Fascicles. 

 Further explanations in tlie adjoiniugtext. 



are larger and possess two pores each, while the smaller plates lb, lib, Ilia, l\b 

 and Ya have only one pore each. Only the ambulacra I and V, i.e. the two 

 posterior ambulacra, are thus bilaterally symmetrical, while the two (paired) 

 anterior ambulacra II and IV, and the two rows of plates of the anterior unpaired 

 ambulacrum III, are asymmetrical. This law holds for all Echinoidea (not only 

 for adults but for their young stages also) ; the plates la, Ila, III?), IVa, Yb are 



