44 SVEN LOVEN, p:chinologica. 



external snrfaee of tlie pyramid, and are inserted npon tlie 

 npper half of its convex sides and npon the epi])liysis, in the 

 narrovv submarginal groove, fig- ^^> 5 h; 41, 5 a. x\bove them, 

 on either side, one of tlie two retractors takes its origin on a 

 line foUowing the npper arehed crest of each brancli, and the 

 two pass downwards undei- the protractors, their fascicles con- 

 verging, become laterally iiattened and compressed, and are 

 inserted nnder the protractors, in the lower continnation of the 

 longitudinal groove that follows the margins of the p3'ramid, 

 fi(f. 40, 5 h; 41, 5 a. Thns every single ])yrainid receives all 

 its mnscles from one and the same interradinm. 



It has been seen, above p. 14, Pl. IV, flg. 2'j, 20, how in the 

 astomons Ectobranehiate the protractors of each pyramid are 

 attached, close together, at iirst to the adoral margin of one 

 solitary peristomal interradial jilate, and latei-, IH. V, fig. 80, 

 wlien this plate has been reabsorbed, to the succeeding pair of 

 iuterradials, — all this as in the Cidaris — bnt that the 

 Ectobranehiate, very dissimilar in this, has tlie two retractors of 

 every pyramid attached on different ambnlacra, each on a ri- 

 sing auricnlar braneh, fig. 26, 30, 81. In the adult, as it is 

 vvell known, for example in the Echinometra Lucnnter L., 

 Pl. VI, flg. 47 — 40, the origin of the broad and powerful 

 protractors is the same as in the young, on the inside of the 

 low interradial ridge. From thence they ascend Avith equal 

 breadth and become inserted on the epiphj^sis and npper sides 

 of the pyramid. AVith the retractors it is otherwise: 



j 

 Tlie pyramid 1 lias its protractors on 1, its retractors on I h, II a; 



■2 ~ * > 2, * II b, III a; 



3 : » » ;-5, . Ill h, IV a; 



»4 4. IV h, v a; 



» ö -> ;'), i > .> \ 11, I a; 



and the two anricnlar branches which support the retractors 

 of one and the same pyramid, in growing npwards have strongly 

 diverged and increased the distance between their muscles in 

 arching över their own ambulacrnm, to such an extent that 

 their expanded tops sometimes have come te coalesce with 

 those of the branches snpporting each a retractor of the nea- 

 rest pyramid, and to bring close together their attachments. 

 The retractors generally have a coarser textnre, of larger fas- 

 cicnli, than the protractors, and a darker colonr. 



