BIHANG TILL K. SY. VET.-AHAD. HANDL. BAND IS. AFD. IV. N:0 1. 43 



These last everywhere arise from tlie peristomal middle of 

 one interradium and are inserted upon the bifureate ends of 

 tlie compasses; their function is as yet not properly under- 

 stood. The combination of movements between every two py- 

 ramids and the interjacent rotula are brought about by means 

 of the ten pairs of small rotnlar muscles; five pairs of ni. rot. 

 int., and five of m. ro. ext., Fl. VI, fig. 39, å2, 44, 45, 46,\\\ 

 the Cidaris papillata Leske; fig. 49 in Eohinometra Lucnnter 

 L.; fig. 51, in Asthenosoma varium Grube. The artieular snr- 

 faces of the rotula and the epiphysis present, in the former, 

 near the external end, a pair of oval condyloid eminenees, cond.. 

 each of them received into a glenoid cavity. gJ., of the ad- 

 joining epiphysis; and in the latter, near its inner end, a tu- 

 bercle, /w., received into one of two corresponding hollows, fo., 

 in the rotula. The m. rot. hit. are attached to either epiphy- 

 sis at the inner, alveolar side of the tubercle, fig. 42, 45, 49, 

 51, and inserted upon the rotula close under its projecting 

 upper margin. The m. rot. ext., fig. 46, are attached to either 

 epiphysis on its inter-muscular side, behind the margin of the 

 glenoidal cavity, and inserted on the sides of the rotula close 

 under its uppei" margin. 



The two principal sets of motor muscles: the protractors 

 and the retractors, are inserted on the outer sides of the py- 

 ramid and have their fixed attachments on the peristome, but 

 in a diflFerent manner in the two groups. The protractors in 

 both originate from the interradium, the retractors from the 

 auricles, but these are in the Cidaridie interradial, in the Ec- 

 tobranchiates ambulacral. We have had a glance at an early 

 stage of their development in a Cidarid, the still astomous 

 Goniocidaris, TI. III, fiig. 22. Both pairs arise from the so- 

 litary first interradial plate, one of each pair, close together, 

 from a slight eminence. But this solitary plate, at this stage 

 their common support, ^vill gradually disappear through re- 

 absorptioh, and their attachments will be removed to the first 

 pair of binary plates, and from thence to the following pairs. 

 This we see to have been done in the adult. In the Cidaris 

 papillata, VI. Yl, fig. 39 — 42, the two long, thin, subtriangu- 

 lar protractors have their narrow, upwards slightly diverging 

 attachments close together near the converging margins of 

 the auricular branches, fig. 39, 5 a; fig. 40, 5 h; fig. 41, 5 a, 

 5 h. They ascend, expanding and applying themselves to the 



