BIHANG TILL K. SV. YET.-AKAD. HANDL. BAND 18. AFD. IV. N:0 1. 45 



Tlie arrangement of these parts in the Asthenosoma va- 

 rium Grube, Pl. VI, fig. 50, is that wliich is typical of tlie 

 Ectobranchiates. The tender auricles have tlieir stånd on tlie 

 inside of the ambulacral columns. Every entire plate is raised 

 outside the pores near its outer end, which projects under tlie 

 edge of the interradium, and forms a groove for the slightly 

 expanded base of the anricular brancli, which also projects 

 into a heel contiguons to the raised margin of the interradium. 

 The branch then bends över the ambulacrum and joins the 

 opposite branch, the two thus forming a low arch. The stri- 

 kingly thin and feeble auricle, based as it is upon the flexible 

 and constantly moving ambulacrum, is hardly equal to the 

 strain produced by the retractor muscles which combine to 

 draw it inward. Tliis is prevented l)y the action of antago- 

 nistic muscles inserted upon the outer edges of the anricular 

 arch and originating from the longitudinal muscles, ^ and others 

 inserted on the inner side of the anricular branches and at- 

 tached to the buccal membrane. The retractors of each pyramid 

 arise from the two diverging anricular branches of the two 

 nearest aml)ulacra, and the protractors from the slight inter- 

 radial ridge; all this as in the other Ectobranchiates. 



The great difference between the Regularia Abranchiata and 

 Ectobranchiata in the arrangement of the retractor muscles is 

 worth some consideration.. It has been seen that in the for- 

 mer the auricles arise from the interradia, and give attachment 

 to the retractors as well as to the protractors, whereas in the 

 Ectobranchiates they arise from the ambulacra, and give attach- 

 ment to the retractors alone. By this the fundamental dis- 

 tinction is heightened between the two coordinated constituent 

 systems of the corona, between ambulacra and interradia. It 

 is already great as it is. The ambulacra are heterotropic; their 

 plates are throughout biseriate; all but exclusively they bear 

 the pedicels, and without exception the spheeridia; simple at 

 their first formation they suecessively unite into compound 

 plates; they grow and move at their own råte, independently; 

 and when they imbricate it is in the adoral direction. The 

 interradia begin in the peristome with a solitary plate, then 

 become binary; they are only exceptionally bearers of pedicels, 

 nowhere of sphairidia; they never are compounded; they grow 

 consistently with the ambulacra, but at a råte of their own, 



' Sarasin, Ergebnisse, Ceylon I, p. 93—95, Pl. XII— XIV. 



