1876.] and Development o/Antedon rosaceus. 219 



(fig. 4), clustered like the carpels of an orange round a central axis ; and 

 near this axis each chamber seems to communicate on its ventral aspect 

 with the surrounding space (an extension of the perivisceral cavity) by 

 a minute orifice (cl) in its wall. "Within the dorsal portion of this organ, 

 lying at the bottom of the centro-dorsal basin, there is a succession of 

 verticils of five triangular leaflets (fig. 5, «), increasing in size from 

 below upwards, from the extremities of some of the upper of which 

 leaflets issue groups of three diverging cords that proceed to the cirrhi. 

 I can scarcely doubt that these verticils mark the origins of the earlier 

 cirrhal cords from the Crinoidal axis ; and this obviously suggests that 

 the five-chambered organ is itself only another and larger verticil, which 

 has come, by the formation of ventricular cavities in its substance (ana- 

 logous to the lateral ventricles of the brain), to occupy the whole cavity 

 of the enlarged centro-dorsal basin. 



This view, which deprives the chambers of the five-chambered organ of 

 any special physiological significance, is confirmed by what I have ascer- 

 tained of the structure of the Crinoidal axis which occupies the intei'ior of 

 the stem of the true Pentacrinus. This has a central portion quite 

 distinct from the peripheral portion, which forms a cylindrical sheath 

 around it. It is from the cylindrical sheath that at every node of the 

 stem there pass off five cords into the whorls of cirrhi or " auxiliary side 

 arms ;" and at every node there is a slight dilatation of the cylinder, 

 which is caused, not by a thickening of the substance of the axis, but by 

 a separation between its central and its peripheral portions — foreshadow- 

 ing the great ventricular dilatation just described at the summit of the 

 axis of Antedon. In Pentacrinus, which has no cirrhi attached either to 

 the uppermost part of the stem or to the base of the calyx, there is neither 

 enlarged centro-dorsal cavity nor special dilatation of the Crinoidal axis. 



The divaricated cords which enter the First Hadials form a sort of 

 " Circle of Willis " within the system of canals completed by their lateral 

 adhesion, as described in my former Memoir (§§ 34, 77) ; and from this 

 circle there issue five radial cords, which speedily divaricate into ten 

 brachial cords that pass continuously through the centra of the segments 

 of the arms. 



This apparatus (consisting of the outer cylinder of the Crinoidal stem, 

 of the five-chambered central organ formed by the dilatation of that axis 

 within the centro-dorsal basin, and of the cords proceeding from it to the 

 arms and cirrhi) I regard as the central portion of a nervous system ; and 

 I shall hereafter draw what seem to me conclusive proofs that such is 

 its character, both from the distribution of branches of the brachial cords 

 to the muscles of the arms, and from the effect of irritation of the central 

 organ in the living animal, even after its complete evisceration, in causing 

 the sudden and general contraction of those muscles (p. 226). 



Axial Prolongation. — I now follow the course of the central or 

 medullary portion of the Crinoidal axis, which passes continuously up- 



