712 DE. W. B. C^VEPENTEE OjS THE STEUCTUEE, PHYSIOLOGY, AND 



a cirrhus which consists of the typical number of segments, and which presents the 

 general characters of matuiity except as regards the form of the claw, but has not 

 attained above three-fifths of the full length ; this may obviously have been produced 

 either by a more advanced development of the large rudimental form represented at f, 

 or by the process of simple increase in the small mature form shown at m. — The augmen- 

 tation in the number of segments I believe to be effected by the interposition of new 

 segments at the base, this bemg the part at which they are of the smallest dimensions 

 and have most the appearance of immaturity. The augmentation in size is produced by 

 addition to every part of the surface of the segment, this being imbedded (so to speak) 

 in the animal basis-substance, into which the calcareous reticulation extends itself from 

 the part previously solidified. It is to the large size of the meshes of that reticulation in 

 the rudimental segments, that the roughness of their surface is due : as they approach 

 maturity, the reticulation formed on the exterior of the old becomes closer, so as to 

 give greater compactness of texture and smoothness of surface ; and this is especially the 

 case, as already mentioned, with that which forms the articular faces. 



31. Pentagonal Base of the Calyx. — When the Centro-dorsal plate has been detached 

 from the rest of the Calyx (which is readily effected by boiling for a short time in a 

 dilute solution of caustic potass), we find the basis of the latter to consist of a penta- 

 gonal disk, formed by the close mutual adhesion of the five First Eadials. The com- 

 position of this disk is obvious enough when we look at the smooth dorsal surface 

 (Plate XXXIII. fig. 2) which was adherent to the margin and inverted lip of the centro- 

 dorsal basin ; the quinary division being clearly marked out by five radiating sutures. 

 But on its ventral aspect (fig. 3) the sutures are less distinguishable, owing to the 

 peculiar inequality of the surface. The inner portion of the pentagonal base forms a 

 sort of funnel, that slopes inwards to the central space ; and the walls of this funnel 

 present an alternation of radiating ridges and furrows, of each of Avhich there are ten. 

 Five of the furrows correspond with the divisions between the component pieces ; and 

 of the ridges which bound them, one belongs to each of the two adjacent Radials. Of 

 the other five furrows, one passes along the middle of each of the five Eadials ; and both 

 the ridges which bound it belong to the same piece. The outer portion of the ventral 

 face of the pentagonal base consists of five surfaces inclining outwai-ds, and marked by 

 peculiar ridges and fossa3 which will be better described when the separate pieces of this 

 disk come under view (§ 33). Turning again to the dorsal aspect, we find the central 

 vacuity of the pentagonal disk to be occupied by a single plate of extremely delicate con- 

 formation and peculiarly inflected shape (Plate XXXIII. figs. 2, 9) ; which I do not find 

 to have been noticed by any of those who have previously described Antedon, and which, 

 for the sake of facility of reference, I shall term the Rosette. I shall hereafter show, 

 however, that it is really a composite structure, formed by the coalescence of outgrowths 

 from the five Basal plates which constituted the primitive foundation of the calyx (§ 59), 

 the original plates having been themselves almost entirely removed by absorption {^ 90). 

 Its peripheral portion is so closely applied to the internal faces of the Eadials of 



