vni ECHINODERMATA MORPHOLOGY OF SKELETON 381 



331, 6). On its outer, i.e. oral surface, this plate shows a more or less deep longi- 

 tudinal furrow, which gives off alternating lateral furrows to right and left. This 

 longitudinal furrow of the lancet plates is universally considered to have the same 

 significance as the food grooves on the tegmen calycis, and on the arms, of Crinoids. 

 Each lancet plate is longitudinally pierced by a canal, the so-called ambulacral 

 canal. 



The space on each side between the lancet plate in the middle, and the lateral 

 margin of the ambulacrum (which latter is formed by the sloping edges of a deltoid 

 plate and of one limb of a fork plate (radial)), is occupied (a) by a longitudinal row 

 of large side plates (5), and (b) by a longitudinal row of smaller accessory side 

 plates (8). The number of side plates and accessory side plates corresponds with 

 that of the lateral branches of the median ambulacral furrow on the lancet 

 plate. Each side plate consists of a narrow portion which is directed towards the 

 edge of the ambulacrum, and of a broad portion which is in contact with the lancet 

 plate. The broad parts of the consecutive side plates in a row are in contact with 

 one another, but between the narrow parts of these same plates are spaces, in each 

 of which lie an accessory side plate, and a hydrospire pore (7). This latter leads 

 below the surface to the hydrospire pouches under the ambulacrum. The hydro- 

 spire pores, the accessory side plates, and the narrow ends of the side plates follow 

 one another regularly in this order. 



The margins of the ambulacra carry thin long jointed appendages, 

 the pinnules (2), which may be compared with the structures of the 

 same name in the Crinoids. 



The pinnules are retained only in rare cases, and are then depressed from the 

 two sides orally over the ambulacral area (ambulacrum B, Fig. 331). There can, 

 however, be no doubt that they could be raised and opened out (ambulacrum A). 

 The number of the pinnules corresponds with that of the side plates in a longitudinal 

 row, as also with that of the accessory side plates and of the hydrospire pores. The 

 points of attachment of the pinnulte lie between the consecutive pores. - Each 

 pinnule consists of a large number of skeletal pieces, which, near the base, alternate 

 in two rows, but above this are arranged in one row. 



If the lancet plate of an ambulacrum is removed (E, Fig. 331), 

 the (smaller and thinner) lower lancet plate (12), which lies close below 

 it, appears at the surface. 



This plate resembles the lancet plate in shape. If the pinnulse, the side plates, 

 and the accessory side plates be removed, the edges of the plates which border the 

 ambulacrum (deltoid plates, limbs of the fork pieces) are seen sloping towards the 

 floor of the ambulacrum. These sloping edges carry a longitudinal row of transverse 

 ridges, alternating with depressions, into which latter the narrower (outer) portions 

 of the side plates fit. On each side, between the lower lancet plate and the sloping 

 lateral walls of the ambulacrum, some of the parallel clefts and folds of the hydro- 

 spire pouches (13) are seen lying parallel to the longitudinal axis of the ambulacrum. 

 At the central or proximal end of the ambulacrum (i.e. near the peristome), the 

 interradial deltoid plates meet, and are joined by a radial suture. An aperture in 

 this suture, the ambulacral aperture (11), leads to the interior of the calyx. Through 

 this ambulacral aperture, the ambulacral canal, which traverses the lancet plate 

 longitudinally, is connected with a circular canal which surrounds the oesophagus. 



In closest proximity to the peristome there are five large inter- 



