16 ON THE ECHINODERMATA OF THE 



specimens, however, they seem to be wanting altogether on the ventral interradia, and 

 are confined to a few isolated spicules on the anterior and posterior extremities of the 

 dorsal area. 



The spicules are wheel-shaped and slightly concave outwardly, varying from '2 to 

 25 of a millim. in diameter. The spokes or radii, which are straight and slender, and 

 are united into a central boss, average about nineteen in number ; but they may vary 

 by three or four more or less. 



The circumference or portion which represents the outer rim is divided by sutures 

 midway between each of the radial spokes ; and prolongations given off from the margin, 

 and bent sharply over at an acute angle, are directed inward towards the centre, and 

 form a continuous series of equal, regular, elongate denticles. These denticles are 

 about one third of a radius in length, generally somewhat three-sided in form ; and their 

 number frequently exceeds that of the spokes of the wheel. 



The manner in which the spicules are developed is interesting, and is easily followed 

 in some examples. The central boss is the first commencement, the margin of which 

 becomes crenulated ; and then denticles develop, which are gradually prolonged in the 

 form of spokes until the full size of the wheel is attained ; processes then form at the 

 extremities at right angles to the spoke, and these eventually meet and coalesce with 

 the corresponding processes from the neighbouring rays or spokes, whereby a con- 

 tinuous rim is formed equal in thickness to the rays ; this rim next increases some- 

 what in breadth, and the inwardly directed denticles begin to develop from the margin. 



The tentacles are twelve in number, and, judging from their structure, are somewhat 

 hand-shaped, and would resemble when extended the form which is found in Chirodota 

 (cf. Steenstrup, loc. cit.) ; when retracted, however, the upper portion is drawn within 

 the basal part or stem of the tentacle itself, the membrane forming a kind of hood 

 which arches over from the inward side, but leaves the extremities of the digitate pro- 

 longations exposed and visible on the outer side. 



Within the broad circlet of the tentacles stretches a muscular membrane, in the 

 centre of which the mouth-aperture is situated. The alimentary canal passes directly 

 from this to the posterior portion of the animal, where it is bent upon itself, then two 

 or three times sharply convoluted, and prolonged as far as the anterior third of the 

 body, when its course is again reversed, and it finally passes direct to the anal 

 aperture, this lower or intestinal portion being several times narrower than the rest of 

 the canal (see fig. 22). The various convolutions are held in position by fine reticulated 

 mesenteries. 



The calcareous mouth-ring is very large and strong, and composed of ten ossicles ; 

 eight of these bear on their upperside a long, three-sided, wedge-shaped prolongation, 

 whilst the remaining two, which are situated on either side of the median dorsal ossicle 

 of the ring, have each two prolongations. Thus the prolongations correspond in number 

 with the tentacles, the bases of the latter being situated between them, whilst a fine 

 muscular band, which would seem to function as their retractor, stretches between the 

 prolongations halfway between the base and tip. A foramen for the passage of the 

 radial nerve is punctured through the prolongation, somewhat above its base. 



