142 Descriptions of Prepa7'ations. 



the circlet of specialized spines, which protect the eye and the 

 contractile tactile organ in relation with it. A third set of spines 

 marks the line of junction of the vertical and dorsal surfaces in 

 each ray ; and the middle line of the anti-ambulacral surface has 

 more or less of a keeled appearance, from the more or less regular 

 longitudinal arrangement there of the spines with which the dorsal 

 surface generally is beset. Remnants of the organs known as 

 ' pedicellariae/ which appear to be spines modified so as to be 

 mobile and prehensilej are to be seen in the interspaces between 

 the spines, and some, though of smaller size and not in especial 

 abundance, may be seen also round the bases of the spines. The 

 spines themselves are immobile in the Asteriae, and, thoiig'h they 

 may carry a coronet of numerous calcified setae on their apices 

 when they are called ' paxillae,' when modified into ' pedicellariae,' 

 they rai-ely carry more than two terminal processes, which make 

 up a pincer-like organ. In the Echinidae, on the other hand, 

 the spines are themselves mobile, and provided with a muscular 

 apparatus ; and the ' pedicellariae,' which are mainly distributed 

 about the oral region, are trivalved. Opposite one of the inter-radial 

 spaces is seen a whitish, circular, raised, concentrically striated 

 disk, the so-called ' madreporic tubercle.' In a fresh specimen 

 of any one of the Asteriae which is provided with sucker-like and 

 not with conical feet, the anus may be found near the centre 

 of the dorsal surface a little to the left of a line drawn from this 

 madreporic tubercle, down the longitudinal axis of the ray, oppo- 

 site to the inter-radial space in which it is lodged. The same 

 line will enable us to divide the five rays into a 'bivium,' between 

 which the madreporic tubercle lies, and a 'trivium,' the two 

 lateral arms of which lie on either side of the arm which is opposite 

 to that tubercle. But we cannot speak properly of an anterior 

 or posterior radius or inter-radius in these Echinodermata, inas- 

 much as, like Echinidae and Ophiuridae, they move in locomotion 

 indifferently in the direction of any one radius or inter-radius. The 

 radius which lies to the right in the madreporic bivium in this 

 specimen, when the central ray of the trivium is placed so as to 

 point away from the observer, is much shorter than any one of the 

 other four; having been reproduced after some injury, but not 

 having attained the size of its fellows. The power of reproduction 

 of injured parts is very great in these animals, having indeed 



