STRUCTURE OF THE SKELETON. 



3 



the compact, flexible mechanism so admirably exhibited in the arms of many species. Fig. 2 

 represents the lateral view of part of a ray of Uraster rubens, Lin,, and exhibits the 

 net-like arrangement of the ossicles on the sides and upper 

 surface ; this reticulate structure forms the inter-ambulacral r-^' f ' 



portion of the skeleton. 



The test of the Echinoidea is formed of [ambulacral 

 and inter-ambulacral areas, and a similar arrangement of the 

 ossicula may be observed in the skeleton of the Asteroidea ; 

 the centrum of the disc or oral ring in the Asteriad^e is the 

 homologue of the centrum and auricular arches in the test of 

 the EcHiNiDiE, to which the muscles of the iaws are attached : t,„h„„,,,„ . 



' J ' Reticulate arrangement of the ossicles 



the centrum in both orders therefore forms the arch to which on the sides and upper surface of 

 the ossicles of the rays in the star-fishes, and on which the ^ ^raster rubens. 

 plates in the sea-urchins, are supported. 



The arches forming the central part of the base of the rays are the homologues of the 

 ambulacral areas in the Echinidte ; they are composed of two central, oblong principal 

 pieces (fig. 1 a) united at the median line, and two smaller transverse pieces on which 

 they rest, having two smaller inferior pieces external to the preceding ; these six elements 

 enter into the composition of a single segment of the ambulacral area. In a specimen of 

 Uraster rubens, prepared as already described, for the purpose of displaying the skeleton, 

 I have counted 140 ambulacral arches in each ray, which multiplied by 6 for the six 

 elements in each arch, 140 X 6 = 840 ossicles in one ray ; this multiplied by 5 for the 

 five rays, 840 X 5 = 4200, is the number of ossicles in the ambulacral portions of the 

 skeleton of this specimen, exclusive of the elements of the centrum. The ossicles forming 

 the lateral and upper portions of the ray are the homologues of the inter-ambulacral areas 

 in the Echinoidea (fig. 2). The number of separate pieces entering into the composition 

 of this part of the skeleton is very great, arising from the smallness of the bones, and the 

 diverse forms of their reticulate arrangement in the different genera, to form a structure 

 at once resistant and flexible, and adapted to the habits of the organism (fig. 2) ; the 

 inter-ambulacral areas of the Asteriad^ are for this reason very unlike the homoloo-ous 

 portion of the test in the Echinid^, where these areas consist of two columns of broad 

 spinigerous plates, between which the narrow ambulacra are placed. In the Asteriadji;, 

 on the contrary, the ossicular elements of the inter-ambulacra, besides their locomotive 

 functions, have assigned to them the formation of the sides and roof of the hollow cylin- 

 drical arms (fig. 1 c). 



The structure of the rays varies so much in the different genera of this order, that 

 any general description would necessitate the enumeration of so many exceptions to 

 the common plan of organization, that I prefer pointing out the differences which 

 several of the genera exhibit, rather than attempt to give a general outline of the entire 

 group. 



