S2 



HAEDWICKE'S SCIENCE -GOSSIP. 



[April 1, 1867. 



SKELETON OE PURPLE URCHIN. 

 {Echinus lividus). 



THE Purple Urchin of our shores is so common 

 an object that few, if any, of my readers — scien- 

 tific or otherwise— can have failed at some time or 

 other to have seen and admired it. Not many, how- 

 ever, I believe, who have seen it know— to use a 

 hackneyed but expressive phrase— what a world of 

 wonder is wrapped up in its prickly box. A little 

 of this wonder I propose to reveal, confining myself, 

 however, entirely to its skeleton. To commence 

 with the shell or test, as it is called : This is made 





Fig. 61. Portion of Shell, x 230. 



up of numerous polygonal plates jointed together in 

 so perfect a manner that the seams or divisions 

 between them can be with difficulty detected. The 

 structure of this shell is everywhere the same ; it 

 is composed of a network of carbonate of lime, with 

 a basis of animal matter. The interspaces or 

 areola of this network vary greatly in size, form, 

 and number, as is shown in fig. 61, which represents 

 a portion of the shell of the Purple Echinus magni- 

 fied. 



Fig. 62. Ambulacral Disc, x ISO. 



Every one who has ever kept an Echinus in the 

 aquarium alive must have noticed its mode of loco- 

 motion — how it protrudes from orifices in its shell 

 sundry curious suckers, by means of which it clings 



to the smooth surface of the glass or stones. Each 

 of these suckers is terminated by a disc, the skele- 

 ton of which presents the same calcareous network 

 as the shell, but in a more beautiful and symmetrical 

 form. Eig. 62 gives a magnified representation of 

 this ambulacral disc, as it is called. 



In many foreign species of Echinus the spines, 

 when examined in section under the microscope, 

 present a very beautiful appearance, somewhat 

 similar to that of exogenous wood. This appear- 

 ance is produced by successive rings of open spaces 

 and solid pillars, which mark the yearly growth of 

 the spine. 



Although the spine of Echinus lividus does not 

 present this beautiful ringed appearance, doubtless, 

 as has been suggested, from its being the result of 

 only one year's growth (being exuriated and repro- 

 duced annually), still it presents a very striking 

 appearance. It is, in common with the species of 

 other echini, composed of solid calcareous ribs, 

 alternating with bands of an open calcareous net- 

 work. 



Fig. 63. Transverse Section of Spine, x 60. 



These ribs and bands in section appear as alter- 

 nating rays, and the network being coloured purple, 

 they are very striking. Eig. 03 represents a trans- 

 verse section of the spine, and exhibits the alter- 

 nating bauds of solid ribs and open network. 



Attached by a peduncle to the spines are to 

 be found the curious bodies called Pedicellarise. 

 These, at one time supposed to be parasites, are 

 now generally considered as simple appendages to 

 the spine, although their function is somewhat 

 doubtful. Each of these consists of a thin cal- 

 careous stalk, surmounted by a curious pincer-like 

 apparatus, the whole being invested by the general 

 animal membrane of the Echinus. Eig. 6± represents 

 the head of a pedicellaria. The pincers are double, 

 and are formed of a fine calcareous network, resem- 

 bling that of the shell. The edges of each limb of 

 the pincers are serrated. 



Thus much for the external portion of the skele- 

 ton. Internally, the only part of the animal requiring 



