HARDWICKE'S SCIENCE-GOSSIP. 



97 



GRAPHIC MICROSCOPY. 



By E. T. DRAPER. 



No. XVII.— Transverse Section of Spine of Echinus. 



HE intimate cor- 

 relation of parts, 

 and the manifest 

 mechanical per- 

 fection of the 

 structures of an 

 Echinus, or sea- 

 urchin, renders the 

 creature peculiarly 

 adapted for inter- 

 esting and contem- 

 plative micro- 

 scopical investi- 

 gation ; every part 

 has a curious fit- 

 ness, and expe- 

 diency may be 

 revealed in all the 

 more important 

 organs ; among 

 other features, the character of the nearly globular 

 box, containing the animal, built up of an enormous 

 number of accurately-fitting plates, the calcareous 

 pieces forming the mouth, the five sharp socketed 

 incisor teeth, of great strength, arranged in a circle, 

 and in such a position as to simultaneously close upon, 

 and crush the hard crustaceous substances, or any- 

 thing that comes in its way, as food ; guided to the 

 centre of the mouth by a similar number of inter- 

 posing osseous processes, with jaws of such complex 

 structure as to establish, with the dental system, a 

 masticatory apparatus, unique in character and 

 adaptability. Supporting this, and enclosing other 

 softer tissues is the "lantern of Aristotle," a frame- 

 work of five symmetrically curved bones with trans- 

 verse ties firmly attached to the interior of the box, 

 strengthened by a similar number of elegantly formed 

 pieces rising from the base ; all the hard parts are 

 formed on a pentagonal principle, a multiple of five ; 

 the whole locked together with mathematical pre- 

 cision ; exteriorly, the interest is sustained by the 

 prehensile suckers emerging through multitudinous 

 No. 245.— May 1885. 



apertures, and where least expected, a disclosure of 

 rare beauty in the structure of any one of the forest 

 of hard spines with which the creature is completely 

 surrounded ; each capable of separate movement. 

 A transverse section of such a spine, ground and 

 polished, is the subject of the plate. 



The case, or envelope of an Echinus consists of 

 a somewhat flattened spherical box, made up of 

 many hundred jointed pieces, the whole ap- 

 pearing like a single shell. In some species the 

 texture is light and porous, in others considering the 

 number of parts of exquisite solidity ; five pairs of 

 " ambulacral " plates, connected by well-marked 

 sutures, traverse the shell in polar lines. This set of 

 segments is perforated with many apertures for the 

 emergence of prehensile locomotive suckers. Between 

 each is a similar number of rather wider segments, 

 the "interambulacral," accurately fitting the others; 

 on these project a double row of knobs, or tubercles, 

 on which the spines are articulated by a ball and 

 socket joint. All the pieces forming the wonderful 

 box are serrated, and compacted with minute precision, 

 giving great strength ; the actual substance of the shell 

 is composed of calcareous material and silicates 

 obtained from the sea, secreted by a soft organic mem- 

 brane which invests and permeates every fissure. The 

 spines are articulated to the tubercles on small 

 polished nipples seen studding the outside of the inter- 

 ambulacral plates, and vary in form and size according 

 to species ; generally they are grooved horizontally. 



Vertical cuttings of these organs are interesting ; 

 but their true beauty is only disclosed when transverse 

 sections are made, carefully ground and polished 

 to a requisite thinness ; and so diversified are the 

 patterns that a collection of many hundred specimens 

 rarely discloses two absolutely alike, differences in 

 appearance and complexity resulting from the position 

 of the cutting, and its distance from the base or the 

 apex ; as the spine consists of a series of cones either 

 of overlapping or inter-deposited growths, necessarily 

 a section reveals annular bands in number equal to 

 the cones included in the part where the cutting is 



