38 



ECHINODEHMATA. 



shell. The gullet and part of the stomach are 

 usually everted, protruded, and applied round 

 the object to be swallowed, which is then drawn 

 in. The hard or indigestible matters, such as 

 the shells of mollusca, are discharged by the 

 mouth. The star-fish is said to be very de- 

 structive to oyster-beds, and is popularly be- 

 lieved to suck the animals out of their shells. 

 Bishop Sprat, in his Histoiy of the Royal 

 Society, informs us that great penalties are 

 laid by the Admiralty Court upon those en- 

 gaged in the oyster-fishery who " do not tread 

 under their feet or throw upon the shore a fish 

 which they call a Five-Jinger, resembling a 

 spur-rowel, because that fish gets into the 

 oysters when they gape, and sucks them out." 

 Tiedemann found the coeca to contain a grey- 

 ish-white fluid which he supposed to be di- 

 gested aliment ; others again, such as Meckel, 

 regard the coeca as secreting organs, analogous 

 to the biliary organs of many invertebrate ani- 

 mals, with which, it must be allowed, they 

 agree in several respects. 



b. The mouth of the Echinus is an orifice 

 situated in the middle of the circular mem- 

 brane which fills up the lower aperture of the 

 shell (Jig. 15, .) The points of the five teeth 

 are seen within it, and at no great distance 

 from its circumference the ten tubular tentacula 

 (d) are observable, which have been already 

 described. The teeth are set in five moveable 

 sockets or jaws which surround the commence- 

 ment of the gullet, and with the addition of 

 some accessory pieces form the singular struc- 

 ture usually named Aristotle's lantern. The 

 lantern (Jigs. 10, 17, and 18) has the appearance 

 pf a five-sided pyramid placed with its apex 



Fig. 17. 



Fig. 18. 



Dental apparatus of the Sea-urchin viewed 

 from above. 



downwards or towards the mouth, the gullet 

 () rising through its centre. It is made up of 

 five smaller hollow pyramids (//,) which are the 

 sockets of the teeth. Each lesser pyramid is 

 three-sided ; its external side (Jig. 18, A',) 

 which forms one of the faces of the greater 

 pyramid, presents an opening in its upper 

 half which is closed by membrane ; its lateral 

 faces (Jig. 18, h, /*,) are applied to the cor- 

 responding sides of the adjacent sockets, with 

 which they are connected by short muscular 

 fibres (/>); they approach each other at (he inner 



A, two sockets with teeth, 

 of Echinus esculentus. 



B, single socket with 

 its tooth viewed on 

 the outside. 



edge of the socket, but do not meet. The 

 tooth (t) is prismatic, very long, and lodged 

 in a groove formed in the external side of the 

 socket ; its point projects beyond the apex of 

 the socket ; its opposite extremity or root rises 

 above the base, where it is bent inwards and 

 downwards and inclosed in a membrane. The 

 teeth are very hard at the point, but softer 

 towards the root, where they are easily sepa- 

 rable into transverse scales or plates with a 

 fine silky or asbestine lustre ; they seem to 

 grow continually at the root, and wear at the 

 point as in the Rodentia. 



Ten additional pieces contribute to form the 

 lantern. Five of these (i) are oblong and 

 flattened, and are placed horizontally, in a ra- 

 diating manner, on the upper surface of the 

 lantern, occupying the intervals between the 

 bases of the lesser pyramids. The other five 

 (/c) are placed directly over the first; they are 

 longer but more slender, and bent in a semi- 

 circular form, the convexity being upwards; 

 their central ends are articulated with the cor- 

 responding extremities of the horizontal pieces ; 

 the outer ends are bifid and give attachment to 

 ligaments. 



The muscles and ligaments belonging to the 

 dental apparatus partly pass between its dif- 

 ferent pieces, and partly connect it with the 

 border of the shell. It will be recollected that 

 the border of the shell forms five processes 

 (jig** 1 and 17, g,g, g,) which rise in the form 

 of arches into its cavity round the lower aper- 

 ture. Ten muscles (m, m,} arise from these 

 arches, and descending inwardly are inserted 

 into the lesser pyramids or sockets near the 

 point. Two of these muscles come from every 

 arch, and diverging are inserted into different 

 pyramids, so that each pyramid receives its 

 two muscles in a converging manner from two 

 adjacent arches. The muscles described draw 

 outwards the sockets separating them and 

 widening the mouth. Other ten muscles (, ?i,) 

 arise in pairs from the border of the shell in 

 the intervals of the arches, and, ascending, are 

 inserted into the outer surface of the sockets 

 near their base, each socket receiving a pair. 

 These are antagonists to the last described; 

 they move the points of the pyramids, and 

 consequently the teeth inwards and against 

 each other. Five muscles composed of short 



