GENERAL CHARACTERS OF THE ECHINODERMATA. 363 



in the body-wall. In the Crinoids, on the other hand, the repro- 

 ductive organs are lodged in the " arms," and permanent generative 

 apertures are rarely, if ever, present. As regards their position, the 

 generative glands (ovaries or testes) alternate in the Echinoderms 

 generally with the radiating nerve-cords and ambulacral vessels, and 

 are therefore " interradial," while the latter are " radial " ; but in the 

 Crinoids the genital cord, being lodged in the arms, is necessarily 

 radial in position. 



The integument of the Echinoderms has the power of secreting 

 carbonate of lime, and a more or less definite integumentary skeleton 

 is thus usually produced. Not only is this exoskeleton in general 

 readily preserved in the fossil state, but its minute structure is so 

 characteristic that even the smallest fragment can usually be recog- 

 nised' with certainty by the help of the microscope. In the first 

 place, the calcareous tissue of the Echinodermal exoskeleton has a 

 very characteristic crystalline structure. " Each plate, each spine, and 

 each joint is mineralogically and optically, as it were, made out of a 

 single crystal of calcite, having its principal axis perpendicular to the 

 plane of the plate, or parallel to the axis of a spine or joint, the 

 growth from first to last being in perfect crystalline continuity" 

 (Sorby). In recent Echinodermal skeletons, this crystalline struc- 

 ture is only recognisable by the optical phenomena displayed by 

 thin sections under the microscope ; but the skeleton of fossil 

 forms, as the result of mineralisation, usually exhibits, in its minutest 

 fragments, the unmistakable rhombohedral cleavage of calcite. In 

 the second place, the skeleton of the Echinoderms has an equally 

 characteristic organic structure, being made up of reticulated and 

 anastomosing calcareous rods, which are produced by the calcifica- 

 tion of a network of organic fibres, the uncalcified intervals of which 

 are filled in the fresh state with living matter. All Echinodermal 

 ossicles and plates exhibit this characteristic netted structure (fig. 

 236), but different forms differ in the size and disposition of the 

 meshes ; the commonest and simplest arrangement being that of 

 parallel strata of perforated calcareous laminae connected with one 

 another by vertical pillars. In other cases, as in the spines of the 

 Echinoids, the structure consists of radially-disposed, vertical, netted 

 plates united by numerous short horizontal rods ; and other types 

 exhibit other special modifications. 



In altered specimens, the minute reticulated structure of the 

 skeleton may be largely obliterated, or even wholly unrecognisable, 

 but the characteristic cleavage usually remains. Owing to the per- 

 sistence of the characteristic micro-structure of the skeleton, even 

 detached and otherwise indeterminable fragments of Echinoderms 

 can commonly be recognised with certainty under the microscope, 

 and valuable assistance is thus often afforded to the palaeontologist 



