ECHINODERMATA. 



149 



of them presenting the characteristic reticulated structure, which are 

 set with greater or less closeness in the substance of the skin. Various 

 forms of the plates which thus present themselves in Holothuria are 

 shown in Fig. 373; and at A is seen an oblique view of the kind marked 

 a, more highly magnified, showing the very peculiar manner wherein 

 one parfc is superposed on the other, which is not at all brought into 

 view when it is merely seen through in the ordinary manner. In the 

 Synapta, one of the long-bodied forms of this order, which abounds in 

 the Adriatic Sea, and of which two species (the S. digitata and 8. in- 

 hcerens) occasionally occur upon our own coasts, 1 the calcareous plates of 

 the integument have the regular form shown at A, Fig. 374 ; and each 



FIG. 374. 



Calcareous Skeleton of Synapta: A., plate imbedded in Skin; B, the same, with its anchor-like 

 spine attached; c , anchor- like spine separated. 



of these carries the curious anchor-like appendage, c, which is articu- 

 lated to it by the notched piece at the foot, in the manner shown (in 

 side view) at B. The anchor-like appendages project from the surface of 

 the skin, and may be considered as representing the spines of Echinida. 

 Nearly allied to the Synapta is the Chirodota, the integument of which 

 is entirely destitute of ' anchors/ but is furnished with very remarkable 

 wheel-like plates; those represented in Fig. 375 are found in the skin of 

 Chirodota violacea, a species inhabiting 

 the Mediterranean. These ' wheels ' are FIG. 375. 



objects of singular beauty and delicacy, 

 being especially remarkable for the very 

 minute notching (scarcely to be discerned 

 in the figures without the aid of a magni- 

 fying-glass) which is traceable around the 

 inner margin of their ' tires ' There can 

 be scarcely any reasonable doubt that 

 every member of this Order has some kind 

 of calcareous skeleton, disposed in a man- 

 ner conformable to the examples now 

 cited ; and it would be very valuable to 

 determine how far the marked peculiar- 

 ities by which they are respectively distinguished, are characteristic of 

 genera and species. The plates may be obtained separately by the usual 

 method of treating the skin with a solution of potass; and they should 

 be mounted in Canada balsam. But their position in the skin can only 

 be ascertained by making sections of the integument, both vertical and 

 parallel to its surface; and these sections, when dry, are most advan- 

 tageously mounted in the same medium, by which their transparence is 



1 See Woodward in " Proceedings of Zoological Society," July 13, 1858. 



Wheel-like plates from Skin of Chiro- 

 dota violacea. 



