506 



CALCAREOUS PLATES OF HOLOTHURIDA. 



are generally so diverse, even in closely-allied Species, as to afford 

 very good differential characters. This subject is one that has 

 been as yet but very little studied, Mr. Stewart being the only 

 Microscopist who has given much attention to it ;* but it is well 

 worthy of much more extended research. 



439. It now remains for us to notice the curious and often very 

 beautiful structures, which represent, in the order Holothurida, 

 the solid Calcareous Skeleton of the orders already noticed. All 

 the Animals belonging to this Order are distinguished by the flexi- 

 bility and absence of firmness of their envelopes; and excepting 

 in the case of certain species which have a set of Calcareous plates, 

 supporting Teeth, disposed around the Mouth, very much as in the 

 Echinida, we do not find among them any representation that is 

 apparent to the unassisted eye, of that Skeleton which constitutes 

 so distinctive a feature of the class generally. But a Microscopic 

 examination of their Integument at once brings to view the exist- 

 ence of great numbers of minute isolated Plates, every one 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. 293 ; and at a is seen an oblique view of the 



Fig. 293. 



Calcareous plates in Skin of Holothuria. 



kind marked «, more highly magnified, showing the very peculiar 

 manner wherein one part is superposed on the other, which is not 

 at all brought into view when it is merely seen-through in the ordi- 

 nary 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. inhcerens) occasionally occur upon our own 

 coasts, f the Calcareous plates of the integument have the regular 



* See his Memoir in the " Linntean Transactions," Vol. xxv. p. 365. 

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



