CALCAREOUS STRUCTURE OF HOLOTHURIDA. 485 



ner. It more frequently happens, however, that, notwithstanding 

 every care, the sections, when ground in a number together, are 

 not of uniform thickness, owing to some of them being under- 

 laid by a thicker stratum of balsam than others are ; and it is then 

 necessary to transfer them to separate slips, before the reducing 

 process is completed, attaching them with hardened balsam, and 

 finishing each section separately. 



319. 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 

 flexibility and absence of firmness of their envelopes ; and ex- 

 cepting in the case of certain species which have a set of cal- 

 careous 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 existence 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. 240 ; and 



FIG. 240. 



Calcareous plates in skin of Holothuria. 



at A is seen an oblique view of the kind marked a, 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 ordinary manner. In the 

 Synapta^ one of the long-bodied forms of this order, which does 

 not occur upon our own coasts, but is abundant in the Adriatic 

 Sea, the calcareous plates of the integument have the regular 

 form shown at A, Fig. 241 ; and each of these carries the curious 

 anchor-like appendage, c, which is articulated 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, of which one species (the 

 C. digitata), although previously accounted a very rare inhabi- 



