14:8 THE MICROSCOPE AND ITS REVELATIONS. 



to be spread over it, so as to be sucked-in by the sections, a moderate 

 heat being at the same time applied to the glass slide; and when this has 

 been increased sufficiently to loosen the sections without overheating 

 the balsam, the sections are to be turned-over, one by one, so that the 

 ground surfaces are now to be attached to the glass slip, special care 

 being taken to press them all into close contact with it. They are then 

 to be very carefully rubbed-down, until they are nearly reduced to the 

 required thinness; and if, on examining them from time to time, their 

 thinness should be found to be uniform throughout, the reduction of the 

 entire set may be completed at once; and when it has been carried 

 sufficiently far, the sections, loosened by warmth, are to be taken-up on 

 a camel-hair brush 'dipped in turpentine, and transferred to separate 

 slips of glass whereon some liquid balsam has been previously laid, in 

 which they are to be mounted in the usual manner. 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 underlaid by a thicker stratum of balsam than others; 

 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. 



540. A very curious internal skeleton, formed of detached plates or 

 spicules, is found in many members of this class; often forming an in- 



FIQ. 373. 



Calcareous plates in skin of Holothuria. 



vestment like a coat of mail to some of the viscera, especially the ovaries. 

 The forms of these plates and spicules 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; 1 but it 

 is well worthy of much more extended research. 



541. 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 others already noticed. All the animals 

 belonging to this Order are distinguished by the flexibility 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 existence of great numbers of minute isolated plates, every one 



1 bee his Memoir in the " Linnsean Transactions," Vol. xxv., p. 365. 



