894 ECHINODERMA 



glass well covered with hardened balsam ; a large proportion of its 

 surface may thus be occupied by the sections attached to it, the 

 chief precaution required being that all the sections come into 

 equally close contact with it. Their surfaces may then be brought 

 to an exact level by rubbing them down, first upon a flat piece of 

 grit (which is very suitable for the rough grinding of such sections) 

 and then upon a large Water-of-Ayr stone whose surface is ; true.' 

 When this level has been attained the ground surface is to be well 

 washed and dried, and some balsam previously hardened is 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 over- 

 heating 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 turpen- 

 tine 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, not- 

 withstanding every care, the sections, when ground in a number 

 together, are not of uniform thickness, owing to some of them being 

 underlain 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. 



A very curious internal skeleton, formed of detached plates or 

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

 investment 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 differ- 

 ential 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. 



It now remains for us to notice the curious and often very beau- 

 tiful structures which represent, in the class ffolothurioidea, the solid 

 calcareous skeleton of the classes already noticed. The greater 

 number of 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 the various species which have a set of cal- 

 careous plates, disposed around the wall of the pharynx, 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 



1 See his memoir in the Liinifan Transactions, xxv. p. 365; see also Bell, 

 Journ. Hoij. Microbe. Soc. 1882, p. '227. 



