SECTIONS OF ECHINUS SPINES. 488 



the calcareous network is uniform, or nearly so, throughout ; but 

 in the pentangular Pentacrini, a certain figure or pattern is 

 formed by variations of texture in different parts of the trans- 

 verse section ; and the patterns, though formed upon one gene- 

 ral plan, are sufficiently diverse in different species, to enable 

 these to be recognized by the examination of a transverse section 

 of a single joint of the stem. 



317. The structure of the shells, spines, and other solid parts 

 of the skeleton of Echinodermata can only be displayed by thin 

 sections, made upon the general plan already described ( 109, 

 110). But their peculiar texture requires that certain precautions 

 should be taken ; in the first place, in order to prevent the sec- 

 tion from breaking, whilst being reduced to the desirable thin- 

 ness ; and in the second, to prevent the interspaces of the net- 

 work from being clogged by the particles abraded in the re- 

 ducing process. A section of the shell, spine, or other portion 

 of the skeleton, should first be cut with a fine saw, and rubbed 

 on a flat file until it is about as thin as an ordinary card, after 

 which it should be smoothed on one side by friction with water 

 on a Water-of-Ayr stone. It should then be carefully dried, first 

 on white blotting-paper, afterwards by exposure for some time 

 to a gentle heat, so that no water may be retained in the inter- 

 stices of the network, which would oppose the complete pene- 

 tration of the balsam. Next, it is to be attached to a glass slip 

 by balsam hardened in the usual manner ; but particular care 

 should be taken, first, that the balsam be brought to exactly the 

 right degree of hardness, and second, that there be enough, not 

 merely to attach the specimen to the glass, but also to saturate 

 its substance throughout. The right degree of hardness is that 

 at which the cement can be with difficulty indented by the 

 thumb-nail ; if it be made harder than this, it is apt to chip off 

 the glass in grinding, so that the specimen also breaks away ; 

 and "if it be softer, it holds the abraded particles, so that the 

 openings of the network becomes clogged with them. If, when 

 rubbed down nearly to the required thinness, the section appears 

 to be uniform and satisfactory throughout, the reduction may be 

 completed without displacing it ; but if (as often happens) some 

 inequality in thickness should be observable, or some minute 

 air-bubbles should present themselves between the glass and the 

 under surface, it is desirable to loosen the specimen by the ap- 

 plication of just enough heat to melt the balsam (special care 

 being taken to avoid the production of fresh air-bubbles), and to 

 turn it over so as to attach the side last polished to the glass, 

 taking care to remove or to break with the needle-point any air- 

 bubbles that there maybe in the balsam covering the part of the 

 glass on which it is laid. The surface now brought uppermost 

 is then to be very carefully ground down ; special care being taken 

 to keep its thickness uniform through every part (which may be 

 even better judged of by the touch than by the eye), and to 



