34^ On the Echinoidea or Sea-Urchins. [Sess. 



at its base an eye-spot or organ of vision, but the whole ten- 

 tacle is now regarded as an organ of touch. I have prepared 

 an egg-urchin shell by painting in distinctive colours the 

 ambulacral and interambulacral zones, the genital and ocular 

 plates, and the madreporite, thus permitting these exterior parts 

 of the structure to be seen at a glance. 



Still dealing with the test or covering of the sea-urchin, the 

 spines next claim attention. These are of various sizes and 

 thicknesses — from a mere thread of less than half-an-inch in 

 length, to several inches, with corresponding thickness. In 

 section, many of the spines show rings similar to the annual 

 growths of an exogenous stem, though, of course, their mode 

 of growth is entirely different. These rings vary in number — 

 not, as one would have expected, according to the dimensions 

 of the spine, but by the distance from the base at which the 

 section is made. The disposition of the rings and rays, and 

 the other ornamentation of the spines, give rise to many 

 beautiful combinations. Taking advantage of the beauty and 

 variety of these spine-sections, preparers of objects for the 

 microscope have in some cases produced " type-slides " which 

 are truly works of art, but, owing to the labour involved, they 

 are usually rather costly. In the third volume of the 

 " Challenger Eeports," some seventy different forms of spines 

 are figured in section, besides many others which are shown in 

 profile. Spine-sections, when large, are not very difficult to 

 prepare for the microscope, being treated very much in the 

 same way as sections of bone, or mineral sections ; but when 

 the spines are small, as in the egg-urchin, considerable care 

 and patience are required. After repeated failures, I hit on 

 the plan of cutting a number of spines and fixing them 

 together by their sides with strong gum. The mass was then 

 held by the fingers and ground fine at one end, and when 

 reduced to about one-eighth of an inch this smooth end was 

 fixed to a glass slip, and the rough end polished on a fine slate 

 till the whole was quite transparent. The slide was then 

 placed in warm water and the sections separated, when each 

 could be examined and the best specimens mounted in balsam. 

 This process gave very good results, as may be seen from the 

 slides now exhibited under the microscope. 



The spines of the sea-urchin are found in greatest number 



