STRUCTURE OF ECHIXOIDS, 171 



The acid has no apparent effect on the green dye, and the 

 results show no serious sign of fading or other degeneration after 

 three years. (The specimen should not be kept for too long 

 exposed to bright sunlight.) Although the process sounds some- 

 what involved, it can be very quickly accomplished. I have 

 completely prepared fifty specimens within two hours of their 

 having been cleaned. The method will probably be found useful 

 for distinguishing sutures in other Echinoids, but I have not 

 obtained siich clear results when applying it to thick plates, such 

 as those of Echimis. For the guidance of any who may adopt 

 the method I add the following particulars : — 



It is inadvisable to reverse the order of procedure outlined 

 above — staining should always precede etching. 



The process destroys the finer surface characters of the plates, 

 and should not be used on rare specimens. 



The etching should be continued until the plate surfaces are 

 almost colourless, but care must be taken that the plates are not 

 eaten through, or distortion of the sutures will result. 



The clearness of the preparation may be increased by slightly 

 etching the inner surface. 



The sutures should be viewed by reflected light, and will not 

 show in perfection unless the specimen is dry. 



If the staining has not been sufficiently intense, but little 

 advantage is gained by applying more. The specimen may often 

 be made useful, however, hj saturation with glycerine and 

 examination by transmitted light. 



III. Description of ambulacrtim III, q/ Echinocardium 

 cordatum. 



The part of the ambulacrum here considered is enclosed by the 

 internal fasciole. In E. cordatum this region is depressed to 

 form a considerable groove passing into the anterior notch. The 

 surface of the area is covered with a uniformly fine granulation, 

 which tends to become more sparse along the perradial suture in 

 the anterior part. A few very small tubercles are sometimes de- 

 veloped on it, although some of the largest tubercles on the test 

 occur on the sides of the groove, which are built of interam- 

 bulacral plates. The shape of the groove (and also of the fasciole) 

 is rarely symmetrical, being normally expanded more on the right 

 side than on the left. 



The character of the pores is strikingly different in various 

 parts of the area : — (i.) Near the apical system the pores are 

 minute, and usually uniserial in arrangement. (ii.) In the 

 middle section they are much larger, more or less pyriform or 

 transversely elongated in shape, and arranged in a very complex 

 order, biserially, triserially, or irregularly. They often form 

 broad " poriferous zones " that occupy half the width of each half 

 of the ambulacrum. (The area in this section expands to its 

 greatest width.) It is here that the longest podia are situated. 

 In both the foregoing sections the pores are closely packed, 



12* 



