1889-90-] Oil the EcJiinoidea or Sea- Urchins. 345 



lists " issued in connection with the Marine Station at Gran- 

 ton, E. sphtera, which, as already said, is plentiful in the Firth 

 of Forth, is absent, while Psammechinus (E.) esculentus, which 

 is not found there, is included. It will be noticed, also, in 

 connection with the pore-bearing zones, tliat in the egg-urchins 

 these pass from pole to pole of the shell ; but in another large 

 group, of which the purple heart-urchin (Spatangics purpureus) 

 of the Firth of Forth is an example, they are not continuous, 

 but form a rosette, resembling a five-petalled flower, on the 

 upper surface of the shell. 



Besides the pentagonal plates which go to make up the 

 covering of a sea-urchin, two special circular rows of plates 

 are found on the summit of the shell, surrounding the anal 

 aperture — viz., the genital and the ocular plates. The genital 

 plates carry the external openings of the reproductive organs, 

 — large racemose bodies, very similar in appearance in the two 

 sexes. It was not till 1840 that the different sexes of the 

 Echinoidea were discovered, it being previously believed that 

 the various individuals composing the class of the Echinoder- 

 mata were all either bisexual or solely female.-^ The ova are 

 fecundated in the water, the spermatozoa moving rapidly about 

 by means of their vibratile filaments. One of the five genital 

 plates, besides carrying a duct or opening like the others, has 

 undergone a curious modification, being larger than the rest, 

 and studded with minute perforations like the fine " rose " of 

 a small watering-pot. This modified genital plate is known 

 as the madreporite — a structure also found in the star-fishes, 

 and generally supposed to serve the purpose of filtering the 

 sea-water before it enters the water-vascular system of the 

 animal. This supposition has been strengthened, if not con- 

 firmed, by the following experiment. A coloured fluid was 

 injected for several hours, at a high pressure, into part of the 

 ambulacral system of a sea-urchin, when the fluid ultimately 

 passed into the tube or " stone-canal " connected with the 

 madreporite, and issued from the perforations of the latter in a 

 finely divided coloured stream. Wedged in between the five 

 genital plates are the five ocular plates, smaller than the 

 genital plates, and each bearing a pore from which a tentacle 

 is extruded. This tentacle was at one time thought to have 



1 See on this point Van der Hoeven's 'Handbook of Zoologj%' vol. i. p. 133 ct seq. 



VOL. II. 2 A 



