1889-90-] On the Echinoidea or Sea-Urchins. 353 



work upon it." This simile gives a very fair idea of the 

 appearance of a pluteiis ; and it may be added to it that the 

 embryo is very transparent, and only about the one-fortieth of 

 an inch in size, so that although the plutei may swarm in the 

 sea during the months of August and September, they are not 

 easily recognised. Strange to say, it is only in warm and 

 temperate waters that this mode of growth by means of a 

 pluteus obtains ; for Sir Wyville Thomson has found that in 

 cold Arctic seas the young echini are produced viviparously 

 from ova, and live as tiny sea-urchins on the body of the 

 mother, by a sort of commensal arrangement, until they are 

 able to fend for themselves.'^ By the kindness of Dr G. Car- 

 rington Purvis, there is shown to-night under the microscope 

 the plutei of a sea-urchin, from the marine-station at Naples. 



Having thus glanced at the general structure of the Echin- 

 oidea, as exhibited by the common egg-urchin of the Firth of 

 Forth and other waters, we may now compare the sea-urchins 

 with the more familiar star- fishes. Taking the common " Five- 

 fingers " {Astcrias ( Uraster) ndiens), we note the following points 

 regarding its structure. There is first a disc and five radiating 

 arms, and on the upper surface of these arms numerous cal- 

 careous nodules which support short spines. One of these 

 nodules is larger than the others, being the madreporite or 

 strainer which is also found on the test of the sea-urchin ; 

 while scattered about amongst the spines of our star-fish will 

 be found the pedicellaria? or pincers already described. On 

 the under surface of the disc, and in its centre, is the mouth ; 

 and from it five grooves extend to the tips of the rays, which 

 carry on each side rows of tube-feet. There is here a remark- 

 able similarity to the structure of the sea-urchin ; and we are 

 forced to admit the truth of the remark made by Dr Eomanes, 

 though at first blush it may be a little startling, that " in all 

 its main features an echinus is merely a star-fish, with its five 

 rays calcified and soldered together, so as to constitute a rigid 

 box." 



Passing from the common egg-iirchin, I shall now notice 

 briefly the other sea-urchins generally found- in or near the 



^ See 'Jour. Linn. Soc.,' vol. xiii., 1876 (Zoology), Art., "Notice of Some 

 Peculiarities in the Mode of Propagation of certain Echinoderms of the Southern 

 Seas," by Sir Wyville Thomson. 



