1889-90-] On the Echinoidea or Sea- Urchins. 347 



on the interambulacral zones, and are moved by an ingenious 

 ball-and-socket arrangement or universal joint. Dots or 

 tubercles, hemispherical in shape, are scattered over the shell, 

 and upon the summit of these the spines revolve at their 

 hoUowed-out extremity, being attached to the muscular coat- 

 ing of the test by a thin flexible membrane. While the tube- 

 feet can only be used under water, the spines are called into 

 play to aid progression on land ; but they also, no doubt, serve 

 as organs of defence, as well as help in scooping out the sand, 

 or even the hard rock, where some species live. The purple 

 egg-urchin (Toxopneustcs (U.) lividus), common in some parts 

 of Ireland, is found in great numbers living in holes scooped 

 out of rocky ledges covered by the tide at high water. The 

 points of the spines in many species are very sharp, and in 

 the egg-urchins have a reprehensible habit of breaking off and 

 burying themselves in the skin of the hands or feet, to the 

 special discomfort of bathers. One of our members, who lived 

 for some time at Gibraltar, where sea-urchins are very common 

 in the surrounding waters, informs me that he has often seen 

 a number of soldiers, after they got back to barracks from 

 their " dip " in the sea, sitting in a row with knife or needle, 

 busily operating on the soles of their feet, in order to extract 

 the sharp tips of the spines that had penetrated so far as to 

 be almost invisible. The sea-urchins are gregarious in their 

 habits ; and members of the Society who have accompanied our 

 dredging excursions will remember that when any appeared 

 in the trawl, there was generally found to be a goodly number 

 of them. It has been calculated that an adult sea-urchin will 

 possess fully 4000 spines ; so that when bathers come upon a 

 shoal of them in the sea-bottom, they have to run the gauntlet 

 of a perfect forest of these tiny spears. 



During the last few months I have examined a considerable 

 number of sea-urchins, brought up by the trawl near the Isle 

 of May, and have been struck by the variety of form assumed 

 by the common egg - urchin. Many were distinctly glob- 

 ular in shape, but others were much flattened at the poles. 

 The colour of the shell, too, varied considerably, ranging from 

 a dull white to a deep orange. While the size and colour of 

 the shell thus varied, there was a similar variation in the 

 spines. Some were long and sharp-pointed, with a very pale 



