AMONG THE SEA URCHINS. 265 



canal, and also a kind of heart ; the latter is the oval canal, shown 

 at b^ in Fig. A, page 15, and is enclosed in the same groove with 

 the sand canal "t-" of the madreporic filter ; but the irregular 

 Urchins {^Spatangida^ etc.) seem to have no organ of that nature. 

 The sexes are distinct in all Echinoids ; the reproductive organs 

 filling the internal spaces between the ambulacral pores, and com- 

 municating with the exterior through the apertures in the genital 

 plates. Fig. C, ^, on page 1 1 . 



We must now, in conclusion, proceed to deal with the 

 developmental history of the Sea Urchins, which, in some respects, 

 is more interesting to the microscopic student than that of any 

 other sub-kingdom. From the ovum proceeds, after segmentation 

 in the usual way, a ciliated oval " planula," which soon differen- 

 tiates a bilateral symmetry, with ciliated shoulder-plates so unlike 

 the parent form that for long the Pliiteus larva was believed to be 

 a distinct genus of marine life. 



It is extremely difficult to follow every step in the life-history 

 of the Echinus larvae, as t^ey take very badly to aquarian imprison- 

 ment. We have often caught them and kept them for days, but 

 they sink to the bottom of the glass and soon dissolve away, 

 lacking apparently the boisterous heaving motion of the open sea, 

 sparkling with oxygen. 



In a previous article ("Wesley Naturalist,^' Vol. III., page 71) 

 we described a lazy, though most enjoyable way of surface netting 

 for Echinoderm larvae, etc. We have tried another method 

 with much greater success. Take a cone-shaped net of fine 

 muslin, i foot diameter of ring, say a yard long ; fasten the small 

 end (open) round the neck of a two-ounce bottle of clear glass. 

 You must then place yourself on a point of jutting rock, or pier 

 end, when a strong tide is running in briskly, and gradually sinking 

 the bottle and net, hold it high or low to get varying samples 

 brought by the incoming flood-tide. If it has been a warm sunny 

 morning in June or July, you will generally find a number of these 

 " pluteus " forms along with a few small jelly fish and innumerable 

 crustacean zooese and copepoda, with possibly one or two "tadpole" 

 larvae of Ascidians^ that curious link of connection between the 

 two great kingdoms of vertebrata and invertebrata, which is such 

 a prize to most naturalists. 



