540 ECHINODERMATA ECHINOIDEA chap. 



Mortensen ^ regards E. norvegicus and E. microstoma as mere 

 variations of E. acutus, and this is probably correct. E. esculcntus 

 has already been described ; its most marked character is the 

 forest of comparatively short, close-packed, reddish or white 

 primary spines with which it is covered, between the bases of 

 which the delicate secondaries are hard to detect. It is essen- 

 tially a shallow - water species. E. acutus is distinguished by 

 having much fewer and longer primaries and numerous delicate 

 secondaries. It is an inhabitant of deeper water, being abundant 

 at 100 fathoms, though stragglers are found in shallower water. 

 At the depths at which it lives wave-disturbance can scarcely be 

 felt, and hence the long primaries are not irritated. 



E. elegans has spines intermediate in character between those 

 of E. esculentus and those of E. acutus. Like the latter it is an 

 inhabitant of the deeper water. It seems to the present author 

 not at all improbable that further research might show that 

 E. acutus, E. elegans, and E. esculcntus are all members of con- 

 tinuous series of forms ; certainly the larvae and early develop- 

 ment of E. acutiis and E. esculentus, the extreme members of the 

 series, are strikingly similar. 



E. miliaris differs somewhat widely from the other species 

 and is closely allied to E. microtuherculatus of the Mediterranean, 

 from which it is distinguished mainly by the greater thickness of 

 the scattered plates on the peristome of the latter species. From 

 the other British species it differs in its much smaller size and 

 in the greenish hue of its primary spines, which are short and 

 thick and possess purple tips. Its larva is markedly distinct 

 from the larva of E. esculentus. E. miliaris is a littoral species, 

 and is found in great numbers in some of the Scottish sea-loclis ; 

 when the tide recedes, under every stone of the gravelly beach 

 several specimens will be found. It has a curious habit of 

 " dressing " itself, i.e. of covering itself with fragments of dead 

 shell, sea-weed, etc., which are held in position by the aboral 

 tube-feet. This habit aids in concealing the animal, and has 

 probably been developed on account of the dangers to which 

 E. miliaris is exposed owing to its littoral habit of life. 



Sphaerechinus differs from Echinus in the structure of the 

 ambulacral plates, in which it agrees with Strongylocentrotus, 

 but it is distinguished from this genus by the very deep gill- 



' Reference on p. 532, note 1. 



