STILIFER. 197 



such a strange habitat, E. lividus excavates holes in 

 slate and gneissic rocks^ within tide-marks^ its mouth 

 and the whole of the loAver surface being pressed closely 

 'to the stone. A parallel instance of the same species 

 of mollusk infesting different Echinoids is that of Mon- 

 tacuta substriata, which has been observed not only on 

 Spatangus purpureus, but also on >S^. meridionalis, Amphi- 

 detus ovatus, Brissus lyrifer, Echinus esculentuSy and 

 Cidaris hystrix. 



The Shetland specimens (a pair) were attached by the 

 foot to an Echinus, on its upper surface at the base of 

 the spines. They did not adhere firmly to the Echinus, 

 like the GaMgus to a codfish^ but frequently shifted their 

 places by creeping between the spines. I gently moved 

 one of the specimens with a stiff cam el^s -hair brushy and 

 placed it in a glass tube with sea-water. It was at first 

 very sluggish or timid, and evidently unaccustomed to 

 its new quarters, and lay at the bottom of the tube ; it 

 afterwards recovered itself, and crept up the side by 

 means of the anterior portion of its foot, very slowly 

 and by an imperceptible movement ; the other part of 

 the foot was not applied to the glass, but rested on the 

 mantle. The foot was occasionally twisted about and 

 contracted, as if through uneasiness. The animal was 

 never wholly withdrawn into the shell, although I irri- 

 tated for that purpose. The slit in the foot probably 

 serves for the admission of water into some tubular cavity 

 or vessels which permeate this organ; it would have 

 the effect of enlarging and swelling the foot, so as to 

 protect the Stilifer from being crushed by the spines of 

 the Echinus. The force exerted by the Echinus in 

 moving its spines may be insufficient to counteract even 

 the slight pressure of the Stilifer against them at 

 their base. The upper part of the sea-egg was covered 



