400 MARSHALL : HABITAT OF MONTACUTA FERRUGINOSA. 



M.ferruginosa, of which I obtained several dozen. Both species 

 occupy the same echinoderm, but the most remarkable thing is 

 that while (as is well known) M. substriata occupies the spines at 

 the ventral end of the Spatangus, and nowhere else, M. Jerru- 

 ginosa as rigidly keeps to the spines at the opposite end, near 

 which the mouth is situate, without a single exception, and never 

 occur on the back or sides. Hence its supposed rarity ; for 

 this being its usual habitat, examples would rarely occur to the 

 dredger, though valves might of course be plentiful. 



In Guernsey almost every Spatangus had its attendant M. 

 substriata, but only about every third one contained M. ferru- 

 ginosa as well, the latter varying in number from two to six — • 

 rarely more than one adult, and the rest immature though of 

 different ages. 



Spatangus purpureus lives at extreme low water mark in 

 the Channel Islands, in shelly or gravelly sand, buried two or 

 three inches deep, but it tilts itself up on the approach of the 

 returning tide, and so reveals its whereabouts. The spines at 

 both ends are about half-an-inch in length, with sufficient space 

 between to allow of the free movement of the Montacutce, who 

 attach themselves by a strong byssal thread to near the extremity 

 of the spines. In this locality the Montacutte are beautifully 

 clean, owing to the sand containing no muddy sediment, unlike 

 specimens procured elsewhere, which are generally coated more 

 or less thickly with a ferruginous deposit. 



Both Montaciita ferruginosa and siibstriata are found under 

 precisely the same conditions in the Scilly Islands, and probably 

 in other places. 



On my return to Torquay I endeavoured to find Spatangus 

 purpureus on the South Devon coast, to see if it harboured the 

 Montacutce as at Guernsey ; but it does not appear to live there 

 so far as I have observed, its place being taken by another 

 echinoderm {Echinocardiutn cordatufn, Pennant), whose habits 

 are somewhat different, though for the purpose of providing a 



J.C, vi., Oct., 1891. 



