MARSHALL : HABITAT OF MONTACUTA FERRUGINOSA. 403 



be the progeny of one adult, as in that case they would be not 

 only more numerous, but also of the same age and size ; but 

 they graduate in the most methodical way in the Torbay habitat 

 — the largest or adult example being next the echinoderm, then 

 the next largest, and so on. 



I have not been able to find Montacuta substriata in con- 

 nection with the Echi7iocardhan in Torbay, and their shells do 

 not occur in the drift sand of the shore as do those of M. ferru- 

 ginosa^ and I take it that the conditions are not so favourable to it 

 as to its congener ; moreover, it has not been recorded as found 

 on this species of Echinus. 



The sand of Torbay contains a large admixture of reddish 

 mud from the triassic rocks, and, as may be expected, the 

 Moniaciitce are thickly coated with it, probably, as Jeffreys 

 supposes, 'by a contmual deposit and accumulation of fsecal 

 matter from the animal, which is not carried off in consequence 

 of its sedentary habits and of the water in which it lives being 

 free from currents.' 



The Torbay locality also indicates that M. ferriiginosa is 

 not parasitic, but that it merely dwells on or near the echino- 

 derms to intercept its food, in the same way as Jeffreys observes 

 with regard to M. substriata. He also says of the latter, which 

 may be taken as correct of the former, that ' in one sense only 

 can it be said to live on echinoderms. The food of Spatangus 

 appears to be animalculse, and to obtain this it swallows large 

 quantities of sand, causing thereby a strong and frequent current 

 in the neighbourhood of its mouth. The Montacutcs probably 

 avails itself of this indraught to partake of the sustenance 

 intended for the Spatangus, placing itself with its alimentary 



tube in the right direction It has no suctorial 



organ, such as is possessed by all animal parasites, .... 

 nor has it once been detected on the back or sides, or elsewhere 

 than in the ventral region of its associate.' (B.C., Vol. II., p. 208.) 



It will be noted that while ' at Guernsey M. ferruginosa 

 occupies the end of the echinoderm at which the mouth is 



