208 KELLIID.E. 



astonishing. Many hundreds of them, packed close to- 

 gether, and glittering like microscopic pearls, might be 

 counted. They occupy at least two-thirds of the space 

 enclosed within the valves of their parent ; and its own 

 body seems to be atrophied and dwindled to a mere 

 skeleton. The shell is in fact turned into a crowded 

 nursery. Perhaps the parent dies, like some insects, 

 immediately after all its progeny have been developed. 

 I do not concur in the general belief that M. substriata 

 is parasitic. In one sense only can it be said to live on 

 Echinoderms. The food of Spatangus purpureus, on 

 which it is usually found, appears to consist of ani- 

 malcula ; and for that purpose it swallows large quan- 

 tities of shell-sand, causing thereby a strong and fre- 

 quent current in the neighbourhood of its mouth. The 

 Montacuta probably avails itself of this indraught, and 

 partakes of the sustenance intended for the Spatangus, 

 placing itself in the way, with its alimentary tube or 

 opening turned in the right direction. No exudation 

 from the Spatangus has been noticed ; and its excretions 

 would scarcely be produced in sufficient quantity for 

 the support of the Montacuta, or perhaps be suitable to 

 it. The latter has no suctorial organ, such as is pos- 

 sessed by all animal parasites : it never attaches itself 

 to the pedicellarise or any other soft part of the echino- 

 derm; nor has it once been detected on the back or 

 sides, or elsewhere than in the ventral region, of its 

 associate. It is only found on the spines close to their 

 points. 



2. M. bidenta'ta *, Montagu. 



Mya bidentata, Mont. Test. Brit. p. 44, tab. 26. f. 5. Montacuta bidentata, 

 F. & H. ii. p. 75, pi. xviii. f. 6 & 6 a. 



Bout clear white : mantle having a bluish tinge ; its margins 



* Two-toothed. 



