lylFE-HISTOEY OP MARGAEITIFERA PANASES^. 411 



of a protected position, and only survive a few months, the 

 secluded spots they ha.ve anchored in proving too small for the 

 growing shells. A large number are smothered in silt or sea- 

 weed ; and amongst the enemies of those that manage to secure 

 themselves to a support may be meutioued the numerous species 

 o£ fist, crabs, starfishes, &c. A peculiarly insidious enemy is 

 found in the person of a small whelk, which, during the first few 

 months of growth, pierces a neat hole in the thin shell, producing, 

 apparently, a paralytic effect on the adductor muscle, with the 

 result that the bivalve loses the power of closing its shell 

 against the unwelcome visitors. Quantities of young dead pearl- 

 shell are washed up on to the beaches with every returning tide, 

 and on a large proportion of these is found this small per- 

 foration, and, curiously enough, in the majority of cases this is 

 situate in the neighbourhood of the attachment of the adductor 

 muscle. 



The species is dioecious, and the embryo is formed by the 

 union of the sexual elements outside the shell. After a free- 

 swimming stage of prohably a few days' duration, a pair of shells 

 is formed; and the embryo, as a result of its increased weight, 

 sinks to the bottom and is washed into, and lodges ia, the 

 crevices hetween layers of seaweed or at the junction of the 

 branches of dead coral and other places. At this stage the shells 

 measure 1 mm. to Ig mm. in diameter, and in appearance are 

 either pure white or else have a white ground with circumferential 

 dark green or black blotches, forming a band either wholly or 

 partly around the margin. 



In many cases, where the situation is an exposed one, the 

 young shell, after deposition, makes a tour of the neigh- 

 bourhood in search of a cranny where it can hide itself from the 

 detection of its enemies. Por the purposes of locomotion, a 

 modified foot is cautiously protruded from the region of the 

 byssal cleft. It has the appearance of a muscular thread, a,nd. 

 its maximum length is about equal to that of the hinge-margin. 

 This organ, after describing circulatory movements and testing 

 the ground in every direction, attaches its extremity in the 

 direction decided on, and, by the contraction of its substance, 

 draws the oyster after it, the latter resting with its flatter side 

 (i. e. the right valve) below. Progress is necessarily slow ; 

 nevertheless, I have known spat under observation travel a foot 

 or so during a few hours previous to anchorage. 



LINN. JOURN. — ZOOLOGY, VOL. XXIX. 29 



