HORNELL— ANATOMY OF PLACUNA 57 



With the direction and extent of the gills. It arises from the anterior ventral angle 

 of the visceral mass close to the lower margin of the palps, and extends thence in a 

 semi-circular and gradually narrowing band to a point midway between the anus and 

 the hinder limit of the adductor muscle. Embedded in it, and showing through it for 

 a considerable part of the distal portion, is the enormously long crystalline style. 



The ventricle of the heart lies free within the angle made by the upper margin 

 of the rectal visceral lobe with the posterior edge of the visceral mass. As the 

 posterior genital lobe lies athwart the base of the angle so formed, an imperfect 

 chamber — the cardiac chamber — is formed, open, be it remembered, on the left lateral 

 face. No trace of pericardium is to be seen. 



The foot (F.) is attached to the anterior surface of the visceral mass between the 

 two pairs of labial palps at about the level of the lower half of their length. The 

 attachment of the anterior extremities of the gills is distinctly ventral to the base of 

 the foot ; in the pearl oyster {M. vulgaris) the apices of the gills bound the foot 

 laterally. 



In Placuna the foot reaches a high degree of specialisation ; it is extremely 

 mobile, and capable of great extension and equally great contraction. The form 

 assumed is that of a cylinder much flattened laterally. The apex is modified to form 

 a well-developed deep cup-shaped sucker with strongly developed muscles in the rim ; 

 in contraction the edges of the sucker are approximated, obliterating the cavity more or 

 less completely. 



Within the walls of the sucker muscle fibres radiate from the centre of the pedal 

 axis. It would seem from this structure that the mechanism is similar to that of 

 a boy's leather sucker — when the rim be everted and the surface of the cup applied 

 to a flattened surface, a vacuum will be created by the contraction of the radial 

 muscles. Were Placuna placenta to live on stony or rocky ground, this arrangement 

 would suggest that the animal employs this organ in locomotion to drag itself from 

 object to object. On the mud flats where it lives, this function cannot be employed. 

 From this consideration, and from observations made on the animal in an aquarium 

 tank, it seems to me that the principal use — if indeed it be not the sole one — the foot 

 here subserves is that of a cleansing organ. 



In dissection the sucker cavity is usually found to be gorged with mud ; where 

 Placuna lives the water is normally turbid with mud and flocculent vegetable debris in 

 suspension. The animal must have an organ to collect and clear away the foreign 

 matter which is continually settling and accumulating upon the surface of the mantle 

 and gills, and lodging in corners and cavities ; an extremely extensible and flexible 

 cylindrical organ, bearing at its extremity a muscular sucker-cup armed with a highly 

 sensitive and mobile rim, is ideal for such purpose, and this appears to be the purpose to 

 which the foot is put by the window-pane oyster. 



In a normal state of contraction, when this organ is at rest, it measures not more 



