ON THE STRUCTDEE OP ENIGMA ^NIGMATICA. 259 



adapted for visual functions. As they are situated at a 

 greater or less distance from the edge of the mantle, they 

 must always be covered by the shell, and the existence of 

 visual organs in such a position is somewhat extraordinary. 

 But, as I have pointed out, the left valve of the shell is thin 

 and translucent enough to allow a considerable amount of 

 light to pass through. As Enigma spends a large part of 

 its existence uncovered by the sea, with its valves tightly 

 closed to prevent evaporation, it is probable that these eyes 

 are efficient in informing the animal of the duration of day- 

 light, or, at any rate, of the incidence of direct sunlight. It 

 is probable enough that after sunset the valves of the shell 

 are slightly opened to admit of the aeration of the water con- 

 tained in the pallial chamber, and are kept tightly closed to 

 prevent evaporation during the heat of the tropical day. 



The right mantle lobe is very irregular in shape, and 

 presents a large anterior sinus corresponding with the sinus 

 of the right valve of the shell, and serviug for the passage 

 of the byssus. In the anterior part of the body the right 

 mantle lobe is attached by a very narrow band of tissue to 

 the visceral mass, the lino of attachment running above and 

 nearly parallel to the upper edge of the byssus cavity. In 

 this part of the body, indeed, the viscera are thrust over to 

 the left side of the body and the visceral mass is adherent to 

 the left pallial lobe. But in the hinder part of the body the 

 rectum and cfecum of the crystalline style, passing respectively 

 above and below the adductor muscle, cross over from the 

 left to the right side, and are here adherent to the right 

 mantle lobe and embedded in the mass of the right gonad 

 and right kidney (compare figs. 11 and 13). The lower and 

 posterior part of the right mantle lobe is exceedingly thick, 

 and its inner surface is pitted and folded in a very irregular 

 manner. As the animal lies with its right valve lowermost 

 these folds and pits must serve for the retention of water 

 during the long periods in which it is uncovered by the 

 tides. 



The mouth, as is shown in fig. 1, lies asymmetrically on the 



