296 A NATURALIST IN BORNEO 



called the byssus, which attaches the animal firmly to 

 the surface on which it is found. The shells are 

 attached to the palm stems well above low-water mark, 

 and in many cases above the high-water mark of neap- 

 tides ; consequently for the greater part of their lives 

 they are exposed to the full glare and heat of a 

 tropical sun. The shells, when above water, fit so 

 closely to the palm stems that they can hardly be 

 removed with the blade of a sharp knife. Having 

 grown in one position, they are perfectly adapted to 

 all the slight irregularities and inequalities of curvature 

 of the palm stems, and the space under the left valve, 

 in which of course the animal lies, must be well-nigh 

 air-tight. When the shells are immersed in water the 

 byssus relaxes slightly, and as this organ is attached 

 at one end to the inside of the left valve, and, passing 

 through the animal and through the right valve, at the 

 other to the palm stem, the effect of its relaxation is 

 the loosening of the left valve so that water can flow 

 under it and bathe the tissues of the Mollusc. It is 

 only during these periods of immersion that the animal 

 can feed, and it is probable that it lives on small particles 

 that are swept into its mouth along some ciliated grooves 

 that traverse the foot. Professor G. C. Bourne has made 

 a careful anatomical study of this animal, 1 and has 

 discovered some interesting features adapted to protect 

 it from desiccation. In bivalve Molluscs there is a 

 fold of the body wall immediately underlying each valve 

 of the shell and enveloping the body of the Mollusc 

 between them ; these folds are termed the mantle-lobes, 

 and the space between each lobe and the actual body 

 is the mantle-cavity. In ^Enigma cenigmatica the lower 

 1 Quart. Journ. Microsc. Sci. (N. Ser.}, LI. (1907), p. 253. 



