Red Sea Shells. 19 



coat which is secreted by the most delicate part of the mantle — 

 that, namely, which contains the viscera. But the best illustration 

 of this subject is to be found in a shell cameo, the manufacture of 

 which consists in the partial destruction of the enamel layers of 

 the shell. The inhabitants of shallow water are as a class more 

 brightly painted than those found in deep seas, which would seem 

 to im})ly that the secretion of colour by the mantle depends greatly 

 upon the action of light. Then, again, it is impossible to help 

 noticing that the part of a shell which is turned towards the light 

 during the life of the creature that inhabits it is more deeply 

 tinted than the under side. Bivalves which moor themselves to 

 reefs, and vegetate there, such as the spondylus and many of the 

 pectens have the upper valve richly dyed, while the lower is almost 

 colourless. The backs of most spiral shells are darker than their 

 under sides, because as the creatures move in their natural state, 

 the apex of the shell points upwards. Conversely, to take an 

 instance from western seas, the purple of the lanthina or violet 

 snail is deeper at the base than on the spire, because as it trails its 

 egg- raft along the top of the waves by means of a projecting foot, 

 it necessarily sails with the aperture upwards towards the light. 



There is a shell, of which I picked up rather a nice specimen, 

 which will serve me for another text. It is the Lima, or file shell, 

 so called, I suppose, from its ribbed and radiated surface. It 

 belongs to the family of the Ostreidae, and might be unscientifically 

 described as something between an oyster and a fanshell. Well, 

 who would suspect this modest looking bivalve, delicate as it is, 

 of belonging to one of the most beautiful of marine animals ? 

 Still less would the scientific description of its owner give one a 

 hint of the fact. The shell is always white, but the live lima is 

 either of a deep or pale crimson colour, with an orange mantle of 

 two lobes, to the edges of which is attached a lovely fringe of the 

 same hue. The animal propels itself with the hinge uppermost, 

 in this position, by opening and closing the valves of its shell. 

 Its locomotion in water is not unlike that of a butterfly in the air, 

 and its progress is no doubt helped by the graceful tentacles of the 

 fringe which keep up their writhing contortions, just as our hearts 

 keep up their action, without any conscious effort on our own part. 

 But besides the fringe, the lima, like all bivalves, has a foot which 

 is exerted through a gap in the anterior side of the shell This 

 limb no doubt has various uses. It manifestly acts as a rudder to 

 the lima ; but in the case of some bivalves it serves for a screw 

 propeller, by absorbing a quantity of water into its spongy texture, 

 and suddenly expelling it again ; then it is the prehensile organ of 

 the animal, by which it captures its food, or a contractile lever 

 by which it hauls itself towards the object it has grasped. In 



