Fig. 69.—Maxillipedes and Maxilla (b) of Shore Crab. (d) First maxillipede. (After Savigny.)
These inner three pairs should also be detached, and the whole of the mouth organs arranged on a card thus:—
- Mandibles.
- First Maxillae.
- Second Maxillae.
- First Maxillipedes.
- Second Maxillipedes.
- Third Maxillipedes.
The first attempt will certainly be unsuccessful; and the first few attempts will probably be unsatisfactory; but we shall gain knowledge with each successive trial. And knowledge is worth the winning.
The stomach is interesting, and the gastric mill may be easily examined. When the mouth organs are removed, there will be no difficulty in taking out the stomach. This should be cut open with a needle, and then we shall see the gastric teeth (g g) which grind up the food against the fixed calcareous plate (b b). The lower end of the stomach is set with fine hairs, which prevent the passage of food into the intestines until it has been ground fine between these living millstones. A similar arrangement is found in all the higher Crustacea. The time spent in comparing the gastric mill of the Crab with the ‘gizzard’ of the Cockroach will not be thrown away.
The Broad-clawed Porcelain Crab (Porcella´na platyche´les) is also worth keeping, for it is a droll little creature. These crabs are generally to be found clinging to the under surface of stones or of ledges of rocks overhanging small pools. The chief interest of these crabs, for us, lies in the exceedingly beautiful arrangement for procuring food with which the outer pair of foot-jaws is furnished.
Fig. 70.—Stomach of Crab laid open. b b b, fixed plate, against which the gastric teeth g g are opposed; b´ b´, gastric teeth enlarged.
‘Watching a Broad-claw beneath a stone close to the side of my tank, I noticed that his long antennae were continually flirted about; these are doubtless sensitive organs of touch, or some analogous sense, which inform the animal of the presence, and perhaps of the nature, of objects within reach. At the same time I remarked that the outer foot-jaws (pedipalps) were employed alternately in making casts, being thrown out deliberately, but without intermission, and drawn in, exactly in the manner of the fringed hand of a Barnacle, of which both the organ and the action strongly reminded me. I looked at this more closely with the aid of a lens: each foot-jaw formed a perfect spoon of hairs, which at every cast expanded and partly closed. That you may understand this better, I must say that the foot-jaw resembles a sickle in form, being composed of five joints, of which the last four are curved like the blade of that implement. Each of these joints is set along its inner edge with a row of parallel bristles, of which those of the last joint arch out in a semicircle, continuing the curve of the limb; the rest of the bristles are curved parallel or concentrical with these, but diminish in length as they recede downwards. It will be seen, therefore, that when the joints of the foot-jaw are thrown out, approaching to a straight line, the curved hairs are made to diverge; but as the cast is made they resume their parallelism, and sweep in, as with a net, the atoms of the embraced water[48].’
All this description may be verified from a spirit specimen, if the foot-jaws be carefully removed. And the examination with the lens will also show that these hairs are plumose, that is, set with smaller hair, like the barbs of a feather.