. Animal life as affected by the natural conditions of existence. Animal ecology. CRABS FORMING SBCH GALLS. 219 the coiul, it may be concluded witTi certainty that the crab moves about very little in the cavity, for otherwise we should not find the very distinct scars which are evidently produced by continual sci-atching in one spot. Since, in all the crabs of this group, the current of water for breathing enters the body close to the mouth, and passes out again at the hinder margin of the branchial cavity, the stream passing through the gall must always flow in one and the same direction. The
. Animal life as affected by the natural conditions of existence. Animal ecology. CRABS FORMING SBCH GALLS. 219 the coiul, it may be concluded witTi certainty that the crab moves about very little in the cavity, for otherwise we should not find the very distinct scars which are evidently produced by continual sci-atching in one spot. Since, in all the crabs of this group, the current of water for breathing enters the body close to the mouth, and passes out again at the hinder margin of the branchial cavity, the stream passing through the gall must always flow in one and the same direction. The results are easily recognisable in the half or wholly t closed gall. The two excrescences on the "^J'X coral grow together quickest in those spots which are least exposed to the current through the gall; there also they first come into contact, till at length only two fissures, more or less wide, are left, which plainly show, by their position opposite to each other, that it is through them that the cur- ^'"i^AM^^rT^m'^l rent for respiration passes ; one fissure serves «â *= normal deveiop- ^ ^ ' ment of the coral: 6, for the influx, the other foi- the exit, of the the gaii with a cavity , . âhere laid open-in water. These two slits remain open so long which a crab was en- as the crab is alive; no living crab is ever found in a closed gall, and they are for the most part perfectly empty. It is impossible not to conclude from this state of things that the fissures are kept open by the force of the current flowing through them; but still this can only occur when the force of the current is exactly commensurate with the strength working in antagonism to it, which is exerted by the growing polyps. These are constantly tending, as is shown by the different stages of the gall, to reduce the space between the two sides of it; at first this may be quite easy, but as the force of the current is gradually increased by the diminution of the fissure, at last a state of equi
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