374 THE INFLUENCE OF LIVING SBEROUNDINGS. 



fluid, but rather a kind of concretion, and the pores for its 

 emission are excessively fine, so as to be hardly discernible. 

 Moreover, these are closely smxounded by a circular muscle, so 

 that by its contraction the openings of the glands are easily 

 closed. Feeble contractions of the skin, such as inevitably 

 occur in the act of creeping, cannot consequently express the 

 minute globules of the secretion out of the glands ; the moisture 

 cannot exude. But supposing that a Periophthalmus approaches 

 suddenly and with rapid leaps (see fig. 100) ; it rises — as I have 

 often seen — several inches into the air, and may thus not un- 

 frequently throw a shadow from some distance off, on the back 

 of the slowly creeping Onchidium, and of course alarm it greatly. 

 The mollusc has all its eyes — and I have positively counted 

 ninety-eight on one specimen — turned upwards in various 

 directions ; suddenly aware of the fish, or of its shadow, it 

 quickly draws up its whole body, thus contracting the glands in 

 the skin on all sides with considerable force. Granting that 

 this force is sufficient to express the globules of secretion from 

 the pores of the glands, these, as the skin contracts, must inevi- 

 tably be expelled from them ; instead of flowing over the skin of 

 the creature's back, they will be shot into the air in hundreds 

 — or thousands — towards the pursuing fish; the fish, now 

 alarmed on its part, and hit by the shower of minute shot, 

 which may be in some way injurious or ofiensive, retires from 

 the pursuit, and the Onchidium is safe. 



Of course, as I have already said, this is merely an hypo- 

 thesis ; the question nevertheless arises whether it may not be 

 possible to show by indirect evidence that it is extremely 

 probable. 



It would, no doubt, be quite conceivable that the Onchidium 

 might be able to defend itself in the mode I have suggested, not 

 merely against the Periophthalmus and Boleophthalmus, but 

 against other foes. But if , as I believe, these two fishes are 

 actually the only, or at any rate the most dangerous, enemies 

 it has to dread, and if the eyes and glands thus serve as 

 weapons of defence against these fishes only, we must expect 

 that wherever these fishes occur Onchidia with dorsal eyes will 

 be found. This is in fact the case. ' The Periophthalmus is 



