86 SOME CURIOUS FACTS CONNECTED WITH 



example hitherto known of an eye so constructed in an invertebrate 

 animal. " During many years of travel," says Semper, " these eyes 

 were totally unknown to me, but I had devoted much attention 

 to the mode of life of the Onchidia. They live exclusively on 

 the seashore or in brackish marshes ; they creep along close to 

 the edge of the water, hiding in clefts of the rocks or under large 

 stones. Together with them in small spots live the genera of 

 fishes, Pcriophthalmus and the nearly allied Bokophthalmiis ; these 

 skip along the strand with long leaps, seeking their food, which 

 consists principally of this very genus of Mollusca. This, it seems 

 to me (continues Semper), seems a way of accounting for the 

 development of these dorsal eyes. 



" The Onchidia are terribly slow creatures, perfectly incapable 

 of escaping or of withdrawing rapidly into a crevice for shelter. 

 They eat nothing but sand, of which, of course, they digest 

 nothing but the nutritious organic particles mixed with the sea- 

 sand. Thus, in order to seek their food, they must often be 

 exposed to the gaze of the swift fish that leap rapidly along the 

 edge of the sea. Fly they cannot ; a house into which to creep, 

 as many molluscs have, they have not ; they have neither spines 

 nor jaws with which to defend themselves, and the eyes on their 

 back can do no more than warn them of the approach of danger. 

 It would be very strange if such eyes were developed in that par- 

 ticular position, unless some weapons were provided, too, for 

 rendering the eyes of service. Such weapons do exist, in point of 

 fact, in every species that has dorsal eyes. The skin of the back 

 is thickly set with minute glands, closely surrounded by circular 

 muscles. Feeble contractions of the skin cannot force out the 

 minute globules which are secreted by the glands. But, supposing a 

 Periophthalmus approaches suddenly and with rapid leaps, it rises 

 — as I have often seen — several inches into the air. The mollusc 

 has all its eyes — and I have positively counted ninety-eight on one 

 specimen — turned upwards in various directions. Suddenly 

 becoming aware of the fish or its shadow, it instantly draws up its 

 whole body, thus contracting the glands on the skin with its whole 

 force. The minute globules of secretion will be thrown into the 

 air, in hundreds and thousands, towards the pursuing fish. The 

 fish, hit by the shower of minute shot, retires from the pursuit, 

 and Onchidium is safe." 



