No. 628] ONCHIDIUM AND ADAPTIVE COLORATION 425 



eyes of Oncliidlum wore employed in sensing]: the approacli 

 of aiiiiiials wliieh niiglit attack it. He had in mind the 

 attacks of the rhilii)])ino PenopthaJmus, a blenny which, 

 leaping along the intcTtidal Ix'acli-'/onc on its ventral 

 fins, feeds npon OmhiJnnn -avA on .-ii-l lircp.xK. Semper 

 thought that tlie evc< ct' the ( )ii<'lii<lia i-cii.leiod them 

 sensitive to the shadow of an approaching lish, where- 

 npon a fluid spray was discharged from the skin glands, 

 driving ot¥ the fish. Whether Semper regarded the secre- 

 tion as repugnant to the tish in a chemical way, or whether 

 it was to act in a mechanical fashion merely, is not alto- 

 gether clear. Sem]Kn' endeavored to make out a correla- 

 tion, illustrative of evolutionary ])rinciples, between the 

 geographical distribution, on the one hand, of the species 

 of Onchidia possessing' dorsal eyes, and, on the other, of 

 the genus Periopihal)irii.<. 



There are at Bernm-li im /'< / n.i,!l.nl nn,^. n..r any other 

 fishes of similar habils. (hicitnlnim ilnn>l(iinnii is, how- 

 ever, (luite reactive 1<. ^liadin-. Tlu' <1.m-;i1 Mirface of 

 the mantle is the part -ruA\\\r in .l..<-i . .i^mi light inten- 

 sity. In a movinu- Oiirhninnii ihc ciiMiin- reaction in- 



nmlcr water, are not photosensitive. An Onchldium 

 shaded when ih.( creeping, but at rest or feeding, ex- 

 hibit- the -auic t\ pe of response, but the mantle is more 

 decidedly humped. But, in our experience at least, no dis- 

 charge of the repugnatorial glands is in(hu'e<l !>> >hailin.u:. 

 This does not mean that Semper's obsei \ at i<)n> were 

 not correct. In 0. florulanmn the type of e\eilation whieh 

 is preeminently and ehai':ieiei'i>tieally etieetive toi- ulaiid 

 discharge is of a mechnnieal natun-, n,nnM> . a rnhhiiiLi or 

 pinching of the apex ot' the dor-uin era di-loilumL; tnrn-t 



upon the ventral 

 tested ))v faradic 



-land 



charge may be elicited when ovn the ^.ie of the foot 



