No. 628] ONCHIDIUM AND ADAPTIVE COLORATION 427 



reference to the use of the glands, is identical throughout 

 the whole series. 



V. Without entering upon a lengthy setting-forth of 

 the several alternative possibilities winch might readilv 

 be suggested m partial explanation of the foregoing series 

 of facts, we mav state our behef that an attempt at such 

 explanation m terms of color-adaptation is confronted b> 

 serious, and indeed fatal, obstacles. From such a stand- 

 point we are required to consider only the period during 

 wliich tlie Onrliidinm is emerging from its nest and creep- 

 iiiii- about in the open. In the ease of a dark Onchidnim it 

 i« quite tiue that the animal is often Aei\ oasiU o\er- 

 looked while m process of creeping over tlie Modiohi.s at 

 the entrance of its nest: the color-match of the snail and 

 the mussel is a fairly exact one. But an Ouchtdmm does 

 not resemble Modiolus, although of about the same size, 

 when creeping upon algre at the rate of 5 cm. per minute- 

 or faster. The paler kinds of Ouclndium are, in almost 

 e\er\ instance iioti d pm.im initdi foi tin u back 



ground, and soitu'lmic^ (•.•iicliil seaivli is necessary m 

 order to detect them. 



. The non-homoclirninic |.i-iiir!it:iti<.ii d llic dark On- 

 chidia would, m view ol llic i.rcM-iicc aii.l hcliavioi- of the 

 poison^glands, be di siiccial ititcnv-l iii coiincct khi with 



iniin(('llmibl<^ 111 the face of the paler, concealmglv-colored 

 t\ pes. Moreover, the Bermudian Oncidiellas to which we 

 iia\(' ali'cadx alluded are verv inconspicuous indeed, and 

 th(\ aKo ])0--e-- lHHM)!i -1 iikK tlu ii mode of lif« quite 

 different fiom tint ol O^/^//' -iiki till oiui hlu wise 

 inhabiting the iiitiMiida! /oiir. liicx rctrcal diifm<>- low 

 water to tinv holes. <load serpiila lubes and the like — 

 commu- forth a'>am oid\- when eo\-ered tiie watei-. 

 The^e On. i li 11 im nn dl in t ,,io,< tli in nun . lU 

 and both liulit and dark iii(h\-iduais are in the limlie-t 

 degree diihciilt to detect when on their natural ha.-k- 

 ground; a delicate mottling of brownish pigment, m vari- 

 ous shades, has a pronounced concealing ettect in these 



