70 THE COLOURS OF ANLVIALS 



interesting example was brought before me by Mr. 

 Garstang, viz. that of the Opisthobranch mollusc, 

 Hermcea, which is transparent and therefore invisible, 

 except for the ' hepatic " canals, which simulate in form 

 and colour the reddish weed (Griffithsia) on which the 

 animal usually lives. Mr. Garstang finds that the 

 colour is purely adventitious, being due to the food 

 undergoing digestion (see pp. 79, 80). Another English 

 species of HermcEci is green, and lives on green weeds. 

 Mr. H. L. Osborn has published a very interesting 

 note on the resemblance between the colour of a coral 

 on the North American coast and the mollusc which 

 habitually lives upon it.^ He states that Dr. E. B. 

 Wilson, working in 1879 in Dr. Brooks's laboratory 

 at Beaufort, N.C., found an orange-yellow coral 

 (Leptogorgia virgulata) invariably attended by a gastro- 

 pod of the same colour {Ovulwm uniplicatum), which 

 was never seen apart from the coral. Dr. Wilson's 

 coral occurred in shallow water. In 1884, Mr. Osborn, 

 also working at Beaufort, found a Leptogorgia in ten 

 fathoms of water, of the same general habit as L. vir- 

 gulata, but of a deep rose colour, almost purple. The 

 ground-colour was mottled with white round the open- 

 ings of the polypes. A large number of molluscs were 

 found on the coral, and these had red-brown shells, with 

 the surrounding skin of a deep rose colour mottled with 

 white. Except for this difference in colour the molluscs 

 exactly resembled O. uniplicatum, and Mr. Osborn con- 



' H. L. Osborn, Science, New York, 1885, vi. pp. 9-10. 



