52 HERDMAN : PLEUROPHVLLIDIA LOVENI IN BRITAIN. 



with minute spiculose papillae, the structure of which has been 

 described above. 



The only external structures in the nudibranch which are 

 altogether unrepresented in the sponge are the two dorsal tentacles 

 or rhinophores ; and these might conceivably nullify the effect of all 

 the resemblances which have here been shown to exist. This 

 obstacle, however, has been overcome in a way that points most 

 decisively to the operation of natural selection. The presence of 

 conspicuous spots on the back of the nudibranch, coloured darkly 

 like the rhinophores, and arranged so that the rhinophores are 

 included in the same series, effectually deceives the eye, and conceals 

 the existence of projecting rhinophores from any but the closest 

 scrutiny. The generally asymmetrical position of the dark spots, 

 and the irregularity of their size and tint, also conspire to prevent 

 the formation of any suspicion. 



In conclusion, it should be stated that the advantage to the 

 species of so close a mimicry of sponges must be considerable, since 

 it has already been shown by me ("Nature," 1890, p. 418) that 

 sponges are highly distasteful to predatory fishes, and are shunned 

 by them under natural as well as artificial conditions. 



AN ADDITIONAL OCCURRENCE OF 

 PLEUROPHYLLIDIA LOVENI IN BRITAIN. 



By \Y. A. HERDMAN, D.Sc, F.R.S., F.R.S.E., 



Professor of Natural History, University College, Liverpool. 



In Professor MTntosh's interesting " Note on the occurrence of 

 Pkiirophyllidia love?ii in Britain," in the " Conchologist " for June, 

 p. 21, one record seems to have escaped notice. It is stated in the 

 Note that since the specimens mentioned in " Forbes and Hanley's 

 British Mollusca," nothing appears to have been heard of the species 

 " till the Trawling Expedition of 1884." But during that interval a 

 specimen was taken by the late Professor F. M. Balfour, off Dunbar, 

 in mud, from a depth of thirty fathoms, as is recorded in Leslie and 

 Herdman's " Invertebrate Fauna of the Firth of Forth," published 



