MOLLUSOA OF PLYMOUTH. 423 



i^awi/y— PHYLLOBRANCHID^. 



11. Hkkm^a, Loven. 



12. Hermj:a bifida, Montagu. 



A single specimen of this very interesting species was discovered 

 by Prof. Johnson creeping over a frond of Delesseria hypoglossum 

 obtained at St. Peter's Point below St. German's River, on September 

 26th, 1889. In length it was -^ inch. The head, tentacles, and 

 body were of a pale, delicate, transparent green colour ; the lateral 

 hepatic canals and their branches to the pleuropodial cerata were of 

 a pink colour, exactly resembling that of the alga upon which the 

 animal was feeding. The right hepatic canal extended almost to 

 the posterior end of the body, the left ceased more anteriorly. The 

 cerata (homologous in a general sense with the pleuropodial folds of 

 Elysia, the lateral fins of Lobiger and Aplysia, and the dorso-lateral 

 processes of Lomanotus and Tritonia) consisted of five large ones on 

 each side alternating with one or sometimes two small ones. I 

 cannot speak with any emphasis, having examined only this one 

 specimen, but if this alternating disposition of the large and small 

 cerata exists regularly in young individuals (cf. Hermsea cruciata, 

 Agassiz^), a comparison can be made between the pleuropodia of this 

 genus and the pleuropodial folds of Tritonia, which are also arranged 

 in a waved line down each side of the back with alternately larger 

 and smaller processes. The cerata were coloured like the hepatic 

 canals, but differed in being transparent. 



As Mr. Poulton has already stated in his recent work. The 

 Colours of Animals (pp. 70, 200), I found that when a shadow was 

 caused to pass over this active little Nudibranch it at once contracted 

 itself, drawing in its head and erecting briskly its cerata. The 

 reaction to shadows is correlated with the unusually large eyes of 

 this species, and is paralleled by a similar reaction, as I shall show 

 below, in a true .<:Eolid, Coryphella gracilis. 



The pink colour of the hepatic canals and their intra-ceratal 

 branches disappeared in my specimen entirely after twelve days' 

 captivity, apparently owing to its refusing to feed any longer upon 

 the Delesseria which was placed in the same dish of sea water with 

 it. On September 29th the colour was paler than at first, and a 

 number of opaque white spots (mucous glands ?) became conspicuous 

 upon the rhinophores. Next day the colour of the hepatic branches 

 was very much paler, hardly noticeable, but the faint rosy colour of 

 the larger cerata and the greenish colour of the rhinophores were 

 still persistent. The opaque white spots had attained a great deve- 

 lopment upon the rhinophores, head, and cerata. On October 8th 

 ' See Verrill, Sep. Invert, yineyard Sound, 1873, pi. xxv, fig. 175. 



