424 THE OPISTHOBRANCHIATB 



the animal was perfectly colourless and quite transparent except for 

 numbers of opaque white spots on the rhinophores and cerata. 



13. Herm^a dendritica, Aid. and Hanc. 



We owe again our only examples of this species to Prof. Johnson, 

 who, while examining some Bryopsis plumosa from a tide-pool from 

 the south-east side of Drake^s Island, discovered two individuals 

 creeping on the weed, August 5th, 1890. 



They applied themselves usually to the stem of the weed, and 

 crawled about actively among the tufts and branches, being excel- 

 lently concealed from observation by their form and the green 

 colour of their hepatic canals. The mucus of the foot is very adhe- 

 sive, and this enables the animals to cling tightly to the weed ; it is 

 indeed a most difficult thing to remove one forcibly from it. I 

 repeatedly tined the experiment of passing a shadow over them, both 

 when in the weed and when creeping openly on the bottom of a 

 capsule, but never obtained the reaction of the cerata described 

 above in H. bifida. The eyes, though conspicuous for a Nudibranch, 

 were not so large, if I remember rightly, as in the latter species. 

 The erection of the cerata may, however, be produced by touching the 

 head with a blunt needle. 



I tried also the experiment of adding a number of differently 

 coloured algge to the dish of sea water in which the Hermasse were 

 living. On the first night the green weeds supplied were Entero- 

 morpha, Ulva, and Bryopsis, and representing the red weeds was a 

 tuft of Antithamnion plumula. Next day the Hermsese. were on the 

 Enteromorpha and Ulva, not on the Antithamnion. Some additional 

 red weeds, RJiody menia laciniata and 8 pondy loth amnion multifida,^ 

 were then put in to afford more chance of the Hermsese meeting the 

 red weeds in their peregrinations. In spite of this, on the next day, 

 August 8th, one was crawling on the sides of the dish, the other was 

 on the Bryopsis. Next day one was again on the Bryopsis, the 

 other was swimming inverted at the surface of the water. Indeed, 

 within the fortnight during which they were kept alive, they were 

 frequently observed upon the green weeds, especially the Bryopsis, 

 never on the red ones. This indicates with much probability that 

 the green species of Hermasa avoids the red weeds upon which it 

 would be conspicuous, and does not avoid the green weeds upon 

 which it is concealed from observation. 



As with H. bifida, the distinctive colour of these specimens faded 

 entirely after a certain time of captivity. By August 13th the green 



' I need hardly say that I owe the identification of many of these algse to my friend 

 Prof. Johnson, who has rendered me much help in this way during his visits to Plymouth. 



