MOLLUSCA OF PLYMOUTH SOUND. 191 



it is obtained occasionally between tide-marks and in shallow water. 

 Prof. Stewart tells me that it used to be abundant at Cremyll. 



Giard has pointed out the great resemblance which an individual 

 of this species bears to a contracted Sagartia troglodytes. It is very- 

 probable that we have here a case approaching one of true mimicry, 

 for Actinians are, as a rule, carefully avoided by shore-fish on account 

 of their nematocysts, which, as I have several times tested, are very 

 irritating to the lining membranes of the mouths of fishes. The fish 

 are additionally deceived because of the possession by the Nudibranch 

 of nematocysts at the tips of the papillge. Some of the shore-fish 

 which frequent rock-pools I have noticed to touch their intended 

 prey, before eating it, with the fleshy margins of the mouth ; and 

 the usefulness of the possession of a certain number of irritating 

 thread-cells is at once obvious. Further, the habit, which has been 

 so often noticed in this and other Eolids, of erecting the papillee when 

 disturbed seems to me to be correlated with this function of the 

 thread- cells, for by the bristling up of the papillas the nematocysts 

 at their tips are placed in the most advantageous position for the 

 assault. 



It is very interesting to note that in the two British species of 

 Sermoea, which are inconspicuously coloured with respect to their 

 surroundings, nematocysts appear to be absent. Hermoea bifida 

 lives among red weeds of the genus Grifiithsia ; the body and papillaB 

 generally are very transparent and of indefinite outline, hut the 

 lateral hepatic canals of the body and the branches to the papilla are 

 highly developed and very conspicuous, having the form and colour of 

 the branches of the weed. The protective resemblance is here very 

 perfect, though there still seems to be another resource for the 

 animal when discovered, viz. the ejection, when touched, of a fetid, 

 colourless fluid (see Alder and Hancock, 2). 



Hermoea dendritica is coloured bright green, with dendritic mark- 

 ings, and lives upon the green weed Codium tomentosum. 



For description of another possible species of this genus see 

 No. 32. 



17. Facelina, Alder and Hancock. 



26. F. CORONATA, Forbes. 



Late in March of this year a specimen of this species, one and a 

 quarter inches in length, was dredged off the Mewstone. On May 

 11th six magnificent specimens were dredged off the same ground, 

 with a mass of Syncoryne. Some spawn was brought up as well, 

 and a young individual was only three eighths of an inch long. 



