MOliLUSCA OF PLYMOUTH. 403 



posteriorly from tlieir origin on the foot to the level of the exit of 

 the pallial (anal) siphon. The edge of the mantle-folds, reflexed 

 over the shell and fused, bounds a circular aperture conducting to 

 the shell, but is never raised up into a tubular prominence. The 

 opaline gland behind the genital aperture is not lobulate (grape- 

 shaped) with a single pore, but consists of a number of large uni- 

 cellular bottle-shaped glands opening separately to the exterior. 

 Anteriorly, however, as Vayssiere has observed in A. punctata 

 (1. c, p. 54), these elongate gland-cells are bunched together, and 

 in my specimens have a single excretory pore which it is quite easy 

 to discover. 



Colour. — The colour of our smallest specimens has always been of 

 a more or less bright and deep rose-red, generally if not always 

 sprinkled with opaque white spots. At this stage our Aplysias 

 correspond with Rathke's " species " rosea and Thompson's nexa. 

 Our largest specimen measures (preserved in alcohol and conse- 

 quently much contracted) 3 inches in length. If inches in height, 

 and 1^ inches in breadth. Its shell is figured of the natural size on 

 PI. XXYIII, fig. 9 ; the structure of the central teeth of its radula 

 is shown in fig. 7. When alive this individual probably measured 

 rather more than 6 inches in length in a state of complete exten- 

 sion. The colour of this specimen and of the other large ones dredged 

 in May last year was olive-green,^ of various shades and intensities. 

 An individual which was living in the tank in the Laboratory for 

 some time in the autumn of last year, and measured about 4 

 inches in length during extension, was of a pure brown colour ; 

 while the specimen dredged on the 8th of May this year was kept 

 alive for some time, and being of a bright pink-red colour at the 

 time of capture had changed in a month's time (June 6th) to a 

 brownish red, and by the 23rd of June to a deep red-brown. Its 

 colour when captured was just that of the olga. Delesseria sang uinea ; 

 on the 6th of June it was exactly that of Iridxa edulis. Thus this 

 species changes its colour with growth from a violet, purplish, or 

 rose-red colour, through brownish red and brown to olive-brown or 

 olive-green. There is considerable variation, as is well known, but 

 these are, I believe, the chief changes which occur. ' Vayssiere 

 attributes the different colours of specimens of A. punctata at 

 Marseilles to the nature of the bottoms upon which they are found 

 (1. c, p. 69), but I may remark that the living Aplysia whose colour- 

 changes I observed was kept under the same conditions for the two 

 months during which it was under observation. 



• In recording a specimen found on the shore at St. Andrews, Prof. Mcintosh remarks, 

 " No spots or other markings were present on the dull olive hue of the body " (Mar. Inv. and 

 Fish, St. Andrews, p. 84). 



