488 F. W. GAMBLE. 



is the otolith, composed of a vesicle containing a central con- 

 cretion bearing two double lateral ones. Immediately in 

 front of this is the single transverse brown " eye/^ and behind 

 it the brain. The male genital pore lies at the commence- 

 ment of the adhesive " tail ;" the female pore between this 

 and the pharynx. The testes are numerous. The vasa 

 deferentia run back to a muscular vesicula wliich opens into 

 a papilla surrounded by accessory glands. This soft papilla is 

 the copulatory organ. The single pair of germaria lie at 

 the base of the pharynx. The vitellaria occupy the sides of 

 the body. 



Habitat. — Hebrides (Claparede, 35) ; Firth of Forth, " on 

 Flustra hispida" (Dalyell, 29); Millport (v. Graff); St. 

 Andrews (Mcintosh, 45); Port Erin, Isle of Man; Plymouth 

 (F. W. G.). 



DiSTKiBUTiON. — Very wide. West coast of Greenland (Le- 

 vinsen), south and west coast of Norway (Claparede), Baltic 

 (Miiller, Schultze), North Sea (van Beneden), Madeira (Lan- 

 gerhans), Naples, Messina, Trieste (v. Graff), Black Sea 

 (Uljanin, Czerniavsky). 



This species is readily distinguished from M. fuse us by its 

 unarmed penis. 



53. MoNOTus Fuscus (Oersted, 16). 



Length 15 — 3 mm. Form similar to M. lineatus. 

 Colour very variable, usually brown, but white, purple, and 

 even dark blue varieties have been recorded by Jensen and 

 v. Graff. Examples 1 mm. in length are usually white and 

 colourless. Larger specimens (2 ram.) are frequently carmine, 

 gradually becoming brown as they grow older. The mean- 

 ing of this change^ in the reticular pigment is not understood. 

 Similar changes in some Opisthobranchiate Molluscs are 

 known (Aplysia, see Garstang, ' Journal Marine Biol. Assoc.,' 

 N.S., vol. i. No. 4, p. 411); and in this case the change in 

 colour appears to go hand in hand with a change of surround- 

 ' Already remarked by v. Graff (53), p. 422. 



