228 F. W. GAMBLE, B.SC. 



Should it meet with an obstacle it retracts with amazing rapidity. 

 The rhabdites are only feebly developed. Owing to the great con- 

 tractility of the body the positions of the organs are difficult to 

 define. Considering, however, the animal to be in a fully extended 

 state, the mouth is a short distance behind the centre of the body. 

 The pharynx is cylindrical, very muscular, its proximal end being 

 almost central. The gut is extensive ; when contracted it becomes 

 distinctly sacculated. In the middle line anteriorly is the otolith, 

 composed of a vesicle containing a central concretion 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 commencement of the adhesive "tail;" the female 

 pore between this and the pharynx. The testes are numerous. The 

 vasa defer entia run back to a muscular vesicula which 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 

 Mspida" (Dalyell, 29); Millport (v. Graff); St. Andrews (Mcintosh, 

 45) ; Port Erin, Isle of Man ; Plymouth (F. W. G.). 



Distribution. — Very wide. West coast of Greenland (Levinsen), 

 south and west coast of Norway (Claparede), Baltic (Muller, Schultze), 

 North Sea (van Beneden), Madeira (Langerhans), Naples, Messina, 

 Trieste (v. Graff), Black Sea (Uljanin, Czerniavsky). 



This species is readily distinguished from M. fuscus by its unarmed 

 penis. 



53. Monotus fuscus (Oersted, 16). 



Length l - 5 — 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 mm.) 

 are frequently carmine, gradually becoming brown as they grow 

 older. The meaning of this change* in the reticular pigment is not 

 understood. Similar changes in some Opisthobranchiate Molluscs are 



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



