39 



In the accompanying plate (Plate IV) the small figure in 

 the left hand corner indicates a smaU detached fragment of 

 Victor ella pavida of the natural size (fig. 1) ; the centre 

 group (fig. 2) is the same considerably enlarged, and beneath 

 that again is a piece of Cordylophora lacustris magnified a 

 few diameters, and showing the mode in which the Polyzoon 

 attaches itself to it (fig. 3) . 



On a Crustacean Parasite o/ Nereis cultrifera, Grube. 

 By W. C. McIntosh, M.D., F.R.S.E., F.L.S. With 

 Plate V. 



Crustacean parasites (Ectozoa) of the Annelids would 

 not appear to be very common ; but this may arise in some 

 measure from their having been overlooked, rather than 

 from their actual rarity. So far as I at present am aware, 

 Dr. H. Kroyer is the only author who has described such a 

 parasite upon an Annelid (' Naturhistorisk Tidsskrift,' 1864, 

 p. 403, Tab. 18, f. 6, a—g). This species, Silenimn polynoes, 

 Kroyer, he found on an example o( Polynoe chirata, O. Fabri- 

 cius, (the well-known and Avidely distributed/farmoMo^' imbri- 

 cata, Lin.) . In Silenium polynoes the body of the female is in 

 the form of a simple saccate mass, with a petiolate process 

 for attachment, and furnished with two flask-shaped ovaries 

 at the posterior extremity. There are neither antennae, 

 rostrum, nor feet. The male, again, is much more minute, 

 though more complex in organisation, for the produced an- 

 terior region of the cephalothorax is supplied with four pairs 

 of feet, and the caudal process is triarticulate and setose. 

 Dr. Baird also mentioned to me that he had seen a small 

 crustacean parasite attached to a foreign Lepidonotus, while 

 examining the collections in the British Museum. 



On the rich shores of the Channel Islands several examples 

 of Nereis cultrif er a, Grnhe, a plentiful species under stones and 

 in other places between tide-marks, were infested by a crus- 

 tacean parasite, which, from its size, and the colour of the 

 ovisacs, was very easily observed. They generally occurred 

 towards the posterior end of the worm, and the largest spe- 

 cimen of the worm had about fifteen examples of the parasite, 

 most of them, however, being small. The usual number was 

 from three to five. They either adhered to the groove be- 

 tween two feet, or to the sides of the feet. In the living 

 state they are of a dull whitish colour, and clings most se- 

 curely to the w^orms, but they do not seem to incommode 

 them to any extent ; and it may be supposed that the Nereis 



