1891.] MR. O. H. LATTER ON ANODON AND UNIO. 57 



pis. i.-iii., show the notch almost without exception. I do not rely 

 strongly on these figures for this particu:lar, as many irregularities 

 of curvature occur, owing to individual injury at some period of life, 

 and it is necessary to examine each specimen personally before 

 deciding whether the notch figured can in every case be assigned to 

 the Glochidian shell-teeth. 



I may take this opportunity of corroborating Schierholz's state- 

 ment (loc. cit.), concerning the absence of sexual distinction in the 

 shape of the shell. It is commonly believed that the shell of the 

 female is far more convex and of greater transverse diameter than 

 that of the male. This is not the case : there is no point by which 

 the shell of the female can be distinguished. On several occasions 

 I have requested persons professing to be able to distinguish the 

 sexes in this way to select a few males from my stock: out of 19 

 thus selected on various occasions only one proved on dissection to 

 be of the male sex, whereas on one occasion a small U. pictorum, 

 which was selected as " undoubtedly female " turned out to be a 

 male ! I have invariably found males very rare and was long unable 

 to procure one ; for instance, of 50 Anodons dredged from a small 

 pond in Norfolk, and averaging between 3 and 4 inches in length, 

 only two were males ; the same was true for Anodons and Unios 

 collected out of the canal at Oxford, though here the proportion of 

 males was slightly higher. So rare in fact were the males and so 

 small were the majority of them, that I was tempted to believe that 

 Anodon is hermaphrodite, functioning in early life as male and later 

 as female ; I made several experiments to investigate this point, but 

 obtained no evidence on either side. Stress of work has prevented 

 me from making any further search in this direction. 



VI. The Cilia on the Foot of Young Anodon. 



While observing young Anodons of 3-6 weeks old (dating from 

 the end of parasitic life), I was struck by the peculiar movements of 

 the cilia covering the foot. While the animal is in n)Otion the foot 

 is first protruded somewhat slowly until it stretches a considerable 

 distance in front of the anterior margin of the shell, the cilia all 

 the while moving with great rapidity and appearing to "feel the 

 way." The foot having been protruded to its utmost extent, the 

 shell is drawn forward by a rapid muscular contraction. As soon 

 as this contraction commences, the cilia suddenly cease moving and 

 stand out from the surface like the bristles of a brush absolutely 

 motionless and rigid. This condition is maintained until the foot 

 again commences to glide forward. I can offer no suggestion as 

 to the meaning or cause of this apparent rigidity other than that 

 the appearances are as though the pressure within the epithelial 

 cells becomes so great that the cilia cannot assume any other position 

 than one perpendicular to the surface, and forming a continuation of 

 the long axis of the cells on which they are severally carried. 



