OCR Text |
Show 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 particular, 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 statement (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 motion 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. |