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Show 56 MR. O. H. LATTER ON ANODON AND UNIO. [Jan. 20, entirely unable to make headway in any direction, even when they are thus temporarily suspended in mid-water. The Glochidia are evidently peculiarly sensitive to the odour ('.) of fish. The tail of a recently killed Stickleback thrust into a watch-glass containing Glochidia throws them all into the wildest agitation for a few seconds ; the valves are violently closed and again opened with astonishing rapidity for 15-25 seconds, and then the animals appear exhausted and lie placid with widely gaping shells- unless they chance to have closed upon any object in the water (e. g. another Glochidium), in which case the valves remain firmly closed. I found this excitement very useful in procuring Glochidia widely open. Flooding with hot corrosive sublimate kills them instantly and the shells remain apart. V. Relation of G\ochid\um-shell to Shell of Adult. So long ago as 1825 it was pointed out by Pfeiffer (Naturg. deutscher Land- und Stisswasser-Mollusken, Weimar, 1825), and more recently by Kobelt and Heynemann, that the shell of the Glochidium sits like a saddle over the dorsal and lateral surfaces of the shell of the young Anodon or may be seen in uninjured specimens close to the hinge-line. It has not, however, been noticed, so far as I can ascertain, that this temporary situation of the Glochidium-shell has a permaneut effect upon the shape of the adult shell. This effect will be at once apparent on referring to Plate VII. figs. 2-5. About 101 days after first attachment to the host and 25-30 days after quitting the host, the shell-teeth of the Glochidimn-shell project ventrally towards the median line, and as a consequence impinge upon the ventral border of the at present soft shell of the adult at a point about halfway along its length, the result being that at this point the permanent shell is prevented from growing so fast as elsewhere. The permanent shell at this stage, therefore, has its otherwise symmetrical curve sharply interrupted by an irregular notch pointing towards the dorsal surface (vide figs. 2 & 3). This notch, in the vast majority of cases, persists through life and causes a slight dorsal turn of the curves marking the lines of growth at a point roughly halfway along their length, but, as a rule, slightly nearer the posterior border of the shell. In each successive line of the growth the notch becomes of greater antero-posterior and less dorso-ventral extent, thus tending to become less evident and to disappear. The notch can therefore be seen most easily near the hinge-line (i. e. on the first lines of growth) in those shells which have escaped corrosion. In 15 species of Unio belonging to the Collection of Admiral Sir John Harvey in the University Museum, Oxford, this notch is evident and undoubtedly caused in the way above described ; it is perhaps present in 2 others ( U. cylindricus and U. triangularis), and is quite clear also in 6 species of Anodon. The figures given by Chenu in his ' Manuel de Conchyliologie,' and by M. Henri Drouet, " Unionidae du Bassin du Rhone," M e m . de l'Acad. des Sci. Aits et Beiles-Lettres de Dijon, (4) i. 1888-89, pp. 27-113, |
