| OCR Text |
Show 1891.] MR. O. H. LATTER ON ANODON AND UNIO. 53 somewhat posterior to the external aperture of the nephridium ; it is slightly anterior to the commencement of the free detached dorsal border of the inner lamella of the internal gill-plate. The ova may be seen through the thin epithelial covering on the dorsal margin of the foot, passing along the oviduct to the genital aperture. After escaping through this pore they are conveyed backwards along the external surface of the nephridium. This surface is densely covered with cilia borne upon tall columnar cells, with a large oval nucleus lying in their lower portion and resting on a definite basement membrane. In the middle line of the nephridial surface the cilia are longer and drive the ova straight backwards ; towards the ventral and dorsal sides of the nephridial surface the cilia are shorter and drive the ova obliquely backwards and towards the line of the longer cilia, so that the latter tend to keep the ova in the middle line where the ciliary currents are strongest. The arrows (Plate VII. fig. 6) show the direction of the currents. The total effect of the cilia is therefore to drive the ova straight backwards along the middle line of the nephridial surface. In the course of about 50 seconds an ovum is thus swept back to the slit between the retractor pedis muscle and the point of fusion of the internal gill-plates. Through this slit the ova pass, meet the stream of ova from the other side of the body, and so reach the exhalant branchial current and the cloaca. The process goes on for several days (10 or 11) in each individual. This being the case, according to von Baer's theory the shell must remain closed during the whole of this period, or, in other words, respiration be suspended for nearly a fortnight. This appears to me incompatible with the continued life of the individual. In order that the ova may reach their final resting-place there must be some reversal of the respiratory currents. I was unable to detect any reversal of ciliary currents by experiments with colouring-matter, and it is improbable that any such reversal occurs. I have, however, observed (v. infra, p. 55) a violent reversion of currents, due, I believe, to suction, during the emission of Glochidia. This suction is probably effected by relaxation of the adductors and consequent partial opening of the shell while the right and left mantle-margins are kept in contact so as to block the aperture at all other parts except the two siphonal notches, of which the exhalant in particular remains open. The thickened margins of the mantle thus serve to temporarily close the aperture between the two valves, and, if m y explanation be correct, the muscle-fibres of the mantle between the point of attachment of the mantle to the shell and its free border may tend to draw the right and left thickened borders together in the middle line, while also increasing their thickness and offering a more solid resistance to the water. Furthermore, when once the thickened borders of the mantle are in apposition and the shell commences to gape, the pressure on the right and left free borders will tend to drive them even more closely together; for the line of the mantle which is attached to the shell must of necessity follow the outward movement of the valves when gaping commences, and the free borders unite to form a bluntly |
