]891.] 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 ot 

 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 tlie 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 sht the ova pass, meet tlie stream of ova from the other side of 

 the body, and so reach the eshalant 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 oi ciliary currents by experiments with colouring- 

 matter, and it is improbable that any such reversal occurs. I have, 

 however, observed (v. itifra, 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 my 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. Further- 

 more, 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 



