50 OSWALD H. LATTER, M.A. 



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 I. 

 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. infrcl, p. 53) 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 aperature 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 



