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Show 54 MR. O. H. LATTER ON ANODON AND UNIO. [Jan. 20, pointed longitudinal ridge with divergent sides ; the pressure of water falls on these divergent sides and drives them together-the whole structure thus acting in the manner of the mitral valve of the human heart. It is probable that the flexible margins of the valves are also driven together by the pressure of water. The diagram exhibited (Plate VII. fig. 7) may make this clearer. I am inclined to think, then, that a suction of this kind is used to swiftly draw the ova forward into the external gill-plate. Direct observation on this point is well nigh impossible owing to the necessity of disturbing the animal or even partly opening the shell in order to ascertain whether or no ova are in transit. The fact that violent suction does take place in the case of the Glochidia is beyond doubt; the exact mode of causing the suction is, for our present purpose, of less importance. The question naturally occurs, why do not the ova find their way into the internal as well as the external gill 1 The reason is, I think, twofold. In the first place, the space between the lamellae of the external gill is considerably greater than that between the lamellae of the internal gill. In the second place, as I have ascertained by careful dissection of many individuals, the inner lamella of each external gill-plate extends further towards the dorsal surface than the outer lamella of each internal gill-plate, and stretches over towards the middle line so as to greatly diminish or even totally close the aperture leading into the space within the internal gill. In some cases the inner lamellae of the external gill-plates of the right and left sides actually come in contact with one another in the median line posteriorly 1. The diagram (Plate VII. fig. 8), which is a modification of Lan-kester's diagram (Encycl. Brit. 9th ed., Art. " Mollusca," fig. 135 D, p. 690), will make these relations clearer. II. The Attachment of the Glochidia to the Parent Gill-plate. It is well known that the epithelium of the external gill-plate secretes a nutritive mucus in which the young are imbedded and thus retained within the gill. This mode of attachment is, however, not permanent; for if, as is often the case, the Glochidia are retained for a long time after they have attained maturity, a large number escape from their egg-capsules, and the so-called " byssus," becoming entangled in the gill-filaments and bars of concrescence, serves to secure them until they are forcibly expelled from the parent. I have found that the number of Glochidia in any given parent which have escaped from their egg-capsules varies with the period during which they have been retained since the attainment of pre-parasitic maturity. It thus appears that as the nutritive mucus is used up, and its power of retaining the Glochidia within the gill is therefore 1 This, of course, applies only to the post-pedal portion of the gill-plates In the region of tbe foot the " labour contractions " close the space between the lamella; of the internal gill, as stated by von Baer. |