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 

 lamellee of the internal gill. In the second place, as I have ascer- 

 tained 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 \ 



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-calied " 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 



^ This, of course, applies only to the post-pedal portion of the gill-platea. 

 In the region of the foot the " labour contractions " close the space between the 

 lamella of the internal gill, as stated by von Baer. 



