162 MR. R. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



between those of the opposite hemibranch, just as one set of cross-folds is 

 intermediate in position to the other. 



The base of each lamella is very broad. From it the lamella itself rises up 

 to project into the rectal cavity in a slantwise direction, with the distal end 

 lying towards the posterior end of the gill-basket. The angle between the 

 rectal wall and the gill-lamella is not constant, but varies with the act of 

 respiration. Also the broad basal part of the lamella is not in the plane of 

 the " blade " or main portion, but curves away from it so as to meet the 

 rectal wall, along its base-line, in a gentle incline (text-fig. 13). Thus, if 

 the lamella be viewed edge-on, the basal portion can be seen rising up above 

 the line of the blade, with the prominent basal pad (J)p) lying upon it. 

 When the rectum is expanded, the gills stand up in serried ranks, nearly 

 normal to the rectal wall. When the rectum is contracted, the laraellse fall 

 down so as to overlap one another, all pointing slantwise tov/ards the 

 posterior end of the basket. 



Svp tp ta lam 





A, 



'" ^y^TT 





■^? ' — ■ 



Text-fig. 13. — Aiistrocordulia refracta, Till. 



Three lamellae viewed edge-on to show arrangement of tnbercles. Semi-diagrammatic 

 from a camera-lucida drawing, (x 90.) b, base of insertion of lamella; bp, basal 

 pad ; lam, blade of lamella ; svj), supravortical papilla ; ta, tubercles on anterior face of 

 lamella ; tp, tubercle on posterior face. 



The lamellae do not, however, lie freely upon one another. They are 

 separated, each from each, by three small tubercles (text-fig. 13) first 

 discovered and described by Sadones in Lihellida. Apart from these 

 tubercles, the cuticle of the lamella is in most cases perfectly smooth. The 

 tubercles are situated as follows : — One (t'p) lies near the tip of the lamella 

 on its posterior or lower face ; a second {ta) lies somewhat distally from the 

 middle on its anterior or upper face ; and a third {ta) lies close to the base, 

 also on the anterior or upper face. By means of this arrangement, each 

 lamella is separated from the one below it by a tripod of three tubercles, of 

 which the outermost belongs to the upper lamella, the two others to the 

 lower. Thus, in all positions of the lamellse, a constant flow of water can 

 circulate between and around each separate lamella. The advantages of this 

 arrangement for the extraction of oxygen by the gills are so obvious as 

 to need no special comment. In Austrocordidia refracta (text-fig. 13) the 



