76 PHYLUM MOLLUSCA 



partitions, the interlamellar partitions. The space between the 

 lamella is thus divided into parallel, vertical chambers, the water 

 tubes, which run from the bottom to the top of the gill and open 

 above into the suprabranchial passage. One of these passages 

 runs along the base of each gill, as a wide canal, to the cloacal 

 chamber. We have already observed the suprabranchial passage 

 of the outer gill. In order to observe that of the inner gill, lift up 

 both gills ; the inner lamella of the inner gill will, in most species 

 of clam, be seen not to be united with the wall of the visceral 

 mass along the hinder portion of the foot, but to have a free edge. 

 The long slitlike opening thus presented leads into the inner 

 suprabranchial passage. Probe it backward to the cloacal cham- 

 ber. Probe it also from the hinder end forward and notice that 

 back of the visceral mass the two inner suprabranchial passages 

 — that is, those belonging to the inner gills of the right and left 

 sides — coalesce, forming a single passage. 



Study the finer structure of the gills. Place a gill on a glass slide 

 in a little water and with forceps and a knife carefully separate 

 the two lamellae. Mount a piece of a lamella in water and study 

 it under a compound microscope. Note the vertical interlamellar 

 partitions. Observe that the lamella is a delicate latticework 

 made up of ridges, the gill filaments, running vertically and thus 

 parallel with the interlamellar partitions, and of cross ridges, the 

 interfilamentary connections, which run between and connect 

 the vertical filaments. The apertures in the latticework place the 

 water tubes in communication with the water in the branchial 

 chamber. The gill filaments are provided with cilia, the action of 

 which causes streams of water to pass into the water tubes. The 

 cilia may easily be seen by mounting a piece of a gill of a freshly 

 killed animal on a slide and examining it under a compound mi- 

 croscope. The course of the respiratory water is from the bran- 

 chial chamber into the water tubes, through which it passes into 

 the suprabranchial passages and through these into the cloacal 

 chamber. 



The outer gill and sometimes the inner gill also act as a brood 

 chamber in the female clam, and during the breeding season will 

 be found to be distended with eggs, or embryos. 



