THE HARD-SHELL CLAM 89 



bottom to the top of the gill and open above into the supra- 

 branchial passage. This is a wide canal running along the base 

 of each gill to the cloacal chamber. The course of the supra- 

 branchial passage of the outer gill has already been noted. In 

 order to observe that of the inner gill, lift up both gills ; the inner 

 suprabranchial passage will be seen at the base of the inner gill. 

 Probe from the cloacal chamber into it. Notice that back of the 

 visceral mass the two inner suprabranchial passages coalesce and 

 form 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 knife carefully separate the 

 lamellae. Mount a piece of a lamella in water and study it under 

 a compound microscope. Note the vertical interlamellar parti- 

 tions. Observe that the lamella is a delicate latticework made 

 up of ridges, the gill filaments, which run 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 of the branchial 

 chamber. The gill filaments are provided with cilia, as may easily 

 be seen if the gill is alive, the action of which causes streams 

 of water to pass into the water tubes. The course of the re- 

 spiratory water is from the branchial chamber into the water 

 tubes, through which it passes to the suprabranchial passages, 

 and through these into the cloacal chamber, whence it is ejected 

 through the cloacal siphon. 



Exercise 6. Draw a diagram of the respiratory system, showing the 

 gills and their relation to the suprabranchial passages. Show the 

 direction of the flow of the respiratory water by means of arrows. 



Exercise 7. Draw a diagram showing the structure of a lamella. 



The Circulatory System. With fine scissors carefully cut open 

 the pericardium by a slit along its dorsal border and expose 

 the heart. Note the heart, with the rectum passing through it. 

 The heart consists of three chambers ; a median, thick-walled ven- 

 tricle and two lateral auricles. These latter are delicate, thin- 

 walled organs, triangular in shape, the base of the triangle lying 



