108 INVERTEBRATE ZOOLOGY 
study it under a compound microscope. Note the vertical 
interlamellar partitions. Observe that the lamella is a delicate 
lattice work 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 lattice 
work 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 be alive, the action of 
which causes streams of water to pass into the water-tubes. 
The course of the respiratory water is from the branchial cham- 
ber into the water-tubes, through which it passes to the supra- 
branchial 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 ventricle 
and two lateral auricles. These latter are delicate, thin-walled 
organs, triangular in shape, the base of the triangle lying along 
the dorsal border of the gills and the apex communicating with 
the ventricle. If the left auricle has been injured in the 
dissection, the right one is easily seen by looking across the 
pericardial space. From the ventricle an anterior and a pos- 
terior artery pass to either end of the body. The posterior 
artery expands, near the posterior end of the pericardium, to 
form a large thick-walled sac, the arterial bulb. These two 
