94 INVERTEBRATE ZOOLOGY 
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 lamelle. Mount a piece of a lamella in water and 
study it under a compound microscope. Note the vertical inter- 
lamellar partitions. Observe that the lamella is a delicate lattice 
work 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 lattice 
work place the water-tubes in communication with the water in 
the branchial chamber. The gill-filaments are provided with 
cilia, as may be easily 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 
chamber into the water-tubes, through which it passes into 
the suprabranchial passages and through these into the cloacal 
chamber. 
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 minute structure of a 
lamella. 
The circulatory system. With fine scissors and great care cut 
open the pericardium by a slit along its dorsal border. 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 
