88 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 lamellae. 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 finer 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 



