98 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 



