border of the anterior adductor muscle, to form a kind of upper 

 lip. The inner palps unite to form a fold corresponding in position 

 to a lower lip. 



Make a drawing showing the arrangement of the soft parts. 



Cut off a bit of a gill with scissors and examine it in sea water 

 under a low power of the microscope. Observe : 



a. The surface is marked by a series of parallel ridges, the 

 gill filaments, which are united together, side by side, by inter- 

 filamentar junctions, to form a lamella; the spaces between the 

 interfilamentar junctions are the ostia. Each gill is composed of 

 two such lamellae, which are united together at intervals by inter- 

 lamellar junctions ; the spaces between these latter junctions form 

 water tubes, which open dorsally into the cloacal chamber. With a 

 high power observe : 



b. The movements of the cilia which cover the filaments. Place 

 a little powdered carmine on the surface of a gill submerged in sea 

 water and see what happens. 



Draw a surface view of a portion of a gill as seen under the loiv 

 power. 



VII. CIRCULATORY SYSTEM. 



Anterior to the posterior adductor muscle on the dorsal side is a 

 clear space, the pericardium, in which the heart lies. Open the 

 pericardium and observe the beating of the heart. The heart con- 

 sists of three parts : 



1. A central ventricle, which surrounds the intestine and gives 

 rise to a blood vessel at each end. 



2. Two triangular auricles, which receive blood from the gills 

 and open into the sides of the ventricle. 



Draw the lie art. 



VIII. EXCRETORY AND GENITAL SYSTEMS. 



Beneath the pericardium is a pair of dark colored excretory 

 organs. Each communicates with the pericardium by a small open- 

 ing and with the cloacal chamber by another small opening. The 

 genital glands are light colored organs that, during the breeding 

 season, extend through the principal part of the visceral mass. 



IX. NERVOUS SYSTEM. 



i. Cerebral ganglia; carefully remove the body-wall by the 

 side of the oesophagus and observe a rounded, slightly yellow or- 

 gan, about the size of a pin-head, lying just posterior to the dorsal 



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