82 A FRESH-WATER MUSSEL. 



Technical Note. With a sharp scalpel shave off portions of the wall of 

 the visceral mass. 



f) Note anteriorly a dark glandular mass, the liver, surrounding a 



cavity just back of the mouth. This cavity is the stomach ; it is 

 connected with the mouth by a short oesophagus. The intestine 

 has several coils in its course from stomach to rectum. To trace 

 these out will probably require more time than the student has at 

 his disposal. 



g) A cream-colored granular substance appears to fill the bulk of the 



visceral mass. This forms the reproductive organs, indis- 

 tinguishable in the two sexes except by microscopic examination. 

 Note, imbedded in the mass, sections of the intestinal coils. 



Exercise 5. Complete Exercise 4, showing the location of the excretory 

 organs, stomach, liver, oesophagus, and reproductive organs, with 

 reference to the other structures. 



THE GILLS. Examine the structure of a gill. Each is composed 

 of two plates or lamellae united along their lateral and ventral margins, 

 but separate above. A gill is thus a deep trough with a V-shaped 

 cross-section. Vertical partitions in the trough (inter-lamellar 

 junctions) divide it off into distinct compartments called water tubes. 

 These partitions can easily be seen on the dorsal border of the gill. 

 The lamellae themselves are wicker-like, being made up of a great many 

 vertical gill filaments connected by horizontal inter-filamentar 

 junctions. The spaces between the filaments and the junctions are 

 called ostia. Through these openings pass the water currents induced 

 by the action of cilia along their borders. Up through the vertical 

 water tubes, back along the suprabranchial passages and out at the 

 exhalant siphon the water continues to flow, ae'rating in its passage the 

 blood in the walls of the gills. The cilia also aid in sweeping forward 

 the food particles to where they can be helped along by the cilia of the 

 labial palps. 



Exercise 6. Study, under the microscope, bits of tissue scraped from the 

 gills and from the inner surface of the palps of a live mussel. Watch 

 the action of the cilia. Make a diagram of the structure of a gill. 



