108 



MOLLUSCA. 



always alike. What would happen if such movements were 

 made by a specimen lying on soft mud? Place a specimen on 

 mud and watch the results. 



2. Leave a specimen in an aquarium in which two inches of 

 bottom mud has been placed, and see if it is feeding in the morn- 

 ing. 



3. Place a young, transparent specimen in a watch-glass of 

 sea-water and study the parts. The foot has already been 

 observed. Its motions will probably be seen again here. It 

 has been considered a creeping organ. Do you find evidence 

 that confirms or opposes the view? With a low power of the 

 microscope notice: 



4. The palps. These are very large. The outer palp on 

 each side is provided with a long appendage that may be pro- 

 truded from between the valves of the shell. This is the feeding 

 appendage. 



5. The gills. These are quite small and are composed of 

 parallel plates that are capable of being moved. They are 

 situated behind the palps, are attached dorsally by muscular 

 membranes to the body- wall, and posteriorly to the wall that 

 separates the siphons. They illustrate what is supposed to be the 

 most primitive type of lamellibranch gill. Watch their move- 

 ments and see if you can determine how they cause the jets of 

 water to leave the cloacal siphon. What reason is there for 

 forming such strong jets of water? 



6. The heart and ganglia are nicely shown in such a speci- 

 men. 



7. Remove one of the shell valves of an adult specimen and 

 examine the organs. An elongated sense tentacle occurs on one 

 or the other side of the base of the branchial siphon, between the 

 wall of the siphon and the corresponding mantle lobe. 



A drawing of the organs will prove profitable. 



Drew: The Anatomy, Habits, and Embryology of Yoldia limatula. Mem. 



Biol. Lab. Johns Hopkins Univ., 4, 1899. 

 : The Life-History of Nucula delphinodonta. Quart. Jour. Mic. Sci., 



44, 1901. 

 Mitsukuri: On the Structure and Significance of some Aberrant Forms of 



Lamellibranchiate Gills. Quart. Jour. Mic. Sci., 21, 1881. 



