134 MOLLUSCA 



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 lower power of 

 the microscope study: 



(a) Palps. These are very large. The outer palp on 

 each side is provided with a long appendage that may be 

 protruded from between the valves of the shell. This is the 

 feeding appendage. 



(6) Gills. These are quite small and are composed of 

 parallel plates. They are situated behind the palps, are at- 

 tached dorsally by muscular membranes to the body wall, and 

 posteriorly to the wall that separates the siphons. They il- 

 lustrate what is supposed to be the most primitive type of 

 lamellibranch gill. Watch their movements 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? 



(c) Heart and Ganglia. Nicely shown in such a speci- 

 men. 



4. 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. 



MYTILUS OR MODIOLUS (Mussels) 



These animals belong to the order Filibranchia, and show 

 comparatively simple gills, as well as interesting modifica- 

 tions for their manner of living. They live attached to 



