YOLDIA LIMATULA. 123 



YOLDIA LIMATULA, 



This form belongs to the order Protobranchia, and is supposed 

 to be one of the most primitive of hving lamellibranchs. It 

 lives in soft mud, such as is found in quiet coves and bays. 

 (It is abundant in the Eel Pond at Woods Hole.) Although it 

 burrows in the mud, it lives near the surface, and frequently 

 has the posterior end above the mud. 



1. Place a specimen in a dish of sea-water, and notice the 

 movements and shape of the foot. See if the movements are 

 always aUke. 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. They are situated behind the palps, are attached 

 dorsally by muscular membranes to the body-wall, and pos- 

 teriorly to the wall that separates ihe siphons. They illustrate 

 what is supposed to be the most primitive type of lamelli- 

 branch gill. Watch their movements and see if you can deter- 

 mine 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. 



