YOLDIA LIMATULA. 99 



order and position. (They should be placed in a dissecting pan 

 in a very little water.) 



Study these sections for the arrangement of organs. The 

 relation of the gills to the branchial and the cloacal chambers 

 should be understood. 



Make drawings of sections that pass through the heart and 

 through the posterior adductor muscle. 



If time permits, it will be desirable to become acquainted with 

 some of the structures of theoretic importance and with some 

 of the adaptations of lamellibranchs for the lives they live. For 

 this purpose a few forms have been selected, and directions for 

 the study of the particular parts in question are given. 



YOLDIA LIMATULA. 



This form belongs to the order Protobranchia, and is supposed 

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

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

 (It is abundant in the Eel Pond at Woods Holl.) 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 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 



