I 4 6 



THURLOW C. NELSON. 



to the outside at the heel of the foot. The cells of this gland are 

 gorged with a transparent secretion which is not found in the 

 much shrunken gland which characterizes the larva immediately 

 after attachment. He believed that fixation was accomplished 

 l>y the larva extending the foot until the heel came to a position 

 near the upper anterior edge of the valve, and that the byssus 

 gland then discharged its secretion, which, flowing between the 

 left valve and the substratum, soon hardened and held the larva 

 fast. Stafford believed that the mantle played no part in the 

 fixation process. 



My own observations show that Stafford was right in consid- 

 ering the foot as the organ of final attachment. It is not thrust 

 forward, however, as he held, but is brought to the median posi- 

 tion. The extrusion of the mantle for a short period evidently 

 aids in the quick and economical distribution of the cementing 



fluid as it is poured out of the byssus gland at the ventral edge 

 of the left valve. This secretion hardens in less than 10 min- 

 utes. In Fig. 3 are shown a number of newly attached larvae 



I'll',. 3. Oyster larvae removed just after attachment and photographed from 

 the left side to show the area covered by the cementing substance from the byssus 

 gland. Magnified about 40 diameters. 



photographed from the left side to show the area covered by, and 

 the distribution of, (he cementing material. 



The use of the foot in creeping over surfaces has been described 

 in a number of larval bivalves, and it is the chief means by which 

 the young molluscs obtain foothold in a favorable locality. 



