82 THE NAUTILUS. 



I have no doubt that a careful histological study of the branchiae 

 and perhaps other parts of the animals of the Unionidfe will furnish 

 additional characters for classification. But it will be many years 

 before this can be done, and when it is, I feel sure that the results of 

 such study will fully agree with the characters of the shells and mar- 

 supia. 



HOW POTAMIDES (CERETHIDEA) CALIFORNIA HALD. TRAVELS. 



BY MRS. M. BURTON WILLIAMSON. 



In traveling over the mud-flats, Cerethidea caUfornica Hald. leaves 

 zigzag marks upon the sandy mud. These lines are made by the 

 apical whorls of the shell as it is dragged forward, or sideways. How 

 .does the animal crawl ? It appears to move forward by the contrac- 

 tion of the foot only, but my observations have led me to the conclu- 

 sion that the foot movement is somewhat secondary. The movement 

 of this tapering shell is one requiring considerable muscular strength. 

 This is very apparent. In order to study the mode of travel of this 

 species, a specimen was studied as it traveled up the sides of a glass 

 jar of sea-water, and this and other specimens were studied for sev- 

 eral hours. The conclusion reached was, that first there was a strong 

 muscular movement forward, then the foot advanced. The Cerethidea 

 pushes its head forward while the tentacles are expanded to their full 

 extent, the body whorl is raised with an effort, then the shell is pro- 

 pelled forward before the foot advances. Immediately, no time is lost, 

 the foot is spread out its full capacity and drawn forward. This is 

 immediately followed by a contraction of the foot in the posterior 

 part, then the head is again advanced. The strength of the animal 

 seems to be concentrated in the movement of the body-whorl as it is 

 raised up and forward. Of course all these movements are rapid, so 

 that it requires close observation to see that all movements are not 

 simultaneous, or at least which is secondary. A homely illustration 

 may be used to make this movement plainer. When a man attempts 

 to step upon a ladder with a hod full of bricks, or plaster, his first 

 movement is apt to be a hunching of the shoulders the weight being 



