that the two valves are frequently seen in the same plane. A long, 

 thread-like organ called a byssus is placed between the valves, and 

 this enables the young clam to anchor itself to a stone when it becomes 

 free. The lower surface of the shell forms two toothed hooks. 



" The young of some clams, as Anodonta, reach this condition in a 

 few days in the fall, and they remain in this state until the following 

 spring, when the parent discharges them into the water. The embryos 

 now swim about by opening and closing their shells and search for 

 a fish, into the fins or gills of which they drive their hooks and close 

 their shells. The fish covers the glochidium with a layer of living 

 cells and it becomes encysted. Here it develops gills, stomach, intes- 

 tines, and heart, and finally breaks the cyst or walls of its prison and 

 falls to the bottom of the water, a perfect young shell. From this 

 point its growth consists in simply enlarging its shell." 



We had all heard of mammals which carried their young in pouches, 

 like the kangaroo of Australia, and Harry said he had recently read 

 of some sea-urchins living in the Pacific Ocean which also carried their 

 young in inarsupia, but that clams should be marsupials was something 

 new to us. 



Professor Parker also pointed out the difference between the male 

 and the female shells, the latter having the lower posterior part of the 

 shell much swollen to accommodate the enlarged gills when they are 

 used as marsupia, while the male shell is thin and perfectly even in 

 this part. We also learned that only a few of the fresh-water clams 

 could be thus distinguished, for in a large number the shells of both 

 sexes are alike. 



After gathering a number of clams and snails we walked several 



miles to the shore of Lake M , where Professor Parker thought 



we might find some species which did not live in the smaller lakes 



and ponds. Just as we were leaving the head of Lake C , Harry 



discovered a pile of clams on the shore, from which the animals had 

 been neatly extracted, leaving the shells as perfect as any collector could 

 wish. The Professor told us that this was the work of the muskrat, 

 that animal being very fond of fresh-water clams. " The vicinity of its 

 burrows," he said, " are always good places in which to find perfect 

 and clean specimens of many comparatively rare species." 



When about half the distance to the lake had been covered, we came 

 to a small ditch. The Professor remarked that this ought to be a good 

 place for the small clams of the genera Sphcerium and Calyculina, 

 and so it proved upon examination. The muddy bottom seemed fairly 

 alive with their delicate, horn-colored shells, which were from a quarter 



8 



