1 882.] Transformations of Planorbis. 447 



are free moving, bilaterally symmetrical, and for a time attached 

 by a byssus, and then lie over on one side. 



Their symmetry is precisely accordant with these changes end- 

 ing with having the lower valve permanently attached and larger 

 and more concave than the upper. 



Fig. 14, PI. vii, shows the outlines of a clam shell above that of 

 an oyster shell, and both in their real positions for comparison 

 with each other, the beaks of the clam shell being upon the back 

 and those of the oyster shell across the anterior or mouth end of 

 the animal which has built the shell. The positions of the mouth 

 in each animal are shown at /, /. 



Fig. 18, PI. vii, shows one side of a clam supposed to be buried 

 in the mud with the siphons extended to the surface and the man- 

 tle border or shell-building organ d +, and the digging foot &, in 

 their natural expanded condition. 



What are the changes which can take place in a member of the 

 oyster family by change of habit P 1 Can an animal of this family, 

 which i.s always unsymmctrical when lying on one side, have a 

 different position and thus return to the normal condition and 

 have valves which are symmetrical and bilateral ? 



This question is answered by Lima, one of the same group as 

 the oyster, a free swimmer but also burrowing into sponges as the 

 clam does into the earth. This change of habit produces a cor- 

 responding change in symmetry, and it becomes like the clam, 

 also perfectly bilateral, Fig. 16,' PI. vn. 



On the other hand, can one of the fresh-water clams, Fig. 3, 

 y, PI. vn, which are the reverse of the Ostreads, being almost 

 invariably free moving or burrowing, and habitually bilateral, be- 

 come attached, and if so does its shell become distorted like that 

 of an oyster ? 



The answer to this is Mulleria and genus Algeria. Mulleria, 

 Fig. 19, becomes attached and is distorted so as to resemble the 

 oyster and not only that, but the animal changes, since there is 

 but one large muscle g, as in the oyster, Fig. 1, PI. VII, in place of 



