448 Transformations of Planorbis. [June, 



the two muscles used to close the shell in the clams, g,g, Fig. 18, 

 PI. vii, and the beaks of the shell have shifted from the middle of 

 the back to the anterior end. Probably all attached animals show 

 this tendency. Their attached and supported parts, the bases of 

 the stems, etc., are irregular in form and growth, and their free 

 upper parts more or less laterally symmetrical. The radiate sym- 

 metry of the soft bodies of corals and of the harder, plate-cov- 

 ered cups of the Crinoids, the attached parts of the Ascidians as 

 compared with their freer bodies above; the perfect bilateral sym- 

 metry of the free moving parts of the Mollusca, as in the Helix, 

 Fig. 8, PI. vii, and Planorbis, Fig. 8 a, as compared with their sup- 

 ported spiral shells, the same perfection in the free Eolis, Fig. 2, 

 PI. vn, which has no shell in the adult, and in most of the Ptero- 

 pods, Fig. 7, PI. vn, free swimming animals, as well as in the 

 Cephalopods, Fig. 5, PI. vn. 



One of the best proofs of this position lies where it is least to 

 be expected. The Brachiopoda are attached by a peduncle, or 

 fleshy stem, and the upper valve, and not the lower, is the larger, 

 just the reverse of the oyster. 



Upon examination, however, it is found that it is the upper 

 valve which is held by the peduncle and the lower valve alone 

 opens and closes. Then again Lingula, another type of Brachi- 

 opod, which is not fixed by its peduncle, but simply occupies a sand 

 burrow, and can move its valves sidewise, one over the other, has 

 equal valves.' Professor E. S. Morse's investigations have shown 

 that the symmetry in these animals changes from a worm-like up- 

 right form of three rings or segments, and becomes laterally sym- 

 metrical by subsequent changes in the form of the first and second 

 segments, the third changing into the peduncle. Here then a 

 round worm-like form exchanges its cylindrical shape for a flatter, 

 shell-covered body in two of its segments, which become bent 

 over into' a horizontal position, while the third, which remains 

 vertical, retains the original round tubular form. 



The Anomia, Figs. 31, 32, PI. vn, presents a series of changes 

 very similar in their meaning, though this animal is closely allied 

 to the oyster. 



It has the lower valve flat, resting upon and taking the form of 

 the surf?ce upon which it grows. The upper valve, b, Figs. 30-31. 

 is convex and larger than the lower concave valve, a t Fig. 3°- and 

 is supported by a plug, h" ', passing through the lower valve. 



