670 LIFE : OUTLINES OF GENERAL BIOLOGY 



valve of an oyster is flatter and thinner than the downtumed, 

 externally very convex left valve. But one would not because of 

 such minor inequalities exclude an oyster from among the Bilateral 

 animals. It is very different, however, with a snail, and with the 

 majority of Gastropods, for in these there is an early acquired 

 asymmetry. In connection with the enclosure of the body in a 

 protecting shell there is a spiral coiling of the viscera; moreover, 

 there is a deep-seated torsion of a large part of the body to the 

 right-hand side forwards; so that the anus, the excretory pore, and 

 the genital aperture come to lie anteriorly towards the mouth — a 

 twisting which is partly connected with the presence of an enclosing 

 shell. The torsion of the snail's body is a difficult problem, beyond 

 our scope here, but it is plain that the asymmetry does not in any 

 way interfere with the creature's efficiency. Among other markedly 

 asymmetrical animeils, which cannot possibly be divided into mir- 

 roring halves, may be mentioned the typical ascidians or sea-squirts. 



Hermit-crabs are interesting in this connection because they are 

 bilateral in their young stages, and become asymmetrical in their 

 abdominal region, and usually as to their great claws (chelae), 

 when they begin to ensconce themselves in borrowed Gastropod 

 shells. The lop-sidedness of the tail may be in some degree an indivi- 

 dual modification, impressed on each generation, but the experi- 

 ment has not been made of supplying the young stages with nothing 

 but symmetrical tubes. On the other hand, an Indian Ocean form, 

 Pylocheles miersii, which inhabits pieces of bamboo stem, is per- 

 fectly symmetrical, even as regards the abdominal appendages and 

 the chelae which form the door of the house. This would suggest 

 that the asymmetry of ordinary hermit-crabs is altogether an 

 individual modification. That this is not the case seems to us to 

 be indicated by the interesting asymmetry in the abdominal region 

 of the Stone-crabs, Lithodes, which do not borrow a shell, and of 

 the very distantly related Robber-crab {Birgus latro), which some- 

 times protects its tail in the broken shell of a coco-nut. Now experts 

 on Crustaccims seem to be agreed that the Stone-crabs and the 

 Robber-crabs are descended from hermit-crabs (Paguridse) that 

 live in shells, though from different stocks of these. But since these 

 two types show distinct traces of the Pagurid asymmetry, the case 

 indicates either [a) that this feature cannot be wholly an individual 

 modification, or [b) that an acquired somatic modification may be 

 in some measure transmitted to forms in which the modifying 

 influence does not operate. (See section on "the transmission of 

 acquired characters" in the chapter on Evolution.) 



Changes of symmetry in the course of the lifetime are not 

 uncommon. Thus all the bony flat-fishes (like plaice and sole) are 

 bilaterally symmetrical in their early larval stages, and the free- 

 swimming Tunicate larva is also bilateral. The conditions in many 



