FALSE SYMMETET. 339 



form of the fully developed sexual animal ; during this stage 

 they have two shells, connected, like those of bivalve mollusca, 

 by an elastic ligament at the dorsal margin. It is probably at 

 this stage that they attach themselves to the abdomen of a 

 crab ; this compels the Cypris-like larva to lie on one side, 

 since there is no room for it to occupy a perpendicular position 

 between the thorax and the folded-up abdomen of the crab. 

 Since, moreover, they establish themselves, almost without excep- 

 tion, in the centre of the crab's body, and ere long cast off 

 the hard shells of the larva form, to allow of the growth of the 

 somewhat soft permanent skin, the back and front of the 

 Pachybdella will continue to grow in similar directions, and it 

 is easily explicable on mechanical grounds that the two edges 

 must be symmetrical in their growth, in consequence of the 

 symmetry of the form of the crab's abdomen. In most animals 

 it is not the back and belly that are symmetrical, but usually 

 the right and left side. Now, since the left side of the larva of 

 the Pachybdella is applied to the thorax of the crab and the 

 right to the surface of the abdomen, it need not surprise us to 

 find that the two sides of the body, which in other animals are 

 usually symmetrical, have become dissimilar in consequence of 

 the unusual pressure upon them, and so their normal symmetry 

 is lost. 'This is actually what takes place, as Kossmann has 

 also proved ; the markings and the hairs of the skin of many 

 species of Pachybdella are extremely diiferent on the right side 

 and on the left. This renders the comparison with the unsym- 

 metrical flat-fish — as Plaice — even more striking, but in them 

 the false symmetry of the front and back is less distinctly 

 marked, than in the Pachybdella. 



The method here suggested as an explanation of the false 

 symmetry of Pachybdella presupposes, however, that only one 

 individual at a time shall have attached itself to the crab, for 

 in that case only can the abdomen of the crab be capable of 

 pressing the Pachybdella, as it grows, into this particular form. 

 And indeed only one individual is commonly found on each 

 crab. But when — as sometimes, though very rarely, happens — 

 more larvae than one establish themselves almost simultaneously 

 on a crab's tail, these must mutually hinder each other in the 



