MOLLUSC A. 31 



that all this, and the complicated structure shown by 

 the organs, occurs in a headless animal.* 



This last fact has a peculiar significance among 

 MoUusca, and can very properly be made the leading 

 idea in treating of the anatomical structures and the 

 general relations of the different forms comprised within 

 this group, the Lamellibranchiata. 



It must not be forgotten, however, that, though head- 

 less, they are not deficient in their nervous system. 



Some of these nerves can be seen readily enough, 

 as opaque white lines running along the bases of the 



* There are certain characteristics of the body of the oysters 

 of the highest interest, Avhich show that the bilateral symmetry 

 of the parts have been modified greatly by the habits of life of 

 the oyster, but which will be better understood after the study 

 of the clam and other forms. 



Though the shell is often curved to the right instead of to the 

 left, the animal is found to be in the same position inside the 

 shell. The young shell is usually, also, even in these reversed 

 specimens, curved in the normal way to the left. This reversion 

 occurs only where there is good mechanical reasons for it, where 

 there is no room for proper growth to the left, and so on. The 

 irregularity of the valves is also probably due to the fact that 

 the animal lies quietly on one side, and naturally produces more 

 shell with the mantle leaf, which is not occupied in the opening 

 or shutting of the valve, or in the function of passing the water 

 through the body. In the same way we can account for the 

 soldering of the mantle on the lower side to the body. The 

 former presence of an opening on this side, leading into the 

 gill cavity, is indicated by a depression near the intestine, which 

 is sometimes quite deep. This, and the perfect symmetry of the 

 young while it is a free animal, and of all the LamelUbranchs 

 which are free, clams, etc., shows that all of these irregularities 

 in the symmetry of the sides must have arisen from the mechani- 

 cal effect of the habit of living on one side. 



