78 STUDIES IN AJSTIMAL LIFE. 



backbone forms, as it were, an axis, on either side 

 of which the organs are disposed ; but the mollusk 

 is said to have no such axis, no such symmetry. I 

 admit the absence of an axis, but I deny the total 

 absence of symmetry. Many of its organs are as 

 symmetrical as those of a vertebrate animal i. e., 

 the eyes, the feelers, the jaws and the gills in Cut- 

 tlefish, Eolids, and Pteropods ; while, on the other 

 hand, several organs in the vertebrate animal are as 

 imsymmetrical as any of those in the mollusk i. e., 

 the liver, spleen, pancreas, stomach, and intestines.* 

 As regards bilateral structure, therefore, it is only 

 a question of degree. The vertebrate animal is not 

 entirely symmetrical, nor is the mollusk entirely 

 unsymmetrical. But there is a characteristic dis- 

 position of the nervous system peculiar to mol- 

 lusks : it neither forms an axis for the body, as it 

 does in the Vertebrata and Articulata, nor a centre, 

 as it does in the Eadiata, but is altogether irregular 

 and unsymmetrical. This will be intelligible from 

 the following diagram of the nervous systems of a 

 mollusk and an insect, with which that of a starfish 

 may be compared (Fig. 18). Here you perceive 

 how the nervous centres and the nerves which 



* In some cases of monstrosity these organs are transposed, the 

 liver being on the left, and the pancreas on the right side. It was 

 in allusion to a case of this kind, then occupying the attention of 

 Paris, that MOLIEEE made his Medecin malgre Lui describe the 

 heart as on the right side, the liver on the left ; on the mistake be- 

 ing noticed, he replies, " Oui, autrefois ; mais nous avons change 

 tout cela.'" 



