NO TES. 43 



such structures could have arisen de novo, we may at least attempt 

 to appreciate the necessity for their existence. 



There is one difference between the general form of the body 

 in Invertebrates and Vertebrates respectively which seems to be 

 of fundamental importance, but which has not been sufficiently 

 emphasised. As a general rule, in the Invertebrates, the body 

 is not bilaterally compressed, but, on the contrary, is either cylin- 

 drical, sub-cylindrical, or flattened dorso-ventrally. Obvious ex- 

 ceptions to this rule are presented by the Lamellibranchiate 

 Molluscs and by many Arthropods ; but these exceptions are 

 readily intelligible as secondary modifications. 



On the other hand, in the more primitive Vertebrates (i.e. 

 fishes), the bilateral compression of the body is one of the car- 

 dinal features of the external form. To this fundamental rule 

 there are of course exceptions afforded, for example, by the 

 skates ; but it is a self-evident fact that these again have arisen 

 by secondary modification from originally bilaterally compressed 

 forms. With the evolution of the pentadactyle appendages and 

 the assumption of a terrestrial existence, the shape of the body 

 in the higher Vertebrates has undergone such changes that the 

 primitive bilateral compression of the body is, as a rule, only 

 present at some period of the embryonic development. 



Amphioxus exhibits the characteristic vertebrate bilateral com- 

 pression of the body in a very typical manner ; while Balano- 

 glossus shows invertebrate affinities in regard to the shape of the 

 body, which is sub-cylindrical. 



The bilateral compression of the primitive vertebrate body did 

 not arise in itself as a special adaptation to a particular mode of 

 life ; but rather in correlation with other characters of the organi- 

 sation. The development of the dorsal medullary tube and the 

 notochord above the digestive tube and the concentration of the 

 myotomes would necessarily lead to a bilaterally compressed form 

 of body. We see this not only in fishes, but in the course of the 

 development of all Vertebrates. 



It is obvious that such a shape of the body is highly unfavourable 

 for the maintenance of the equilibrium except with the assistance 

 of some special mechanical and sensory apparatus. 



Now in Amphioxus, the metapleural folds, whatever their exact 



