NO TES. 



43 



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

 tt) appreciate the necessil\' for their existence. 



'I'here is one differenee between the general form of the body 

 in Invertebrates and Vertebrates respectively which seems to be 

 of fundamental importance, b\it which has not been sufficiently 

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

 is not bilaterally com|)ressed, but, on the contrary, is either c\lin- 

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

 ceptions to this rule are presented by the Lamellibranchiate 

 Molluscs and by many Arthropods; but these exceptions are 

 readih' intelligible as secondary moilihcations. 



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

 hshes), the bilateral com[iression of the body is one of the car- 

 dinal features of the external form. To this fundamental rule 

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

 skates ; but it is a self-e\ident fact that these again have arisen 

 by secondary modification from originally bilaterally compressed 

 forms. With the evolution of the pentailactyle appendages and 

 the assumption of a terrestrial existence, the sha[)e 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 develoi)ment. 



.Vmphioxus exhibits the characteristic vertebrate bilateral com- 

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

 glossus shows invertebrate affinities in regard to the shape of the 

 bodv, which is sid)-c\'lindrical. 



'I'he bilateral compression of the primitive vertebrate body did 

 not arise in itself as a si)ecial adaptation to a particular mode of 

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

 sation. The tlevelopmcnt of the dorsal medullary tube and the 

 notochord above the digestive tube ami the concentration of the 

 myotomes would uecessaril)' lead to a bilaterally compressed form 

 ol bod)'. We see this not onlv 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 

 tor the maintenance of the cipnlibrium except with the assistance 

 of some special mechanical antl sensory apparatus. 



Now in .\ui|ihioxus, the metapleural folds, whatever their exact 



