HOMOLOGY. 



31 



In both Vertebrates and Invertebrates, as a rule, the parial 

 limbs diverge from their arches nearer the neural than the 

 haemal sides of the trunk — nearer to the centres whence their 

 nerves originate. In Vertebrates the joints or segments of 

 the limbs bend toward the hsemal aspect ; in Invertebrates 

 they bend from the haemal aspect : and thus the most frail 

 and precious of the organic systems, namely the neural axis, 

 is brought in Arthropods towards the least exposed and safest 

 surface of the body, that, viz., which is downward, next the 

 ground — therefore called the " belly," or ventral surface or 

 aspect. When the myelencephalous tract runs along the most 

 exposed, dorsal side, it receives an immediate protection by a 

 vertebral column. But the surfaces or aspects of the body 

 which are truly homologous in the Snake and Caterpillar are 

 the 7teural and the hmmal, not the dorsal and i\\Q ventral. 



