126 Kansas Academy of Science. 



muscles in the lower chordates, fish and higher animals. The 

 ventral nerve-cord does this in the arthropods, in addition to 

 managing digestion and circulation; but this double work on 

 the part of the ventral cord in crustaceans and insects has 

 been fatal to their higher development; and vertebrates with 

 two nerve systems and division of labor have far outstripped 

 anthropods in the race to greater complexity. The dorsal cord 

 must have appeared with and in the lower chordates and fish 

 of the Silurian era, twenty million years ago. A specia: de- 

 velopment of this dorsal nerve-cord at its front end and near 

 the organs of special sense, which we call the brain, appeared 

 in fish, so the geologist tells us, in the Silurian era, over twenty 

 million years ago. This instinct for brain-building did not 

 improve much till mammals appeared, twelve or fifteen million 

 years later, in the Jura-Trias. The intricate land life of the 

 true mammals of the Tertiary era induced a rapid development 

 of the brain-building instinct, especially in the production of 

 a larger cerebrum and cerebellum, till it culminated in man 

 after he appeared, 100,000 years ago. The organs of special 

 sense have developed with the brain, as we would naturally 

 expect. 



The instinct for building a backbone also appeared with and 

 in the primitive fish; but the first backbone was not at all 

 bony, for it was merely a cartilaginous rod produced from a 

 longitudinal furrow-invagination of the alimentary canal. 

 Millions of years later it became segmented and bony, as we 

 find it in the higher vertebrates, and received its peculiar 

 curves as in man. 



The aquatic habits of the early animals permitted the use 

 of the entire ectoderm of the body, increased by evaginations 

 or invaginations in some cases, in oxygenating the blood ; but 

 the increasing activity of the more and more complex aquatic 

 animals made it necessary to expose some of the entoderm to 

 water containing oxygen. This was more especially true of 

 those animals which thickened the outer skin for service as an 

 armor to protect them from their foes. Obviously the entire 

 entoderm could not be used for purposes of respiration, so 

 certain anterior portions near the mouth, as we know, were 

 invaginated, forming sacs or pouches which communicated 

 with the exterior through pores or slits. It is true that the 

 gastrula-animals, such as the hydra and sponge, probably did 

 attempt to use the entire entoderm for both digestion and 



