THE ORIGIN OF THE ANIMAL KINGDOM. 73 



undergone some considerable modification within 

 recent years, but these modifications do not alter the 

 wide significance of the common appearance of this 

 stage in the embryology of animals. While then the 

 Gastraea as originally conceived may be modified in 

 future years, there is no probability that future study 

 will make any less significant the early ancestor 

 whose fundamental structure was essentially that of 

 the gastrula. 



We may start the history of multicellular animals 

 then by supposing that the unicellular animals first 

 became aggregates of cells, and then that those cells 

 which were situated on the outside of the mass 

 acquired special functions connected with the rela- 

 tion of the animal to the external world, while 

 the cells which were in the interior of the mass took 

 for their duty the digestion of food which was 

 passed to them from the exterior. In other words, 

 there was a division of labor between the cells of the 

 animal, and when this had appeared, there arose the 

 first true multicellular animal. While we have no 

 fossil evidence of the form of the first multicellular 

 animal which appeared in the world, the evidence 

 derived from embryology tells us pretty surely that 

 it must have been one which could be described as a 

 two-layered sac, probably open at one end, for the 

 injection of food and the ejection of waste, whose 

 inner lining served for digestion, and whose outer 

 covering possessed motor and nervous functions. 



As a further confirmation of the conclusion thus 

 reached from embryology, it is highly important to 

 notice that there is still in existence, among the 



