INSECT 'S STOMACH SNODGRASS 



375 



the stomach, say just above the level of the table; but, thanks to our 

 horizontal animal ancestors, we are able to take our food without 

 embarrassment in family groups and social gatherings. In short, 

 it is much better that we have evolved from worms than from jelly- 

 fish. Such considerations may seem frivolous, but many ideas are 

 funny to us simply because we take our accustomed facts too se- 

 riously. Historians are often given to speculating on what might 

 be true today if something had not happened in the past that did 

 happen. "VYlien we begin to inquire why we are not like what 

 human beings might have been, we are likely to find that in most 

 ways we are simply made-over animals, that we have very little 

 in our organization that was designed in the first place for our 

 particular use. 



/\ Bpr Msd He g 



\ BC 



Msd 



c 



\^C 



FiQOBE 8. — Diagrams Illustrating in cross-section the probable historic, or ancestral, 

 method of formation and growth of the mesoderm In invertebrate animals. 



A, the mesoderm being formed as hollow ingrowths along the lips of the blastopore. 

 B, the blastopore closed and the mesoderm sacs extended upward around the alimentary 

 canal. C, a later stage in which the walls of the mesoderm sacs form an inner lining 

 (EMad) of the body wall, and an outer covering (IMsd) of the alimentary canal, their 

 cavities united ventrally with the blood cavity to form the definitive body cavity (DC) 

 of the arthropods. 



BC, definitive body cavity ; Bpr, blastopore ; Coel, mesoderm cavity ; Ecd, ectoderm ; 

 EMnd, outer mesoderm layer ; End, endoderm ; Qc, stomach cavity, or gastrocoele ; HG, 

 blood cavity, or haemocoele ; Itnsd, Inner mesoderm layer ; Msd, mesoderm ; VNC, ventral 

 nerve cord. 



Another feature common to all the metazoic animals other than 

 the coelenterates is the possession of a third fundamental cell layer 

 interposed between the ectoderm and the endoderm. This layer is 

 the mesoderm. The mesoderm gives rise to the muscles, connective 

 tissue, heart, blood vessels, fat, excretory organs, bones, and other 

 internal parts that the coelenterates do not have. It originates as 

 ingrowths of cells in each side of the body along the margins of the 

 blastopore, or of the blastoporic area when the blastopore is closed. 

 Tlie history of the mesoderm is usually more or less obscured in 

 embryonic development, but the probable primitive method of 

 mesoderm formation in annelids and arthropods is well shown in 

 Perlpatus^ and is here illustrated in a series of diagrams (fig. 8). 



