374 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1933 



openings of these two ectodermal invaginations added to the food 

 canal become the permanent mouth (Mth) and anus (An). The 

 entire alimentary canal, it is to be seen, consists now of three parts, 

 of which only the middle one {Ment) is derived from the primitive 

 endodermal archenteron. The new anterior ectodermal part is 

 called the stomodaeum (jStom), the posterior part the proctodaeum 

 (Proc) . The middle part, which is still the true stomach, is rechrist- 

 ened the tnesenteron {Ment). It should be observed that, while the 

 anal opening is terminal on the rear end of the body, the mouth aper- 

 ture {Mth) is on the ventral surface a short distance behind the 

 anterior pole of the animal. The part of the trunk before the mouth 

 is termed the frostomiuiii {Prst). This condition holds for all the 

 Annelida and the Arthropoda. Some of the unsegmented worms 

 have the mouth situated near the middle of the under surface of the 

 body. 



Eod 



-An 



Prst 



Mth' J\. ''"^Mth Mth' b An 



Figure 7. — Diagrams of the probable, evolution of the alimentary canal in the ancestors 



of the arthropods. 



An', primary anus ; An, definitive anus ; Arc, primitive brain, or archicerebrum ; Ecd, 

 ectoderm End, endoderm ; Ment, stomach, or mesenteron ; Mth' , primary mouth ; Mth, 

 definitive mouth ; Proc. proctodaeum ; Prst, prostomium ; ^tom, stomodaeum. 



The elongation of the body in the direction of movement has had 

 a profound effect on the general organization of all animals that 

 have evolved from this basic principle of structure. It has led 

 particularly to many features characteristic of the annelid worms 

 and the arthropods. The elongate form in itself is clearly an 

 adaptation to greater efficiency of movement. Habitual movement 

 in one direction has established one end of the trunk as the head 

 end, and the other as the tail end. For this reason the nervous 

 system originates at the same end as that containing the mouth, 

 since the forward pole is the best place for the food intake orifice, 

 and is also the end chiefly in need of sensory perceptive organs, 

 from which the brain (fig. 7 B, Arc) has had its inception. This 

 explains why with all us elongated animals, vertebrates and inver- 

 tebrates, our eyes, organs of smell, and the brain are in our heads 

 along with the intake aperture of our food canal. It explains also 

 many of our habits and social customs. Of course, in our own 

 particular case, since we as human beings have departed from the 

 horizontal position, except for purposes of repose, it would be quite 

 convenient for us now to have our mouth located somewhere nearer 



