Chap, in.] METAZOA. 47 



The archenteron ordinarily closes up, so that the 

 blastopore disappears ; a fresh mouth, and in most 

 cases also, an anus, are developed at either end of 

 the tube ; these are lined by inpushings of the epi- 

 blast; the epiblastic pits, deepening and elongating, 

 finally become continuous with the original or arch- 

 enteric cavity, which is, it wil] be remembered, 

 lined by hypoblastic cells. In a fully developed di- 

 gestive tract we have now to distinguish three regions : 

 (1) a mouth passage (stomodceum) which is lined 

 by epiblast ; (2) a mid-intestine (mesenteroii) lined 

 by hypoblast ; and (3) an anal passage (procto- 

 <8<rum) lined again by epiblast. 



The greater number of the Metazoa are free 

 animals, and no doubt the ancestors of all the terres- 

 trial were aquatic forms; organisms moving freely 

 in such a medium as water would clearly have 

 one end which was anterior and one which was pos- 

 terior, and as these would be differently affected by 

 the water through which they moved, the one end 

 w r ould become differently constituted to the other ; the 

 anterior end would be that at which food would be 

 taken in, and at which the prey or an enemy would 

 l>e first met. This end would then be primarily the 

 sensitive end, and we find that it is here that sense 

 organs of various kinds are best developed. In other 

 words, we have henceforward to look for a definite re- 

 gion, specially sensitive in function, developed in front 

 of the mouth ; this may be called the praBStomiiim. 



On either side of the moving body the water would 

 exert equal pressure, and the two sides would come to 

 exhibit similar characters, or bilateral symmetry 

 would become apparent. In shallow waters one as- 

 pect of the body would be more exposed to the in- 

 fluence of light than the other, and we should there- 

 fore distinguish between an upper or dorsal and a 

 lower or ventral surface. 



