MORPHOLOGY OF ANIMALS 



is bent, as shown in Fig. 3, and the gastrula becomes strik- 

 ingly bilateral in symmetry. 



The apical pole of the gastrula usually becomes the 

 anterior pole of the adult. There is frequently a sense 

 organ at this pole of the gastrula (Fig. 3, so) and it is 

 often called the ''sense pole" ; the brain and higher sense 

 organs of the adult usually develop at or near this pole. 

 When the gastrular axis bends through approximately 

 90° the oral pole of the gastrula (blastopore) comes to lie 

 on the ventral side of the adult in many invertebrates 

 (some flatworms, Fig. 3, C, and arthropods) ; if it bends 

 through more than 90° the blastopore approaches the an- 

 terior end (apical pole) of the adult (annelids, Fig. 3, D, 

 mollusks). These relations are reversed in the lower verte- 

 brates (Amphioxus, ascidians, amphibians) where the 

 blastopore is at iirst dorsal and later posterior in position 

 (Fig. 4, C-E). In general among invertebrates the blasto- 

 pore becomes the mouth of the adult, and the anus, when 

 present, is a new formation ; in the echinoderms and lower 

 vertebrates it becomes the anus (in part), the mouth being 

 a new formation; in arthropods it usually closes com- 

 pletely, the mouth and anus being new formations. 



These relations of gastrula and adult may be sum- 

 marized as follows : 



GASTRULA ADULT 



Apical Pole becomes Anterior Pole 



i ventral side (invertebrates) 



( dorsal " (vertebrates) 



i mouth (most invertebrates) 



/ ( anus (echinoderms, some chordates) 



1: 17 3 



