GENERAL ORGAXOLOGY. 



important for the well-being of the animal, it is to be ex- 

 pected that of all the organs in the animal series the diges- 

 tive tract should be formed first, and also in almost every 

 case should be earliest established in the embryo. The 

 fact that many worms (Cestodes] and crabs (Rhizocephala) 

 have no digestive tract does not alter this statement; for 

 with respect to these it can be definitely affirmed that, in 

 conformity to special conditions of life, particularly as a 

 result of parasitism (compare page 203), the digestive 

 tract has degenerated. Very commonly the lowest-organ- 

 ized, multicellular, free-living animals are merely simple or 

 branched digestive pouches which have only a single open- 

 ing, functioning both as mouth and anus (Fig. 54). Such 

 an animal has necessarily two epithelial layers, one of which 

 lines the digestive tract, the other covers the surface 

 of the body. These two fundamental cell-layers, which 

 are found in all cases except in degenerated animals, are 

 called entoderm and ectoderm. The primitive digestive 

 cavity lined by entoderm is called the archenteron. In 

 the case of niednscz and polyps it forms the entire digestive 

 tract. 



Stomodceum and Proctodceum. In case of most 

 animals the digestive tract undergoes an enlargement, 

 areas of the surface of the body, the ectoderm, becom- 

 ing invaginated. Even in many coclaiterates and lower 

 worms there arises an invagination at the anterior end of 

 the cylindrical digestive tract, forming the ectodermal 

 foregut or stomodoeum (Fig. 55). From the higher worms 

 onwards, it is accompanied by a second invagination at the 

 hinder end, the ectodermal end-gut, or proctoihvutii (Fig. 

 56); embryologically, this is formed as a blind sac whose, 

 closed end is bounded by the likewise closed posterior 

 part of the archenteron, now called also mcsentcron or 

 mid-gut, until the separating wall disappears, whereupon 

 mid- and end-gut communicate with each other, and the 

 digestive tract becomes a canal extending through the 

 entire body. 



