INSECT 'S STOMACH SNODGRASS 



373 



of a short tube on the under surface of the body (B, Mth), and 

 in both forms it is usually surrounded by a circle of tentacles (Tl). 

 The entire inside of the body is occupied by a large stomach cavity, 

 except that the jellyfish have a mass of transparent gelatinous ma- 

 terial between the ectoderm and the endoderm. The stomach cells 

 of the coelenterates are said to retain the primitive function of 

 intracellular digestion; by extending protoplasmic arms they grasp 

 and engulf food particles in the manner of the amoeba, and then 

 eject the refuse back into the stomach. 



The other metazoic animals are typically elongate in form but the 

 elongation appears to be crosswise through the blastopore instead 

 of through its axis. The lengthening of the animal, therefore, has 

 drawn the blastopore out into a narow slit on the under side of the 

 body. This conclusion, at least, appears to be warranted from a 



Mth 



ABC D^-Arf 



FiQUBE 6. — Diagrams showing the probable evolution of the blastopore in the arthropods. 

 An', primary anus ; Bpr, blastopore ; Mth' , primary mouth. 



comparative study of embryonic development, but, as in other things, 

 the embryo is so prone to depart from its race history that its story 

 may require a considerable amount of interpretation. Yet there are 

 some forms that adhere very nearly to what we might expect to be 

 a close approximation to historic truth. For example, the wormlike 

 animal mentioned above, known as Peripatus^ at an early stage of de- 

 velopment has an open blastopore slit on the under side of its body, 

 which gradually closes in the middle until only the two ends are left 

 as openings into the stomach. The usual arthropod development, in 

 which the blastopore is closed from the beginning, may, therefore, 

 be interpreted in the light of Peripatus as shown diagrammatically 

 in figure 6. The anterior remnant of the blastopore becomes the 

 primary mouth (D. Mth'), the posterior remnant the primary anus 

 {An). The gastrulation cavity, or archentron, is thus converted 

 into an elongate stomach sac open to the exterior at each end (fig. 

 7A). 



The primitive mouth and the primitive anus (fig. 7, A, Mth\ An), 

 however, do not remain at the surface of the body. Each is carried 

 internally by an inward growth of the ectoderm (B), and the outer 



