104 



GENERAL PRINCIPLES OF ZOOLOGY. 



layers of the body. In most animals they are separated by inter- 

 mediate tissues, called collectively mesoderm. The higher the 

 animal, the more differentiated is the mesodermal layer. The 

 primitive digestive cavity lined by entoderm is called the archen- 



FIG. 57. 



FIG. 58. 



FIG. 57. Longitudinal section through the nutritive polyp of a siphonophore. (After 



Haeckel.) o, mouth-opening; en, entoderm ; efc, ectoderm. 

 FIG. 58. Stenostoma leucops, in division, a, ectodermal fore-gut, at a' forming anew 



for the hinder animal; rn, the blindly ending entodermal mid-gut; e, ectodermal 



ciliated epithelium ; 0, ganglion with ciliated pit ; to, water-vascular canal ; y', 



ganglion of the hinder animal. 



teron. In the case of medusae and polyps it forms the entire diges- 

 tive tract, but in most animals this is not sufficient for the needs 

 of digestion and the alimentary tract is increased by invaginations 

 of parts of the surface of the body. 



Stomodaeum and Proctodaeum. Even in many coelenterates 

 and lower worms an invagination arises at the anterior end of the 

 digestive tract, forming the ectodermal fore-gut or stomodmim 

 (fig. 58). From the higher worms onwards, it is accompanied by 

 a second invagination at the hinder end, the ectodermal hind-gut, 

 or proctodceum (fig. 59) ; embryologically, this is formed as a blind 



