INTRODUCTION TO THE METAZOA 71 



tion to meet the entoderm. This second ectodermal invagination 

 is normally located at the posterior extremity as a proctodaeum 

 terminating in an anal aperture. Forms in which both stomo- 

 daeum and proctodaeum occur are said to have a complete 

 digestive system. In its simplest condition, this consists of a 

 continuous tube the extremities of which are ectodermal and 

 the true digestive middle portion of which is formed of, or at 

 least is lined with, entoderm. Most of the highly developed 

 digestive systems represent only modifications of this simple 

 tubular condition. Different regions become modified for 

 limited functions. With these specializations, changes both 

 in form and in structure arise and regions such as esophagus, 

 stomach, and intestine are marked off. Jaw^s or organs for the 

 comminution of food are frequently developed in the stomodaeum. 

 Glands grow out from the walls of the tube and remain attached 

 to the digestive tract only through ducts. Muscles, connective 

 tissue, and vessels of the circulatory system become associated 

 with the entodermal tube to form a more highly complicated 

 digestive system. In some instances, the proctodaeum receives 

 the ducts of the reproductive system and is then termed a cloaca. 



Circulatory System. — Whether digestion is intercellular or 

 intracellular, the products of digestion are directly available to 

 only part of the cells of the body while the remainder of the cells 

 must rely upon some agency external to themselves for furnishing 

 them with the materials essential for anabolism. In some of the 

 simpler Metazoa, this is accomplished by direct transfer of the 

 materials from cell to cell, but in the highly complicated organ- 

 isms such transfer is not possible because some cells are so far 

 removed from the organs where digestion takes place. In such 

 instances, a special system for the distribution of digested food 

 has made its appearance as a specialized circulatory sj^stem. 



Many steps in the development of complexity of the cir- 

 culatory system are observable in the different animal groups. 

 It has already been pointed out that the gastro vascular system of 

 coelenterates serves for both digestion and distribution but even 

 in this phylum many steps in the differentiation of the two sys- 

 tems are found. Trematodes and Turbellaria among the flat- 

 worms also represent the condition of a combined gastro vascular 

 system. 



In its simplest form, as exemplified by the hydroid polyp, 

 this system is a simple sac or pouch, the walls of which are formed 



