70 INVERTEBRATE ZOOLOGY 



for the entire digestive process is performed by the organs of 

 the host. 



In some of the Protozoa and in a few of the Metazoa, sohd 

 food substances are taken directly into the protoplasm of the 

 organism. These ingested particles, though within the proto- 

 plasm, remain foreign to it until they have undergone physical 

 and chemical changes which render them capable of assimilation 

 by the protoplasm. This type of digestion is termed intra- 

 cellular digestion. Among the sponges and some of the flat- 

 worms, in addition to the Protozoa, the entire digestive process 

 is intracellular. 



Ontogenetically, the archenteron of the gastrula stage repre- 

 sents the first organ specialized for digestion. In many animals, 

 the larva upon reaching the gastrula stage is an independent 

 organism capable of self-maintenance. Food taken through the 

 blastopore of such a gastrula is rendered liquid through the action 

 of secretions formed by the entoderm cells. Thus the gastrula 

 cavity becomes a space within which intercellular digestion takes 

 place and is termed an archenteron or primitive digestive system. 



Among the Metazoa, there are some animals in which the 

 digestive system of the adult never reaches a state of complexity 

 appreciably higher than that of the archenteron. Among the 

 Coelenterata, the polyp of the Hydrozoa is essentially similar to 

 the archenteron, for it is a simple entodermal sac with but a single 

 opening, the mouth. In many other coelenterates, there is a 

 beginning of a separation of the distributive function from the 

 central digestive cavity. Thus in the jellyfishes, the gastro- 

 vascular system consists of a central cavity from which a periph- 

 eral system of pouches or canals passes to the outlying parts of 

 the body and thereby aids in the distribution of the digested food. 



In most of the Metazoa above the coelenterates, the blastopore 

 of the gastrula is not retained, as the mouth of the adult for 

 the blastopore closes during development of the embryo and for a 

 time the entodermal digestive tube is but a sac without communi- 

 cation with the outside. Communication is later established by 

 an infolding of the body-covering at the anterior extremity to 

 form an ectodermal antechamber to the entodermal digestive sac. 

 Such an entodermal digestive sac with a single opening lined with 

 ectoderm (the stomodaeum) is characteristic of the Turbellaria. 



In the nemertines and in all organisms higher than the flat- 

 worms, there is characteristically a second ectodermal invagina- 



