GENERAL ZOOLOGY 



gullet, and the anal spot where egestion occurs are permanent features of the 

 cell, along with certain modifications of the oral ciliation used in feeding. 

 The food vacuoles, on the other hand, are not permanent but transitory. The 

 amoeba possesses only temporary structures, the food vacuoles and the 

 pseudopodia. The available evidence indicates that the particles of food 

 contained in food vacuoles are subjected to enzymatic hydrolysis, or digestion, 

 and that the functions of the food vacuoles are thus those of "temporary 

 stomachs." 



In the Mesozoa and Porifera there is no enteron, or digestive cavity; 

 mesozoans absorb food in solution through the cells of the outer layer, and 

 sponges ingest particulate matter through the activities of individual collar 

 cells lining the water channels. In the Coelenterata, Ctenophora, and 

 Platyhelminthes, the enteron is a cavity, usually with only one external open- 

 ing. If we define a digestive tract as a continuous tube through which food 

 moves from mouth to anus, these groups have not evolved true digestive 

 tracts. In the other phyla of invertebrates there is, typically, a complete 

 enteron with both mouth and anus, and the term digestive tract is applicable. 

 It is interesting to note that in some protozoans a tract-like course is followed 

 by the food vacuoles as they traverse the cytoplasm of the single-celled body. 

 The intracellular digestion which occurs in protozoans and in the digestive 

 cells of many metazoans presents another parallel between unicellular and 

 multicellular animals. 



As we pass on to the invertebrates with complete digestive tracts, with both 

 mouth and anus, it is clear that a fundamental plan is characteristic of all. 

 Once the tubular digestive tract had been evolved in its basic aspects, it was 

 apparently preserved in all subsequent groups through the selective ad- 

 vantages it presents. Comparisons may be made by means of a generalized 

 diagram such as Figure 17.1. This is a schematic representation of an in- 

 vertebrate digestive system with all the important parts that might be found 

 in a single species. Certain invertebrates have all these parts, but in most 

 species some are missing whereas others are highly developed. There is 

 probably no organ system of animals which demonstrates more clearly than 

 the digestive system the relation between structure and function. Digestive 

 systems are adapted to many different kinds of food, therefore to many ways of 

 feeding and the related requirements of digestion. Parts that are necessary 

 for ingestion and digestion in some species may be unnecessary in others, and 

 so may be absent or reduced, even in species within the same phylum. For 

 example, compare the digestive system of the clamworm, a predaceous, free- 

 swimming annelid (pp. 398-399), with that of the earthworm, a compara- 

 tively sedentary, herbivorous annelid (pp. 405 406). The digestive systems 

 of forms described in preceding chapters may be effectively reviewed by 

 comparisons with the schematic figure. 



With respect to functions, the tubular digestive tract shows regional 

 specializations related to various aspects of the digestive process. The 

 mouth parts, oral cavity, and pharynx are concerned chiefly with ingestion; 



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