William A. Hilton 487 



cause villi in mammals are well formed at a comparatively early stage 

 in the embryo. 



Brief Summaey. 



1. Simple folds, villi and valvule conniventes involve the mucosa 

 alone. 



2. Simple mucosal folds and villi of the intestine are homologous; 

 villi are the more specialized of these and usually occur in higher verte- 

 brates, as mammals and birds. 



3. True villi are found in a number of lower forms, although folds 

 of different types are the usual elevations of the mucosa in all classes 

 except birds and mammals. 



4. The individual variation in shape, size and number of folds, and 

 villi is marked in all groups, but is less characteristic of mammals. 



5. Although a number of divisions of the shape of folds or villi may 

 be made, there are intermediate conditions which connect the different 

 divisions with each other. 



6. Four general types of folds are : 



1. Long, straight, parallel. 



2. Wav}^, parallel folds. ' 



3. Zigzag parallel folds. 



4. Net-arranged folds. 



7. Four general forms of villi are : 



1. Thin, leaf-like. 



2. Thread-like or long, cylindrical. 



3. Cylindrical or tinger-shaped. 



4. Low columnar or wart-like. 



8. The largest villi are usually found in the largest animals. 



9. The number of villi per square millimeter in many cases is largely 

 determined by the size of the villi. 



10. Carnivorous mammals usually have finger-like or cylindrical villi. 

 Herbivora, leaf -like, thread-like or mound-like shapes. Omnivora have 

 either the carnivorous or herbivorous type of vilhis or both types in the 

 same intestine. 



11. By an examination of many species of vertebrates, is is found that 

 the following, in general, represent the steps of specialization : 



1. Few longitudinal, straight folds beginning near the pyloric 



valve. 



2. ISTumerovis straight, longitudinal folds, more extensive. 



3. Slightly wavy folds throughout the intestine. 



4. Very wavy folds, numerous and thickly placed. 

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