Feeding and Digestion I47 



stylets tor piercing. If blood is the material ingested as food, the presence of an 

 anticoagulant is not uncommon. Yonge further subdivides this class into three 

 groups: 



FOR PIERCING AND SUCKING. Included in this group are the sucking mech- 

 anisms of hookworms, leeches, parasitic copepods, tardigiades, pycnogonids, 

 many Diptera, mites, and ticks, and many cyclostomes. 



FOR SUCKING ONLY. Examples of this category are the feeding mechanisms 

 of the suctorians, trematodes, most nematodes, lepidopterans, Muscidae, pen- 

 tastomids, and suckHng mammals. 



FOR ABSORPTION THROUGH THE GENERAL BODY SURFACE. This mechanism 



was discussed earlier (see p. 144). Species possessing this means of feeding 

 have no digestive organs or mechanisms and depend wholly on absorption of 

 nutritive materials from their surroundings. Many parasites are included in 

 this category. 



MECHANICAL FACTORS IN DIGESTION 



The digestive tracts in animals, like the feeding mechanisms, show striking 

 adaptations to the character of the foods and the feeding habits of the organ- 

 ism. The general problem of the evolution and adaptation of the digestive 

 systems of animals has been ably discussed by Yonge. ^''^ The digestive tract 

 in several animals is diagrammatically shown in Figure 29. Yonge divides the 

 tract into five functional regions. 



Functional Regions of Alimentary Tract. Reception. This includes the 

 mouth and mouth cavity. This region, except in fine-particle feeders, usually 

 possesses glands which have received various names, largely depending on 

 their particular position, such as labial, buccal, or sublingual. These glands 

 are primarily lubricating, although special functions have been secondarily 

 assumed by them. These glands in many blood-sucking species, such as 

 leeches, ticks, and some insects, are the sources of anticoagulants. Many insects 

 and the snail Helix secrete carbohydrases from such glands. Some snails pro- 

 duce acid which is used in dissolution of food. Some nematodes and carnivor- 

 ous gastropods produce proteases in the oral glands. Toxins for the paralysis 

 or killing of prey are found in a wide range of species from many groups. 



Conduction and Storage. Following the region of reception is one of con- 

 duction and storage. This includes the esophagus and crop of many species. 

 The crops of leeches, of some insects, and of herbivorous birds are simply 

 dilatations of the esophageal region. Digestive glands may be present in this 

 region, or digestive enzymes may be regurgitated from more distal regions. 



Digestion and Internal Trituration. The third general region is for diges- 

 tion and internal trituration. In this region one often finds the differentiation 

 of gizzards or gastric mills. Such special grinding mechanisms are seen in 

 rotifers, some clams, certain annelids, crustaceans, some insects such as the 

 cockroach (Fig. 29, A), and birds. It is not uncommon to find associated with 

 the grinding mechanism a filtering mechanism which maintains the food par- 

 ticles in the region of the triturating mechanism until they have been reduced 

 to a particular degree of fineness, as in the crayfish. In this region also the 

 greater portion of the digestive enzymes are liberated and the lining may be 

 rich in unicellular glands, distributed uniformly through the region or dis- 

 tributed in a characteristic pattern. This region also often possesses conspicu- 



