VI. Biology. 2. Different Food, etc. — Para.'iitism. G3 



In many cases it is evident that the kind of food exercises a very 

 ■great influence on the structure of the animal. This influence 

 manifests itself primarily in the form of the digestive tract. 

 In allied animals, its length is generally greater in herbivorous 

 than in carnivorous individuals ; among the Mammalia, for example, 

 it is much longer in Ruminants than in Carnivora. Many other 

 variations in the alimentary canal result from the differences in food ; 

 this is specially marked in the buccal cavity, in the hard structures 

 serving for the prehension and comminution of the food, e.g., the teeth 

 of Vertebrata. Among the Mammalia this is very apparent; the 

 teeth of a Cat, for instance, may be compared with those of a Horse, 

 or again, the teeth of different Carnivora, Cat, Dog, Bear, etc., with one 

 another. Since the modification and insertion of the teeth again 

 affects, for example, the structure of the skull, the influence of the 

 food extends indirectly to other systems. 



The nature of the food, further, often affects the locomotor 

 organs, and the entire external conformation. This is very evident 

 in the Inseota, where the slender, active larva of the Carabus, with 

 its elongate limbs, running about after its food, may be contrasted 

 with the maggot-like larvse of the Curculionidse and Longicornia, 

 which either have only rudimentary legs or are entirely apodous, and 

 live in the midst of plenty. Frequently, again, the limbs of 

 predaceous animals function secondarily as prehensile organs 

 and are correspondingly modified. As the limbs are affected, so 

 also are the sense organs, especially the eyes, by the nature, and 

 particularly by the greater or less activity of the prey. Carnivores 

 often possess large, well developed eyes, whilst herbivores, whose food 

 is plentiful, have smaller or even degenerate optic organs. Insects 

 also afford characteristic illustrations of this. 



TMs does not mean that the locomotor or sensory organs of herbivorous 

 animals, whose food is plentiful, are always poorly developed. Amongst the 

 Mam.malia there are numerous herhivores rivalling Oai-nivora in this respect, 

 as Stags, Antelopes. In these cases the high state of development of these 

 parts affords a means of escaping the pursuit of the Carnivora. 



A discussion of the direct and indirect influence of food leads on to 

 a consideration of Parasites. Those animals are called parasites 

 which live upon or in other living animals known as the hosts, and 

 feed at their expense ; either upon some part of the body [e.g., its 

 blood), or upon food taken in and digested by it, as in the Tape- 

 worms. They may be temporary or stationary. The former 

 do not stay continuously on the host, but seek it out at intervals 

 simply for the sake of food, whilst the stationary parasites remain 

 constantly upon or within it. Parasites have, indeed, been 

 classified according as they live on the outer surface or in the 

 interior of the host, as ecto- or endo-parasitio. A sharp 

 line cannot be drawn between these two groups, since the 



