168 HOMOLOGY 



parts, the useless portions degenerating, the useful parts in- 

 creasing in usefulness by continued activity. Another group of 

 biologists, however, take the very opposite view, viz., that the 

 function or use of an organ depends upon its position, the max- 

 illae and maxillipedes for example, crowded together in the 

 thorax, have the functions of seizing, sifting and propelling food 

 matters forced upon them and could not do otherwise. In 

 either case there* is general agreement that all appendages are 

 derived from one ancestral biramous type of appendage, which 

 is regarded as a generalized organ capable of differentiation and 

 development along different lines until structures result of 

 widely different appearance, although homologous throughout. 

 The study of comparative anatomy is, in large part, only the 

 ferreting out of such homologies in animals of the same or allied 

 groups (see Chapter IX). 



DIGESTIVE SYSTEM. The lobster is primarily a scavenger, 

 and eats all forms of dead and decaying proteid matter. For 

 this purpose it has a highly developed digestive apparatus capa- 

 ble of extracting the nutrient material out of all sorts of food. 



The most conspicuous part of the digestive system is the 

 chitinous fore-stomach or cardiac stomach into which the 

 oesophagus opens (Fig. 69). In the walls of this organ special 

 chitinous processes are developed forming tooth-like accumula- 

 tions which are worked by special muscles attached to the body 

 wall. These teeth form a grinding machine known as the 

 gastric mill which triturates the food passed on by the jaws. 

 They also form a sieve, guarding the opening into the functional 

 or pyloric stomach and preventing all but digestible materials 

 from entering the physiological stomach in which digestive 

 juices are poured from the large digestive glands known as the 

 hepato-pancreas. After action by the digestive fluids the food 

 is passed on to the intestine which lies over the dorsal sides of 

 the ventral muscles. 



Not only are digestive fluids poured into the pyloric stomach 

 but some of it also goes into the cardiac stomach where the food 

 particles are sotened and prepared for passing the gastric filter 

 between the cardiac and pyloric portions. Food thus passed 



