172 HOMOLOGY 



tions degenerating, the useful parts increasing 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 maxillae 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 general- 

 ized organ capable of differentiation and development along 

 different lines, until structures result of widely different appear- 

 ance, although homologous throughout. The study of com- 

 parative 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 protein matter. For 

 this purpose, it has a highly developed digestive apparatus, 

 capable 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 bones or large 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 these fluids, the undigested 

 residue 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 softened and prepared for passing the gastric 



