126 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the other that, when at rest, the two make an open 

 angle towards the dorsal aspect, while the more pos- 

 terior has at its lower edge a strong tooth (mt) which 

 takes a backward direction. To the fore and hind 

 bars of the hexagonal framework there are attached 

 strong muscles, which, by contracting, draw these two 

 bars away from one another. This separation of the 

 terminal naturally requires an approximation of the 

 lateral faces (pc, zc), two of which bear strong teeth. 

 While these teeth are thus brought closer to one 

 another, the angulated bar which connects the fore and 

 hind pieces of the hexagon becomes straightened out ; 

 the result of this straightening is seen in the downward 

 and forward movement of the tooth which is developed 

 on the hinder median bar (mt) and which is thereby 

 brought into closer relation with the approximating 

 teeth on the side-pieces. (See Fig. 59.) This elaborate 

 " gastric mill " must break up" the food-masses taken 

 in by the crayfish ; but, as if this were not enough, 

 the hinder part of the so-called pyloric region of the 

 stomach is provided with cushions covered with hairs, 

 and longitudinal ridges with still finer hairs, which 

 form a most efficient filtering apparatus. This may, 

 from a physiological point of view, be compared with 

 the sieve of hairs which lies at the entrance to the in- 

 testine of the fish-eating bird, the darter (Plotus). 



As far as this filter the whole of the enteric tract 

 will be found to have its inner face lined with chitin ; 

 the next succeeding portion, which forms the com- 

 mencement of the delicate " intestine," has no such 

 internal layer ; but this in the crayfish, though not in 

 the lobster, is quite short. On it there follows the 

 remainder of the " intestine," and this will be found 

 to be again lined with chitin. 



When we come to ask ourselves why so much, yet 

 not all, of the enteric tract of the crayfish is thus lined 

 by the same dense body as that which forms the outer 



