chap, iv.] DIGESTION IN ANNUL AT A. 117 



stones the gizzard becomes the organ of the earth- 

 worm in which the food is triturated, or ground up. 

 Beyond the gizzard the intestine runs straight back- 

 wards to the anus, which is placed quite at the end of 

 the body. In this intestine we first meet with a 

 structure which will reappear in other groups, and 

 affords us the first example of a method by which the 

 absorbing capacity of the intestine may be increased 

 with the greatest economy of space. A transverse 

 section of an intestine reveals the presence of a fold 

 which runs along the median dorsal line and projects 

 into the enteric cavity. This is the so-called typhlo- 

 sole, or blind tube. Around the intestine are a 

 number of granular greenish cells (Fig. 57; m), which 

 become specially aggregated together on the dorsal 

 surface to form the so-called " liver " (o) ; the function 

 of this aggregation of cells is unknown, but it is un- 

 doubtedly misleading to apply to it a term of such 

 definite significance as that by which it is known. 

 This remark will apply also to the so-called livers of 

 other invertebrate animals. 



We may easily pass from the intestinal tract of 

 the earthworm to those of the other ringed worms. 

 The absolutely unarmed condition of the mouth is not, 

 of course, to be expected in a blood- sucking or vora- 

 cious form, and thus it is that we find the leech 

 provided with three chitinous "jaws," hardened by a 

 little carbonate of lime, the edges of which are 

 minutely serrated, and which are provided with a 

 special system of muscles by means of which they are 

 able to work on one another ; so, again, one or more 

 pairs of hard chitinous or even calcareous teeth are 

 developed in the free-living marine worms ; these, 

 which are generally hooked and serrated on their con- 

 cave edge, work from side to side.. The earthworm is 

 enabled to push its pharynx forwards when seizing 

 food, but the voracious sea-worms can protrude their 



