EARTHWORM. 215 
animal is made aware of the differences between light and 
darkness, aid of the approaching tread of human feet, not 
to speak of the hostile advances of a hungry blackbird. 
The sense of smell is also developed. The afferent or 
sensory nerve fibres from the nervous cells of the skin enter 
the nerve-cord and bifurcate into longitudinal branches, 
which end freely in the nearest ganglia. In this the earth- 
worm’s nervous system suggests that of Vertebrates. 
The nerve cells, instead of being confined to special centres or 
ganglia, as they are in Arthropods, also occur diffusely along with the 
nerve fibres throughout the course of the cord. Along the dorsal 
surface of the nerve-cord there run three peculiar tubular “giant 
fibres,” with firm walls and clear contents. They are probably 
comparable to the medullated nerve fibres of Vertebrates. 
Alimentary system.—Earthworms eat the soil for the sake 
of the plant débris which it may contain, and also because 
one of the modes of burrowing involves swallowing the 
earth. In eating they are greatly helped by the muscular 
nature of the pharynx; from it the soil passes down the 
gullet or cesophagus, first into a swollen crop, then into a 
strong-walled grinding gizzard, and finally through a long 
digestive and absorptive stomach-intestine. There are 
three pairs of cesophageal glands. Canals from the posterior 
two pairs open into the anterior pair, and thus into the 
gullet. Their contents are limy, and perhaps counteract 
the acidity of the decaying vegetable matter. It may be 
that they are in part excretory; or it may be that they 
serve to fix some of the carbon dioxide formed by the 
animal. The long intestine has its internal surface 
increased by a dorsal fold, which projects inwards along 
the whole length. In this “typhlosole,” and over the outer 
surface of the gut, there are crowded yellow cells. 
There is no warrant for calling the yellow cells hepatic or digestive. 
Structurally they are pigmented cells of the peritoneal epithelium, which 
here, as in most other animals, lines the body cavity and covers the 
gut. As to their function, they absorb particles from the intestine, 
and go free into the body cavity, whence, as they break up, their 
débris may pass out by the excretory tubes. When a worm has been 
made to eat powdered carmine, the passage of these useless particles 
from gut to yellow cells, from yellow cells to body cavity, and thence 
out by the excretory tubes, can be traced. The amceboid cells of the 
body cavity fluid act as phagocytes. Various ferments have been 
