212 PHYLUM ANNELIDA. 
precisely, that the formation of vegetable mould was mainly 
to be placed to their credit. 
Though without eyes, earthworms are sensitive to light 
and persistently avoid it, remaining underground during 
the day, unless rain floods their burrows, and reserving 
their active life for the night. Then, prompted by “love” 
and hunger, they roam about on the surface, leaving on 
the moist roadway the trails which we see in the morning. 
More cautiously, however, they often remain with their tails 
fixed in their holes, while with the rest of their body they 
move slowly round and round. The nocturnal peregrina- 
tions, the labour of eating and burrowing, the transport of 
leaves to their holes, the collection of little stones to pro- 
tect the entrance to the burrows, include most of the 
activities of earthworms, except as regards pairing and egg- 
laying, of which something will afterwards be said. When 
an earthworm is halved with the spade, it does not neces- 
sarily die, for the head portion may grow a new tail, while 
a Gecapitated worm may even grow a new head and brain. 
Phagocytes help as usual in the regeneration. ‘The earth- 
worm is much persecuted by numerous enemies, ¢.g. centi- 
pedes, moles, and birds. The male reproductive organs 
are always infested by unicellular parasites—Gregarines of 
the genus Monocystis ; and minute thread-worms (Lelodera 
pellio) usually occur in the nephridia and body cavity, and 
often in the ventral blood vessels. 
Form and external characters.—The earthworm is often 
about 6 in. long, with a pointed head end, and a cylindrical body 
rather flattened posteriorly. The successive rings seen on the surface 
mark true segments. The mouth is overarched by a small lobe called 
the prostomium, and the food canal terminates at the blunt posterior 
end. The skin is covered bya thin transparent cuticle, traversed by 
two sets of fine lines, which break up the light and produce a slight 
iridescence. On a region extending from the 31st to the 38th ring, 
the skin of mature worms is swollen and glandular, forming the 
clitellum or saddle, which helps the worms as they unite in pairs, and 
also forms the slimy stuff which hardens into cocoons. The middle 
line of the back is marked by a special redness of the skin. On the 
sides and ventral surface we feel and see four rows of tiny bristles or 
setae, which project from little sacs, are worked by muscles, and assist 
in locomotion. These bristles are fixed like pins into the ground, at 
times so firmly that even a bird finds it difficult to pull the worm 
from its hole. As each of the four longitudinal rows is double, there 
are obviously eight bristles to each ring. On the skin of the ventral 
