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45 s 
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from 3 to over 6 inches. It is a long, flat, whitish-coloured creature, occurring on 
muddy ground and between the branches of coral; and has received its name from 
the habit of breaking up into pieces at the least touch. The 
vitality of the severed pieces is so great that the head end 
has the power to re-form a new tail, and the tail end a new 
head, and the intermediate pieces a new head and tail. 
Another common kind is Folia crucigera, so-called because 
its greenish body is marked with five longitudinal white 
bands and transverse white stripes, forming together a 
series of crosses. These worms are long and slender, reaching 
a length of about 16 inches. The proboscis, moreover, when 
protruded, adds another 6 inches to their extent. They are 
found most abundantly in pieces of rock riddled with holes 
and galleries by boring sponges, and they also intertwine 
themselves amongst the prongs of branching-corals, as shown 
in the illustration. 
Most marine nemertines prefer rather shallow water; 
but some occur at considerable depths; and a pelagic species 
from the Indian Ocean, originally described as a mollusc, 
under the name Pterosoma planum, is a transparent 
creature, whose internal organs, especially the chestnut- 
brown digestive apparatus, are visible through the colour¬ 
less integument. The body diminishes from the front 
towards the hinder end, and at the sides is marked out by 
deep notches into a series of five lobes, of which the first 
pair are enormously large, and have the form of two 
semicircular wings. The use of these is doubtless to enable 
the creature to float or swim in the water. All the fore¬ 
going are free living types, but we now come to forms 
(Malacobdella) not unfrequently found living parasitically 
under the gills of various marine molluscs. In these, the 
body is short and broad and capable of but little change 
in shape at its hinder end; it is furnished with a sucking 
apparatus, by means of which the animal adheres to its 
host. As stated above, almost all nemertines are of separate 
sexes; and in some marine species the development of the 
young is so remarkable that it is impossible to pass it by 
without notice. The young which issue from the egg are 
so unlike the parent, that no one would at first sight suppose 
them to belong to this group. The larva, as shown on 
p. 461, bears some resemblance to a helmet, and has been 
named Pilidium. This creature, which is covered with 
cilia, swims near the shore for some time, while the young 
nemertine is developed inside. As soon as this has acquired 
four-byed nemertine, c pi a ant [ attained a certain stage of maturity, it breaks 
Tetrastemma obscurum ... ° J 
(enlarged). from the pilidium and starts an independent life. 
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