154 ZOOLOGY. 
Crass VI.—NemeErtTINA (Nemertean Worms). 
General Characters of Nemerteans. The Nemertean 
worms occur abundantly under stones, etc., between tide- 
marks and below low-water mark; they are of various col- 
ors, dull red, dull green and yellowish, and are distinguished 
by the soft, very extensile, more or less flattened, long and 
slender body, which is soft and ciliated over the surface, 
the skin being thick and glandular. A few forms, such as 
Prorhynchus (Fig. 105), live in fresh water. 
The mouth forms a small slit on the ventral surface im- 
mediately behind the aperture for the exit of the proboscis. 
The proboscis is, when protruded, a long tubular organ, 
sometimes armed with stylet-shaped rods; it is thrust out of 
a special opening in front of the mouth, and when retracted 
within the body lies in a special muscular sheath. The 
cesophagus leads to a large digestive tract, ending posteriorly 
with an anus, and often with short lateral ceca. In Pela- 
gonemertes and Avenardia the numerous ceca are much 
branched. 
The nervous system is quite simple, consisting of two 
ganglia in the head united by a double commissure; from 
each ganglion a thread composed of nerve-fibres and ganglion 
cells passes back to the end of the body. 
The brain is well developed; the two halves are connected 
by a double commissure surrounding the throat, and each 
half is composed at least of a dorsal and ventral lobe. 
While the Nemerteans are much like the flat worms, 
most of them approach the Annzzlata, such as the earth- 
worm, in their highly complicated circulatory system, which 
is composed of a series of closed contractile vessels. There 
are three great longitudinal trunks, one median and two 
lateral, and connecting with each other. The blood is pale, 
rarely red, with corpuscles. Another feature characteristic of 
many Nemerteans is the “‘ proboscis,” nothing like it being 
found in other worms. Along the back of the head-end is 
a special muscular sheath containing the complicated probos- 
cis, which is extended through a pore situated above the 
mouth. The sheath contains a corpusculated fluid, and 
