NEMERTEA 119 
invariably in front of the mouth; more rarely it opens with it 
(Malacobdella). 
Tetrastemma has a well-developed spine at the end of its 
proboscis, and on each side a couple of small secondary ones. 
Certain glandular structures open by a duct near the base of 
the spine, and possibly secrete a poison. In those Nemertines 
in which the introvert is constantly in use, the walls of the 
proboscis sheath are extremely muscular; and this defensive 
organ can be shot out with the greatest velocity, and at times 
with such force as to break off. When this is the case, it 
retains its vitality for some time, and crawls about independ- 
ently. This may be accounted for by the enormous develope- 
ment of nervous tissue found in its walls. The animal is 
capable of reproducing its lost introvert. 
A closed system of blood-vessels lined with an epithelium is 
present (Figs. 76 and 80). It comprises a median dorsal vessel 
5 
Fia. 79,—Anterior portion of the body of a Nemertine. 
After Hubrecht. 
= Bram. 
. Lateral nerves. 
. Proboscis sheath, 
. Proboscis. 
ore wd 
. External aperture through which the intro- 
vert is everted. The mouth and oesophagus 
are indicated by dotted lines. 
which runs along the body just below the proboscis sheath, at 
the posterior end this divides into two branches above the 
anus, and the branches run forward as two lateral trunks 
situated in the longitudinal muscle layer. At the anterior 
end the three trunks again unite, and from their point of 
union give off a loop which in many species encircles the 
proboscis sheath. The dorsal and lateral trunks in most forms 
communicate by transverse vessels which le between the 
diverticula of the alimentary canal. The blood is colourless in 
Tetrastemma, but in some other species it contains haemoglobin. 
