NEMERTEA 



119 



invariably in front of the mouth ; more rarely it opens with it 

 (Malacohddla). 



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 



Fig. 79. — Anterior portion of the tody of a Nemertine. 

 After Hubrecht. 



1. Brain. 



2. Lateral nerves. 



3. Proboscis sheath. 



4. Proboscis. 



5. 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 lie between the 

 diverticula of the alimentary canal. The blood is colourless in 

 Tetrastemma, but in some other species it contains haemoglobin. 



