154 ZOOLOGY. 



CLASS VI. NEMERTINA (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 

 oesophagus leads to a large digestive tract, ending posteriorly 

 with an anus, and often with short lateral cceca. In Pela- 

 gotiemertes and Avenardia the numerous cceca 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 ilat worms, 

 most of them approach the Annnlata, 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 



