216 ZOOLOGY. 



Laboratory Work. The Tunicates can well be studied only in 

 a living state ; or sections of hardened SalpaB may be made. The 

 young, caught with the tow-net, should be immediately examined, as 

 they are very short-lived. Delicate sections of hardened eggs and 

 larvae are made with great difficulty, but are necessary to examine in 

 connection with the living more or less transparent animals. 



CLASS VII. 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. 141), 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 oesophagus leads to a large digestive tract, ending pos- 

 teriorly with an anus, and often with short lateral ceeca. In 

 Pelayonemertes 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. 



While the Nemerteans are much like the flat worms, 

 most of them approach the Annulata, 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 



