PHYLUM PLATHELMINTHES 



123 



the development of an anal opening at the posterior extremity of 

 the digestive tract, and (3) the presence of a closed sac, the 

 proboscis sheath, which surrounds the proboscis and by some is 

 considered as representing the beginning of a coelom. In 

 attempting to show relationships with the coelomate animals, 

 this last point is of considerable importance. Other workers 

 have thought that the cavities of the gonads of nemertines are 

 really coelomic sacs. 



Nemertines are chiefly marine, living usually in burrows in 

 mud or sand, and in some species attaining a length of 90 feet. 

 Some of the smaller forms inhabit fresh water or live in moist 

 soil. The body surface is ciliated and frequently brilliantly 



ps pm 



pn po 



Fig. 69. — Diagram of a nemertine. b, brain; c, ciliated pit; d, dorsal nerve 

 trunk; di, dorsal blood vessel; g, gastric ceca; i, intestine; /, lateral nerve trunk; 

 Iv, lateral blood-vessel; p, proboscis, retracted; pm, proboscis muscles; pn, proto- 

 nephridial tube; po, its opening; ps, cavity of proboscis sheath. {After Kingsley, 

 courtesy of Henry Holt and Co.), 



colored. Numerous mucous glands produce secretions which 

 may form a tube within which the animal dwells. The body 

 wall contains an outer circular and an inner longitudinal layer 

 of muscles which are so effective that a worm which is 15 feet or 

 more when fully extended may shorten to less than 2 feet in 

 length. 



The proboscis (Fig. 69) is one of the most characteristic struc- 

 tures of the nemertine. This is a hollow muscular tube turned 

 into the body at the anterior extremity and when thus inverted 

 extends far back through the body within a saclike cavity called 

 the proboscis sheath. By contraction of the fluid-filled sheath, 

 the proboscis is everted and thrust out from the anterior part 

 of the body. At the tip of the extruded proboscis, there is fre- 

 quently a sharp-pointed stylet which, of course, is at the extreme 

 posterior end of the proboscis when it is retracted and inverted 

 within the sheath. Retraction of the everted proboscis is accom- 

 plished by means of a retractor muscle which runs from the tip 



