168 MARINE INVERTEBRATES 
lar covering; others remain naked. All are covered with vibrat- 
ing cilia, which is a universal feature of flatworms. In color 
they may be white, yellow, green, red, purple, etc., and some- 
times they are banded or striped with a contrasting color. The 
ventral is usually lighter than the dorsal surface. 
The principal characteristic of the nemertean worms is a long 
thread-like organ, known as the proboscis. This lies in a sheath 
along the center of the dorsal surface, and is quickly thrown 
out to a great length, and as quickly completely withdrawn within 
the body. The proboscis is slender, hollow, muscular, and full 
of nerves. It reaches the outside through a pore at the ante- 
rior end of the body, and has no connection with the alimen- 
tary system. It is used as a feeler and as a weapon. Some 
species have a sharp spine at the end of the proboscis, others 
have stinging-cells. The proboscis is sometimes so forcibly 
ejected that it breaks off, in which case it retains its vitality for 
some time, and seems as if it were itself a worm. A new pro- 
boscis is quickly grown by the worm to take the place of a lost 
one. The head is a little broader than the body, and has eyes 
arranged in one or several pairs on each side. The mouth is on 
the ventral surface, near the anterior end, has thick lips, and is 
very dilatable. Through the mouth the animal ejects a part of 
the esophagus and envelops its prey, which is often of consider- 
able size. Cheetopod worms they often swallow whole; the soft 
parts, after being digested, are carried off through the regular 
passages, ending in the ciliary flames (page 164), while the indi- 
gestible parts are ejected at the anus. Often the spines and 
bristles find their way out by perforating the intestine and the 
body-wall, without apparently doing the worm any injury. The 
perforated parts quickly heal. 
These worms have the strange power of regenerating lost parts ; 
mutilated portions are soon repaired. The anterior end, when 
severed from the rest of the body, grows again into a complete 
individual, while the posterior end continues to perform part of 
its functions and retains its vitality for a considerable time be- 
fore dying. One species, Lineus sanguineus, is capable, after being 
broken in pieces, of regenerating each section into a perfect worm. 
