CHAPTER VII 
NEMERTEA 
Palaeonemertea—Carinella, Polia. 
NEMERTEA~ Schizonemertea—Lineus, Cerebratulus. 
Hoplonemertea— Tetrastemma, Geonemertes, Malacobdella, 
CHARACTERISTICS. — This class is characterised by a ciliated 
ectoderm, which at the anterior end of the body is sunk in, 
and forms a pair of ciliated grooves or pits. There is a 
protrusible introvert, which may be armed with hooks and 
spines, opening above the mouth. A nerve comnvissure sur- 
rounds it. The nervous system consists of two cerebral 
ganglia, gwing off two lateral nerves which extend through- 
out the body, and may unite above the anus. The alimentary 
canal is not branched, but may bear lateral caeca ; it ter- 
minates in an anus. The generative organs are simple 
and paired ; the sexes are usually distinct. With few excep- 
tions, the Nemertea are marine. 
The Nemertines are mostly found amongst seaweed and 
coral rock, and they are frequently of the most brilliant colour. 
Pelagonemertes is, however, pelagic, and like most pelagic 
organisms is transparent, and JMJalacobdella is parasitic, living 
in certain Lamellibranchs. A few inhabit fresh water, such 
as Tetrastemma aquarum dulcium from North America, and 
an unknown species of the same genus recently found in 
England. Two species of TZetrastemma and two species of 
Geonemertes are terrestrial. 
Probably the members of no class vary so much in size as 
do the Nemertines; many of them are quite small, whilst 
others attain the length of many feet, and Professor M‘Intosh 
records finding a Lineus marinus which measured thirty yards, 
and even then was only half uncoiled. They possess extreme 
