118 ZOOLOGY 
is produced into a certain number of lateral diverticula, but 
these do not seem to be very definite in number or size 
(Fig. 77). The stomach is lined by a layer of cells which 
are capable of assuming very 
different outlines at different 
times; they often break away 
from the wall and are seen 
floating in the lumen of the 
digestive canal. There is no 
special muscular coat, but some 
of the muscle-fibres running 
through the parenchyma are 
attached to the walls of the 
stomach. In most Nemertines 
the alimentary canal is ciliated. 
The anus is terminal. 
Fic. 78.—Introvert of a Hoplonemertine, ; 
with stylet, ‘‘reserve” sacs, and The most characteristic 
muscular bulb, After Hubrecht. organ in the Nemertines is the 
A. Retracted, Cet ; ‘ : : 
Bo Werte introvert or proboscis, which 
consists of the hollow eversible 
anterior end of the animal. In its retracted condition this is 
invaginated into a cavity, the lumen of the proboscis sheath, 
just as the finger of a glove may be inverted into the glove. 
The cavity of the proboscis sheath is a closed one, and full of 
a corpusculated fluid; the walls of this cavity, ae. the pro- 
boscis sheath, are extremely muscular, when they contract 
the pressure of the fluid drives the introvert forward and it is 
everted. In many Nemertines the proboscis sheath extends 
the whole length of the animal, and only ends just above the 
anus. The method of the eversion of this proboscis is interest- 
ing; when it begins to protrude, it is the walls of the organ 
which first grow forward, and the extreme end of the proboscis 
—often armed with a spine—is the last part to appear, and 
is therefore only to be seen when the proboscis is fully ex- 
tended. It is retracted by a special muscle inserted into the 
tip of the proboscis behind the spine, and arising from the 
base of the proboscis sheath; when this contracts, the first 
portion to disappear is the tip. The aperture through which 
the proboscis appears is either terminal or ventral, but almost 
