116 ZOOLOGY 
powers of contractility, but are very easily broken into frag- 
ments. 
Tetrastemma flavida, which may serve as a type of the 
class, is a pinkish Nemertine an inch or more in length, which 
is found under stones or in fissures of the rock when the tide 
is down. It occurs in all European waters from Scotland to 
the Red Sea; it is extremely delicate, and the slightest touch 
may serve to break it. 
The ectoderm of this animal is columnar and ciliated. 
Beneath the ectoderm, and separated from it by a basement 
membrane, is the thin layer of circular muscles, and this layer 
in its turn surrounds a thickish layer of longitudinal muscles 
Fig. 76.—Diagrammatic section to 
show disposition of parts in a 
Hoplonemertean (such as Tetra- 
stemma). After Hubrecht. 
Cellular portion of integument. 
Basement membrane. 
Circular muscular layer. 
Longitudinal muscular layer. 
Lateral nerve. 
Cavity of proboscis sheath, 
Proboscis. 
Intestine. 
Lateral blood-vessels. 
Dorsal blood-vessel. 
Connective tissue. 
SS) SE) ESTED SES oS) I 
a 
(Fig. 76). From the latter, fibres arise and pass in various 
directions through the body, certain of these are especially 
conspicuous, running in a dorso-ventral direction between the 
diverticula of the alimentary canal, and these give the body 
the appearance of being segmented, but the bundles of muscles 
are not very regular, nor are they always opposite each other. 
A layer of diagonal muscle-fibres usually occurs within the 
longitudinal. 
The integument of Nemertines encloses a more or less 
solid mass of parenchyma, in which the various organs of the 
body are embedded. In this there are two kinds of spaces. 
Firstly, there is the space into which the proboscis is with- 
drawn, and the blood-vessels. These contain a fluid in which 
corpuscles float, and they represent that kind of coelom known 
as an archicoel. And then there are the spaces in which the 
