NEMERTEA. 
Class NEMERTEA. Nemertines 
197 
The ribbon-worms or Nemertines are interesting in many 
ways, ¢.g. in being the simplest animals to have an open 
gut, a closed blood-system, and, occa- 
sionally, hemoglobin ; in having some 
very peculiar structures, notably a pro- 
trusible proboscis and ciliated head 
slits; in being in many cases extra- 
ordinarily extensile and liable to break 
into pieces. 
The Nemertines are worm-like ant- 
mals, unsegmented and generally elongate 
in form; they are almost all marine, 
and most, if not all, are carnivorous. 
The ectoderm is ciliated. There is a 
remarkable retractile proboscis, uncon-: 
nected with the alimentary canal, and 
Jorming a tactile organ or a weapon. 
The nervous system consists of a brain, 
a commissure round the proboscis, and 
two lateral nerve-cords ; in connection 
with the brain there ts a pair of ciliated 
pits. The gut terminates in a posterior 
anus, and ts furnished with lateral 
pockets. |There is no body cavity in the 
adult, but the closed vascular system ts 
probably of cwelomic origin. The ex- 
cretory system is apparently of the 
Platyhelminth type. The sexes ave usu- 
ally separate and the organs simple. The 
development is in some cases direct, while 
in others there is a peculiar pelagic larva. 
Fig. 103.—Diagrammatic longitudinal section 
of a Nemertean (Amphzporus lactifloreus), 
dorsal view.—After M‘Intosh. 
#.£-, Proboscis pore; 4., brain giving off the lateral 
nerve-cords (.); #o., cesophageal pocket; %., pro- 
boscis lying within its sheath 3. s¢., stilet of proboscis ; 
m., retractor muscles of proboscis; g., gut shown in 
outline at the sides of the proboscis; ¢, the three 
main longitudinal blood vessels, which unite both 
anteriorly and posteriorly. 
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