PLATVHELMINTHES 89 
The position of the mouth in the different species of Tur- 
BELLARIA varies greatly ; in some it is anterior, in others it is 
ventral and median, and it may be nearly at the posterior end 
of the body. True nematocysts, 
such as are found in the COELEN- 
TERATA, exist in the ectoderm of 
a few genera, amongst others in 
Microstoma. Some of the ecto- 
dermic cells are also modified, and 
form adhesive or glutinous areas 
which make up for the want of 
suckers. 
The pigment may be in the 
ectodermal cells, or, as in 
Mesostoma, in the parenchyma ; 
Vortex viridis and Convoluta 
Schultzii_ contain cells coloured 
with chlorophyll and with starch 
in them. These are probably 
symbiotic algae. 
The arrangement of the 
muscle fibres in Jesostoma is 
very simple; the diagonal layer 
exists Im many genera, and in 
some of the Dendrocoels there 
may be as many as six separate 
layers. 
The disposition of the phar- 
ynx and stomach in Turbellarians 
is very various, and has served as 
a basis for the classification of 
the class. The muscular part of 
the pharynx may be very much 
enlarged, and capable of being 
protruded through the mouth, 
and acting as a powerful sucker. 
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Fic. 62.—Plan of a Triclad Turbel- 
larian. After Von Graff. 
1. Anterior. 
1’,1”. Posterior branches of alimentary 
canal. 
5. Oviduct. 
9. Tentacle. 
10. Vas deferens. 
2, 3, 4, 6, 7, 8, 11, 12, 13, as in Fig. 59. 
Q. Female copulatory organs. 
é 9. Common sexual aperture. 
In one division of the Rhabdocoelida, the Acoela (Fig. 61), 
the pharynx ends in the solid parenchyma, into which the food 
passes, and is there digested ; in the Rhabdocoela the alimentary 
