88 ZOOLOGY 
larvae. When it has caught a Daphnia, for instance, it will 
wrap its body round it and attack it with its proboscis; very 
frequently it makes prisoners of its prey by ensnaring them 
with threads of mucus. In the autumn these animals become 
of an opaque white colour, lose their power of movement, and 
sink to the ground. The survival of the species through the 
cold of winter is ensured by the existence of the well- 
protected winter eggs, a device frequently met with amongst 
fresh-water creatures. 
Mesostoma is not a large Turbellarian; the marine forms 
often measure more than an inch in length, and are usually 
flattened and leaf-like (Fig. 55); the land Planarians, on the 
Fra. 61.—Plan of an Acoelous Turbellarian. 
After Von Graff. 
. Eye. 
Mouth. 
Otolith. 
Ovary. 
Digestive parenchyma. 
Testicular follicles. 
Vesicula seminalis. 
. Male copulatory organ. 
, Common sexual aperture. 
+0 OF 1m OT BP Oo DD OH 
Oy 
other hand, are elongated and linear: one of this has been 
described which attains a length of nine inches. The marine 
forms are often brilliantly coloured, and their dorsal surface 
beset with papillae. 
