56 
Tank No. 20. 
One of the finest animals belonging to this group is Tethys (Fig. 92, 
Tank 20). Its body is white and semi-transparent, and carries an excee- 
Fig. 92. Tethys leporina, V 2 nat. size. 
dingly broad head. On both sides of the back are attached the delicate 
transparent gills, and besides these there are large spotted appendages 
Fig. 93. Aeolis papillosa. 
which fall off very easily and were therefore formerly considered as para 
sites. This animal, like the pelagic forms is only occasionally found in 
the Aquarium; the fishermen scoop them up from the surface of the sea 
with buckets. They swim by throwing their head first to one side and 
