694 
THE VOYAGE OF H.M.S. CHALLENGER. 
“ Most of the families belonging to this group are found in all seas ; some, however 
(■ e.g ., the Phyllidiadse), occur only in the tropics. The largest group, the Dorididse, is 
represented by a large number of generic forms especially in tropical regions, whilst the 
iEolidiadse, on the other hand, are more conspicuous in more northern seas. 
“ Most Nudibranchiata live on algse and coral reefs in the neighbourhood of coasts or 
on the surface of the open seas ; it was therefore not to be expected that the Challenger 
Fig. 238. — Back view of Batliydoris abyssorum, Bergli (natural size), showing the five branchial tufts, one having been 
probably accidentally lost from the right side ; close to the branchiae on the right side is the renal pore, and below 
in the middle line the anal papilla ; at the bottom is the expanded foot. 
Expedition, which was chiefly concerned with deep-sea investigation, should bring back 
a large collection of these animals, nor many new species. 
“ In fact, only twenty-four (or twenty-five) forms altogether were obtained, of which, 
however, twelve or thirteen were new species, which give rise to the constitution of 
four new genera. 
“ The most remarkable form was dredged from the great depth of 2425 fathoms ; the 
Nudibranchiata called ‘ deep-sea ’ by M. Sars came from much shallower water, This 
