694 
THE YOYAGE 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 Nu dibran chiata 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 Bathycloris abyssorum, Bergh (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 
