ALIMENTARY CANAL IN LEPIDOSIEEN AND PEOTOPTEEUS. 499 
encloses anterior as well as posterior nares, and forms one 
of the most distinctive features of the Dipnoi. How this 
ai’rangement has come about in Phylogeny does not seem 
quite certain, but most probably it has been by a backward 
migration of the narial rudiment, rather than by an extension 
forwai’ds of the mouth boundary. It will be noticed that the 
upper lip, while very prominent laterally, can hardly be said 
to exist in the region near the mesial plane. This gives a 
characteristic gaping, almost cyclostomatous, appearance to 
the mouth of the young Dipnoan,^ and if it be assumed that 
this is a repetition of a phylogenetic condition, it is clear that 
a backward migration of the narial openings into the buccal 
cavity could readily have taken place. The physiological 
significance of the iutrabuccal position of the narial openings 
is clearly in all probability adaptive to the mud-burrowing 
habits. The olfactory organ is in the living Dipnoan used, 
so far as my observations go, entirely as a sense organ, its 
respiratory function not yet having developed. The sense of 
“smell” affords the principal means by which the living 
Lepidosiren or Protopterus finds its food; a little colouring 
matter, e. g. blood, in the water, shows how they actively 
“sniff” about, with snout sharply bent down, in search of 
food particles at the bottom of the water. 
Glands and Sense Organs. — Unicellular glands and 
sense buds are scattered, as already shown by Parker for 
Protopterus, over the lining of mouth and pharynx as on the 
outer skin, while the flask-shaped glands so characteristic of 
the external ectoderm are normally absent from the buccal 
cavity. 
Thyroid. — The thyroid makes its appearance about stage 
XXX in Lepidosiren as a solid keel-like projection from 
the ventral side of the solid buccopharyngeal rudiment. The 
study of sagittal sections (text-fig. 7) show that the thyroid 
rudiment becomes gradually cut off from the buccopharyn- 
geal mass from behind. 
‘ Even in the adult Lepidosiren the month is actively suctorial, and 
food is drawn into the mouth by a strong sucking action. 
VOL. 54, PART 4. NEW SERIES. 
36 
