SOME OF THE WAYS IN WHICH ANIMALS BREATHE. 317 
is, indeed, more spongy than in some Amphibians (e. g., Meno- 
branchus). 
While there are fishes that breathe by lungs as well as by 
gills, so are there some allies of the frog, which, unlike it, 
retain their gills throughout life; here, again, we get an 
instructive series of gradations. The lowest Amphibia, or those 
which retain their tail throughout life (Urodela), may, like 
Menobranchus and Proteus, have persistent gills in addition to 
lungs ; in Menopoma, Amphiuma, and Megalobatrachus the gills 
disappear, but one or two of the clefts through which, in larval 
life, the gills protruded, persist in the adult condition, though 
the gills themselves are lost. 
It is very instructive to remark with what difficulty the gills 
are lost; this is especially well illustrated by the developmental 
history of Hpicrium glutinosum, the Cecilian of Ceylon. Epicrium, 
as Messrs. P. B. and C. F. Sarasin have lately told us, lays eggs 
within which the young are hatched. While still within the 
shell the young develop, on either side of the head, a tuft of 
three blood-red gills which move about constantly in the 
surrounding fluid. When these gills disappear the young 
escape from the egg-shell and pass into water; here the gill- 
clefts are at first open, just as they are in the adult Menopoma; 
later on the clefts disappear, and the Cecilian takes to a 
terrestrial mode of life. 
As I am confining myself to breathing-organs, I must not now 
trace the fate of the gill-clefts in those higher Vertebrates in 
which gills never serve as organs of respiration; but you who 
have heard Professor Parker speak from this place know, doubt- 
less, of the important functions which remain for the clefts, and 
the indications of them that are to be seen even in ourselves. 
Quite another form of terrestrial respiratory apparatus is 
found in that great group, the members of which outnumber all 
the rest of the Animal Kingdom ; I speak, of course, of insects 
and their allies. Here we have to do with thin tubes instead of 
with wide sacs; these tubes are called trachex, and the mode of 
respiration is not pulmonary but tracheal. The simplest con- 
dition of this mode is found in that most interesting and 
instructive animal Peripatus ; this instructive creature shows by 
its wide geographical distribution that it is an exceedingly 
archaic form, and we might well suppose from that one indication 
