1908-9.] THE Hapits OF PLETHODON CINEREUS ERYTHRONOTUS. 489 
represent the average thickness of the epidermis over the whole body. 
These averages were 22.4 micra for Plethodon, 46 micra for Amblystoma, 
and 44 micra for Diemyctylus. Consequently as regards this factor 
unless we assume a most improbable thing, namely that the epidermis 
of Plethodon is of a material less permeable than that of Amblystoma 
and Diemyctylus, we must conclude that of the three the skin of the 
lungless form Plethodon is fitted to be by far the best respiratory 
organ. 
Second. The bucco-pharyngeal respiration is indeed established 
very soon after escape from the egg yet even in the adult it may be sus- 
pended for a considerable time without serious inconvenience. When 
confined in a glass vessel the animal will occasionally rest the ventral 
surface of head, neck, and more or less of the trunk against the glass; 
the adhesion between the glass and the moist and sticky skin is sufficient 
to prevent the respiratory movements. I have frequently seen this 
continue for two or three hours and have found the position of the body 
apparently unchanged after even much longer intervals, but in these 
cases observation not being continuous it is possible that the animal 
may have moved for a time and then resumed its exact original position, 
but such a thing is unlikely. 
Third. There is a period of a few days in the life of a Plethodon 
during which such respiration as occurs at all must take place through 
the skin. ‘This is the period just prior to the escape from the egg envelopes. 
The gills attain their maximum of development—a length of 3 mm.— 
in larve of about 15 mm.; after this they decrease in size and for some 
time before the escape of the larva are reduced to small points not over 
.25 mm. in length. No movements of the ventral pharyngeal wall are 
to be observed at this time so it cannot be that there exists a mode of 
respiration similar to the aquatic pharyngeal respiration noted in Diemy- 
ctylus, both viridescens (Gage ’o1) and torosus (Ritter ’97), the fluid 
within the egg envelope playing for Plethodon the part of the surrounding 
water for Diemyctylus. Consequently whatever oxygen is used by 
the larva must be absorbed through the skin. The amount of oxygen 
required by the larva may well be less than that required by the free- 
living animal yet it is by no means an inconsiderable fraction of it. The 
muscular activity in the beating of the heart and the wriggling of the 
larva within the envelopes that occurs not infrequently is probably little 
less than that of the free-living animal, which though capable of active 
movement rarely indulges in it, forming in this respect a marked contrast 
to Diemyctylus. If the larva can carry on its respiration through its 
skin alone, hampered as it is by the surrounding fluid and envelopes, 
