110 NUDIBRANCHIATA.—TRITONIADA. 
the surface of the water, now and then lifting and 
puckering up the edges of the mantle, and allowmg 
the air to bathe the sides of the body. 
FAMILY TRITONIADZ. 
This is a small group considered in the number 
of its component species, though some of these are 
of unusually large size. Its distinctive character- 
istics are that the gills, which are either laminated, 
plumose, or papillose, are arranged down the sides 
of the back, and that the stomach is simple. By 
the former character it is distinguished from the 
Doridide, the feathery gill-leaves of which are 
retained in several of these, but never arranged 
in the form of a flower around the vent. By 
the latter it is severed from the Holidide, in 
which the stomach sends off branching tubes on 
each side. 
There is no proper mantle distinguished as such 
from the general surface of the body ; but there is 
often an elevated ridge running down each side, 
along which the gill-tufts are placed. In the 
Scyllea, an oceanic genus found crawling among 
the stems and weeds of the floating gulf-weed, 
there are two or three erect, square lobes of flesh, 
projecting from each side of the back, and on the 
imner side of these the small tufted gills are 
scattered. In Glaucus, another oceanic animal of 
exquisite beauty, the gills take the form of fan-like 
ae: of filaments, diverging from the tips of long 
oot-stalks. 
The head, in most of the species, is protected by 
a veil or expansion of membrane, sometimes cleft 
