220 
CLASS VII. 
occasion to the name, was first, as it seems, observed by Ducrs in 
Planarie, although he did not refer it to cilia (Ann. des Se. nat. 
Tom. xv. p. 165), whilst Von Bazr observed at the same time an- 
other phenomenon which could only be an effect of these cilia, 
that when a portion of these animals is cut off it continues to rotate 
circularly in the water (Wov. Act. Acad. Caesar. Leop. Carol. Tom. 
ties, ps 711). 
The internal structure of these creatures was first recognised with 
precision in this century, especially through the investigations of 
Duce&s, V. Barr, Foxxe, OrrstTED, and QUATREFAGES; it presents 
important differences in different genera whilst the external habitus 
is similar. We must confine ourselves to the notice of a few general 
traits. 
Besides the cilia on the external surface, the external integument 
in many is distinguished by the presence of cells with nettle-threads, 
like as we stated in Acalephe. Beneath the integument there is 
a layer of transparent, homogeneous tissue, which, according to 
QUATREFAGES, supplies, as it seems, the place of muscle, and by its 
contraction effects the movement of the body. The motion occurs 
by swimming in the water, the lateral margins beating to and fro, 
by creeping with bending and straightening of the body, or by equably 
gliding, much as the gasteropod molluscs move with their so-named 
foot. 
The oral aperture is sometimes more forward, sometimes more in 
the middle of the body on the ventral surface. The intestinal canal 
is in some straight, and extends itself, when the mouth is not placed 
quite forward, anteriorly as well as posteriorly, with blind termina- 
tions in both directions. In others the intestinal canal is like a 
tree divided into branches ; in our fresh-water species one branch is 
seen to run forwards, on both sides provided with blind appendages, 
and two stems or main branches backwards at the sides of the body 
(in Planaria lactea the branches may be readily distinguished 
externally by their dark colour). In other species from sea-water 
the division of the branches is somewhat different ; sometimes quite 
retiform. In the cavity of the mouth is situated a part that can 
be extended by eversion, serving for the seizure of food, and various 
in form. It is able, when severed from the living creature, to move 
independently for some time whilst it swallows greedily surrounding 
substances which are seen to pass out by the posterior open ex- 
tremity as through a funnel. With respect to the vascular system 
little is known ; that which is described as such by some writers, 
