PLANARIAN WORMS. 
391 
generative organs (PI. XIX. figs. 3, 4, 6pe). They differ notably 
from both the Trematode and Cestode Flat- worms in the complete 
and permanent ciliation of the surface of their bodies, which is 
one of their most characteristic features; also in having no suck- 
ing discs or 'prehensile hooks, such as both these other orders pos- 
sess, used for locomotion and adhesion, but only a sucker-like 
mouth. In their mode of life they present the most striking 
contrast to both Trematodes and Cestodes ; for whilst the mem- 
bers of these orders are one and all parasitic (chiefly internally), 
no Turbellarians are ever so, though they are found in the sea, 
in ponds, and fresh-water streams, and in moist earth and decay- 
ing wood. The integument of the Turbellaria is remarkable 
not only as being densely covered with cilia, but as containing 
very numerous oblong corpuscles and clear cells, some of which 
have been compared to the “ nettle-cells ” possessed by the 
Polyps, jelly-fish, and Nudibranchiate Molluscs. The muscular 
fibre is not separate or collected into masses apart from the skin 
and viscera, is in some cases faintly striated transversely, and pre- 
sents, with the rest of the worm’s organs, a most singular softness 
and fluidity ; so that while living most Turbellarians are almost 
incredibly elastic, and when dead decompose and become a 
watery mass in the course of one or two minutes. The digestive 
organs are very simple : the suckqr-like mouth, placed in almost 
any position (except terminally) on the ventral aspect of the 
body — towards the front, centrally, or quite posteriorly — opens 
into a pharynx, which is very frequently muscular and everti- 
ble ; from this a straight or arborescent cavity extends through- 
out the body, opening in an obscure terminal anus in the 
Nemertines, but having no outlet in the Planarians. Little 
glandular masses, assisting by their secretion in digestion, sur- 
round the digestive tract ; and by Leydig are said to act as 
kidneys also. In both Nemertians and Planarians the region 
in front of the mouth is often (but not always) modified to 
form what is called the proboscis, a most extraordinary organ, 
which has caused the greatest differences of opinion, and has 
prevented and does still prevent the proper understanding of 
the anatomy of these animals. The real mouth varies so much 
in its position, and is often so very backwardly placed (especially 
in the Planarians), that for a long time it was thought to be a 
sucker like those of the Fluke-worms, and the proboscis being 
in front was thought to carry the true mouth. It is now ad- 
mitted by Schmidt and Schultze (though themselves at one 
time mistaken) that no Turbellaria have any sucking discs; and 
that the proboscis is quite separate from the mouth, though it 
may be perforated in some cases. It is probably in most cases 
a prehensile appendage, but its use is very obscure. Suppose 
you have a Turbellarian as I have described — with ventrally 
