82 ZOOLOGY 
are simpler in their organisation, Mesostoma, is, however, not 
infrequently found in ponds and streams, and the transparency 
of its body permits a more thorough examination of its internal 
organs than is the case with most of the members of the group. 
Mesostoma Ehrenbergii is a flat leaf-shaped Rhabdocoel, 
its dorsal surface being but very slightly vaulted. Its length 
does not exceed 15 mm., and its greatest breadth 5 mm. 
Another member of this genus has a square cross-section, 
with the angles somewhat produced. The body is very trans- 
parent, and sometimes nothing of it can be seen but the brown 
stomach and occasionally the red eggs. 
The epidermis is composed of flat cells, with irregular out- 
lines and conspicuous nuclei. The whole surface of the body 
is covered with fine cilia, somewhat longer on the ventral side 
than on the dorsal. Within the layer of ectoderm cells, but 
separated from it by a basement membrane, is a thin sheet of 
Fig. 56.—Integument of 
Mesostoma lingua, O. Sch. 
After Von Graff. 
1. Epidermis with perfora- 
tions (2) through which 
the rhabdites (3) project. 
Beneath is the basement 
membrane (4), and _ be- 
neath this again the 
muscular layers consist- 
ing of circular (5), longi- 
tudinal (6), and diagonal 
(7) fibres. 
circular muscle fibres, and within this a layer of longitudinal 
fibres, crossing the outer layer at right angles, and a few 
oblique fibres (Fig. 56). These tissues form the integument, 
and surround a mass of tissue, the parenchyma, in which the 
various organs of the body are embedded. The coelom is 
broken up into irregular spaces or splits by the presence of 
numerous dorso-ventral muscle fibres and connective tissue 
strands. The lacunae left between these branching fibres 
