90 ZOOLOGY 
canal is a straight and unbranched cavity, such as exists in 
Mesostoma. The other great division of the class, the Den- 
drocoela, have a branched stomach, the main division of which in 
the Polycladida is ciliated, the branches only being lined with 
an amoeboid epithelium, which digests intracellularly. The 
branches may anastomose, and in rare instances, Yungia and 
Cycloporus, they open on to the exterior. 
The excretory system is absent in Acoela; in Rhabdo- 
coela the main trunk may be single and open posteriorly, 
or double, and then the two ducts may, as in Jfesostoma, open 
separately and near the mouth, or together at the posterior end. 
In the Dendrocoels the main trunks open by paired apertures 
situated on the dorsal surface. The main trunks may be 
ciliated, the finer branches are probably always intracellular, 
piercing the cells and not lying between them. 
The two main nerve cords are rarely connected by trans- 
verse commissures in the Rhabdocoels, but in the Triclads this 
is the usual arrangement. In the Polyclads there are many, 
usually eight, nerve cords which diverge from the central cere- 
bral ganglion (Fig. 63). Sensory cells provided with tactile 
hairs oecur in the ectoderm. The eyes are usually two or four 
in number, but they may be more numerous, and in the Poly- 
clads they increase by division. Auditory vesicles also occur, 
though they are rare in the Dendrocoels; they are often single, 
and consist of a vesicle full of fluid in which a calcareous 
otolith floats. The anterior end of the body is remarkably 
sensitive, and in some genera forms a tactile proboscis which can 
be retracted into a sheath; this, together with a pair of lateral 
ciliated grooves which lie one on each side of the brain in many 
Rhabdocoels, affords matter for a comparison with the members 
of the class Nemertea. 
Turbellarians, with the exception of two genera, Miero- 
stoma and Stenostoma, are hermaphrodite, but many of them 
are protandrous—that is to say, their male reproductive cells 
mature before the female. Most Rhabdocoels and all Tri- 
clads have a common genital opening for both male and female 
duets ; others have separate apertures, and then the male is 
usually anterior to the female. Self-fertilisation is said to 
occur in the summer eggs of some Rhabdocoels. 
