PLATVHELMINTHES 93 
middle of the dorsal surface, 1s a structure resembling the aboral 
sense organ of Ctenophors ; on each side of the body is a tentacle 
with short side branches. In the neighbourhood of the ten- 
tacles is the opening of a branching system of canals, which 
have been compared with the excretory system of Turbellarians. 
Beneath the ectoderm is a basal membrane which encloses an 
outer layer of longitudinal muscles and an inner of transverse 
muscle fibres; there are also dorso-ventral muscles. 
From this short description it will be seen that these two 
animals resemble Ctenophors in (1.) their aboral sense organ ; 
(ii.) the presence of ctenophoral plates (Ctenoplana) ; (iii.) the 
pair of solid, retractile, feathered tentacles; (iv.) the general 
arrangement of their body. On the other hand they resemble 
the Polyclads in (i) their flat shape, uniform ciliation, and 
creeping motion ; (i.) their basal membrane (Ctenoplana) and 
disposition of muscle fibres; (i1.) the branched stomach with 
ventral mouth; (iv.) the possession of an excretory system 
(Ctenoplana ?). 
These two animals serve to show that there may have 
been a connection between the Ctenophora and the Polyclads ; 
and the resemblance is emphasised by the fact that both 
groups are hermaphrodite. On the other hand, it is not easy 
to homologise the nervous system of the two groups. The 
Ctenophors have nothing corresponding with the excretory 
system of Turbellarians, and the mesogloea of the former does 
not necessarily represent the mesoderm of the latter. Further, 
if the view that there is a connection between the CTENOPHORA 
and the TURBELLARIA be accepted as sound, it must be through 
the Polyclads, in many respects the most complicated tribe, 
and in this case the other members of the group must be 
regarded as degenerate. 
The Turbellaria are classified as follows : 
A, RHABDOCOELIDA.—Of small size, body cylindrical or de- 
pressed, with or without an intestine ; when it is present 
iw is simple and unbranched. Female genital glands 
compact, not follicular. Genital orifice single or double. 
Tribe 1. Acoela.— No intestine ; digestion takes place in 
general parenchyma, No nervous system or excretory 
