PLA T YHELMINTHES 93 



middle of the dorsal surface, is 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 (i.) 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 ; (ii.) their basal membrane (Ctenoplana) and 

 disposition of muscle fibres ; (iii.) 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 Ctenophoea 

 and the Turbellaeia be accepted as sound, it must be through 

 the Polyclads, in many respects the most comphcated tribe, 

 and in this case the other members of the group must be 

 regarded as degenerate. 



The TurbeUaria are classified as follows: 



A. Khabdocoelida. — Of small size, body cylindrical or de- 

 pressed, with or without an intestine ; when it is present 

 it is simple and uniranched. Female genital glands 

 compact, not follicular. Genital orifice single or double. 



Tribe 1. Acoela. — JVo intestine; digestion takes place in 

 general parenchyma. No nervous system or excretory 



