100 PLATYHELMINTHES [CH. 



fused at the base so as to resemble the combs of the Ctenophora. The 

 resemblance of these ciliated loops to the "ribs " of the Ctenophora 

 suggested to Lang the idea that Turbellaria were Ctenophora which 

 had become adapted to a creeping life, in which a marked bilateral 

 symmetry had replaced the generally radial arrangement of the 

 organs of the normal Ctenophora, though traces of the latter 

 arrangement remain in the Polyclada. The brain on this view 

 would be the apical plate which had shifted forward; the stomach 

 with its radiating branches would correspond to the funnel of the 

 Ctenophora and the canals in connection therewith ; the pharynx 

 sheath would represent the stomodaeum, the so-called " stomach " 

 of the Ctenophora ; but the eversible pharynx, the accessory genital 

 organs, i.e. genital ducts, penis, bursa copulatrix, uteri, etc., and 

 above all the excretory system must be regarded as new acquisitions. 

 This view, which at first was not received with much favour, has 



FIG. 42. Planaria polychroa x about 4. 



1. Eye. 2. Ciliated slit at side of head. 3. Mouth of proboscis. 4. Out- 

 line of the pharynx sheath into which the pharynx can be withdrawn. 

 5. Eeproductive pore. 



received strong support by the investigation of a marine organism 

 known as Ctenoplana. This is a flattened animal resembling a 

 Polyclade in shape and in the circumstance that the ventral surface 

 is covered with cilia with which it creeps. It possesses however an 

 apical plate of thickened nervous ectoderm supporting a mass of 

 otoliths on bars of fused cilia, and there are eight short "ribs" 

 radiating from this plate. These ribs as in Ctenophora are thickened 

 bands of ectoderm bearing combs of cilia fused at the base. The 

 funnel and its canals are represented by a lobed alimentary canal, 

 continued on each side into a tentacle canal, from the end of which 

 springs a long retractile tentacle. The genital organs have their 

 independent ducts opening directly to the exterior. In all these 

 respects therefore Ctenoplana is intermediate between the Poly- 

 clada and the Ctenophora. The evolution of the more complicated 

 systems of genital organs amongst the Turbellaria out of the simpler 



