TURBELLARIA 



215 



coloured with stripes down the dorsal surface. Bipalium kewense is 

 a cosmopolitan tropical form that often turns up in greenhouses. It 

 is often a foot long and is easily recognized by the axe-shaped head. 

 Rhynchodemus terrestris, a small form 6-8 mm. long, is a British 

 representative of this division. It is found in damp situations under 

 the bark of decaying trees and fallen timber. 



Order POLYCLADIDA 



These are entirely marine. The gut has many diverticula leading out 

 from a not very conspicuous main stem. The mouth has shifted to the 



ciL' 



— ciV 



Fig. 155. Miiller's larva of a Polyclad, Cycloporus papillosus Lang. Ventral 

 view, cil.^ large cilium at anterior end; e. eyes; l.^ L^ l.^ projecting lobes, 

 the edges of which have cilia longer than those on the general body surface 

 (there are eight of these lobes, there being one similar to 1} and another pair 

 similar to /.^ on the dorsal surface); m. mouth; cil} large cilium at posterior 

 end. (Altered from Kiikenthal.) 



posterior end. The germarium and the vitellarium are combined into 

 one organ but there are separate male and female openings. The ovum 

 is entolecithal, i.e. it has the yolk inside it as in the Acoela. In all other 

 Platyhelminthes the ovum is ectolecithal, i.e. it has no yolk inside it 

 but is surrounded inside the egg shell with yolk cells, which break 

 down when development begins. The early embryological stages in 

 the development of the Polycladida resemble, as might be expected, 

 those of the Acoela, but there are however four macromeres instead 

 of two as in the Acoela. A further point of difference is that in the 



