TURBELLARIA. 669 



the Tricladida^ but in the latter they are sometimes more numerous, and in 

 a terrestrial form from Brazil extend along the lateral margins. They are 

 generally numerous in Polycladida, two groups lying constantly above the 

 ganglia, others upon the tentacles, or where these are absent on the anterior 

 and lateral margins of the body. The most complex eye consists of a 

 pigmented cup into which project visual rods with retinal cells at their 

 outer ends. These cells are continuous with the optic nerves. Such eyes 

 are found in Tricladida and all Polycladida. In the latter group they 

 increase by division, whereas in some fresh-water Tricladida^ e. g. Polycelis 

 nigra, the eyes which are simple in structure have been observed to fuse. 

 Otolithic vesicles are very rare in Dendrocoelida^ and when present in the 

 Rhabdocoelida there is usually only one. It lies anteriorly and consists of 

 a vesicle with fluid contents and suspended calcareous otoliths. A pair of 

 ciliated grooves, one on each side, lie close to the ganglia in many Rhabdo- 

 coela. In fresh-water Tricladida and the Polycladida a ciliated groove 

 extends along the anterior margin of the body, and backwards laterally on 

 the dorsal aspect. These structures appear to be sensory. 



The mouth varies much in position : it may be anterior, median, or 

 posterior. It leads, as a rule, into a muscular pharyngeal sac, within 

 which lies a muscular pharynx. The pharynx can be extended from the 

 mouth, and is an organ for the prehension of food. It assumes various 

 forms. In the Acoela the mouth leads into a short tube which ends in the 

 parenchyma, into which the food passes. In other Turbellaria there is an 

 intestine, simple and saccular in Rhabdocoela, lobed or irregular in Alloio- 

 coela, branched in Dendrocoelida. In the last-named group it consists, 

 in Tricladida of three chief branches, one extending forwards above the 

 ganglia, and one backwards on each side, all beset with secondary branches ; 

 in Polycladida of a main intestine, which usually sends a branch forwards 

 above the ganglia, and gives off a variable number of paired lateral 

 branches, never, however, less than four. These branches divide in their 

 turn, and the points of division are often constricted. In some instances 

 the ultimate divisions form an anastomosing network. The epithelium of 

 the main intestine of Polycladida is ciliated ; that of its branches, like the 

 intestinal epithelium of other Turbellaria, emits pseudopodial processes 

 which absorb nutriment. Digestion is therefore parenchymatous. The 

 cells often contain vacuoles, concretions, and coloured granules. The main 

 intestine of Polycladida possesses a well developed musculature, while the 

 constrictions of the lateral branches are provided with both constrictor and 

 dilator muscles. The branches of the intestine in three genera of Polycladida 

 open externally through the ectoderm 1 . A nervous commissure generally 

 surrounds the branch of the intestine extending forwards above the ganglia. 



1 Yungia with dorsal, Cycloporus with marginal apertures. Oligodadus has a doubtfully 

 present pore situated dorsally and posteriorly. 



