26 PLATYHELMINTHES TURBELLARIA chap. 



nitrogeneovis waste substances, which are then discharged into the 

 capillaries, whence the cilia of the main vessels drive them pre- 

 sumably to the exterior, though external openings of the excretory 

 system are not known. Traces of this system have been observed 

 in young Zeptoplana (first by Schultze in 1854) and also in 

 Cestoplana. 



Sensation. A nervous sheath, with scattered ganglion cells, 

 everywhere underlies the musculature. It is exceedingly faintly 

 marked on the dorsal surface, but laterally and ventrally forms 

 a dense network with polygonal meshes. Thickenings of this 

 sheath give rise to lateral nerves, and also to a pair of stout longi- 

 tudinal nerves from which the internal organs are probably in- 

 nervated. The brain, hardly distinct in pelagic Polyclads, in most 

 forms does not differ greatly from that of Zeptoplana (p. 13). 



The sense organs of Polyclads have the form of tentacles, 

 eyes, otocysts (in Zeptoplana otophora), and stiff tactile cilia. The 

 solid dorsal tentacles of Planoceridae contrast strongly with the 

 folded or pointed hollow processes of the Cotylea. The former 

 (Fig. 8, A, T) are muscular and very contractile, and are placed 

 near the brain some distance from the anterior end. The latter 

 are outgrowths of the front margin of the body, and are some- 

 times (Yungici) provided superficially with olfactory pits and 

 internally with eyes and intestinal coeca. 



The eyes which occur in Polyclads may be divided into (a) a 

 pair of cerebral groups overlying the brain ; (b) those embedded 

 in the tentacles (tentacular group) ; and (c) the marginal eyes, 

 which in Anonymus occur all round the margin. A complex 

 form is sometimes assumed by the cerebral eyes of Pseudoceridae, 

 resulting probably from incomplete fission (Fig. 11). Zepto- 

 plana otophora was obtained by Schmarda on the south coast of 

 Ceylon. On each side of the brain is a capsule containing two 

 otoliths. This is the only known case of the occurrence of 

 these organs in Polyclads. 



Reproduction. Although Polyclads are able to repair the 

 result of injuries to a very considerable extent, they are not known 

 to multiply asexually. The two processes are intimately associated, 

 but, though probably all Turbellaria can regenerate certain lost 

 parts, asexual reproduction only occurs sporadically. 



All known Polyclads are hermaphrodite. The male organs, 

 scattered, like the testes of Zeptoplana, over the ventral surface, 



