Ill PLATODESSEXSORY ORGANS 149 



backward are always the thicker nerves, while the lateral and front nerves are the 

 shorter and thinner. The greater development of the musculature on the creeping 

 surface has as a consequence the greater development of the ventral trunks. The 

 elongation of the body further causes a retrogression of the outer longitudinal nerve 

 trunks, so that finally the 2 inner longitudinal trunks remain as the main trunks, 

 which from the first are very strong, since they have to innervate the most import- 

 ant muscular organs which lie in the median line between them (pharynx, eopulatory 

 apparatus, suckers). Thus in the end the regular ladder nervous system of Gunda 

 is deducible from a general nerve plexus in the integument, determined by the elonga-_ 

 tion of the body, the localisation of the sensory organs at its anterior end, and the 

 strong development of the musculature on the ventral surface. 



The brain in the Platodes appears the more evidently to consist of 2 lateral halves 

 or ganglia connected by a bridge of fibres, the more strongly the 2 longitudinal trunks 

 of the nervous system are developed in comparison with the whole nervous system. 



IX. The Sensory Organs. 



The Platodes possess sensory organs, which according to their 

 structure may be described as eyes, auditory organs, or organs of touch. 

 To these belong also ciliated pits, whose function is unknown. Many 

 consider them to be olfactory organs. The development of the sensory 

 organs stands in direct relation to the mode of life. They are best 

 developed in the free-living Turbellaria. In the parasitic Tremadoia 

 their degeneration begins, and the Cestoda no longer possess any 

 specific sensory organs. 



A. Eyes. 



These are found in most TurMlaria and ectoparasitic Trematoda. 

 They are wanting in adult endoparasitic Trematoda, but, on the other 

 hand, are found in their young stages, which are free-living, at least 

 for a time. 



All Pohjclcula possess eyes in large numbers, often as many as several 

 hundreds. In their arrangement the following points are worthy of 

 note. A group of eyes is always found above the brain and in the 

 tentacles. Many forms possess, besides these, other eyes at the anterior 

 margin of the body or all round the margin. In the Triclada there 

 are either 2 eyes near the anterior end, or numerous eyes along the 

 margin of the whole body, or along its anterior margin. In the Rhabdo- 

 ccelidce there are generally 2 or 4 eyes (less frequently one unpaired 

 eye) directly on or over the brain ; this is also the position of the 

 4 eyes of the ectoparasitic Trematoda and the 2 eyes of the free-livmg 

 larv£e of the endoparasitic Trematoda. 



In all the Platodes (except the Accela and Microstomidce) the eyes 

 lie under the epithelium in the parenchyma ; in the Bhabdoccela and 

 Trematoda directly on or in the brain. Yet in the case of the Polyclada 

 at least, it can be ontogenetically proved that the eyes at their first 

 appearance arise in the ectoderm of the embryo and only secondarily 

 sink below the surface. 



