196 SUMMARY OF OUEKENT RESEARCHES RELATING TO 



takes on an arrangement which strikingly calls to mind those found 

 in the higher segmented forms ; in consequence of the reduction of 

 the lateral portions of the body and the simplification of the digestive 

 and generative systems, the anterior and lateral nerve-trunks, as com- 

 pared with the longitudinal trunks, have become quite inconspicuous ; 

 and in many cases the same fate is reserved for the brain. 



The fresh-water Triclades come nearest to the Polyclades, but 

 the longitudinal trunks unite posteriorly. The land forms, as repre- 

 sented by Ehjnchodesmus, present us with an arrangement in which the 

 brain is nothing more than a somewhat well-developed portion of the 

 longitudinal trimks with transverse commissures somewhat thicker 

 than in the other parts of the body. The regular arrangement of the 

 peripheral portion is best seen in the marine Triclades, Gunda 

 having longitudinal trunks which, at perfectly regular distances, are 

 connected by simple unbranched commissures, and, so far as seg- 

 ments can be made out at all, there is a transverse commissure for each 

 segment. The homology of this system is fully discussed. 



The mesenchymatous nervous system consists, in the Polyclades, 

 of a fine network of nervous substance which is closely applied to 

 the ventral and dorsal muscular layers; the meshes are generally 

 polygonal, and the system is best developed in the region of the 

 sucker. In the terrestrial Triclades the meshes are generally quad- 

 rangular ; in the Trematoda the system is best developed in connection 

 with the large ventral sucker, and ganglion-cells of considerable size 

 may here and there be detected in it. Among the Cestoda Pleurocotyle 

 has the plexus largely developed near the proboscis. 



No sensory organs, other than eyes, have been detected in the 

 Platyhelminthes ; a large number of these are always to be found in 

 the Polyclades, in the Trematoda they are less numerous, and in the 

 Cestoda they are either absent or are confined to the free-living 

 stages. In most of the fresh-water and terrestrial Triclades two are 

 alone found. In all cases there is presented a marked similarity of 

 structure ; optic cells, formed from the ends of the optic nerve, pig- 

 ment-cups, and a crystalline body can always be made out. 



No complete series of observations have been made by the author 

 on either the Ehabdoccela or the Nemertinea. 



The nervous system of the Triclades, the more general characters 

 of which have already been pointed out, is dealt with in detail ; in 

 treating of the fresh-water forms the author has especially studied 

 Planaria torva, and he finds himself in essential agreement with Graff, 

 Kennel, and the Hertwigs. 



In dealing with the land forms he has the advantage of Moseley's 

 investigations into the land Planarians of Ceylon ; a study which, he 

 says, he has daily learnt to value more and more, though that author 

 has called the nervous the primitive vascular system. This he 

 regards as an error of interpretation which has been corrected by 

 others, though the details have not been essentially altered. 



Gunda has been the chief example of the marine forms, and the 

 author has been able to distinguish in it a motor portion which is 

 formed by two ventral enlargements, from which there arise the anterior 



