28 BULLETIN OF THE 



anterior longitudinal trunks. I could find no trace of a similar plexus 

 in connection with the less developed muscles of the dorsal side. 



The nervous system of planarians may be readily understood, it 

 seems to me, if we regard it as composed of two more or less distinct 

 parts, — a deep-seated and a more superficial portion. The deep-seated 

 and more central part is present in all planarians hitherto investigated, 

 and consists of the brain, longitudinal nerve trunks, their commissures, 

 and the lateral nerves given off from them. The superficial portion 

 consists of a nerve plexus which lies just underneath the longitudinal 

 muscles, and may be confined to one or the other of the two surfaces, or 

 may be wholly wanting. A conspicuous part of this superficial system, 

 whenever it exists, is the marginal nerve. The connection between the 

 deep and superficial portions of the nervous system is effected by means 

 of vertical nerves running between the two, and, as I have found in 

 Phagocata, the marginal nerve also serves in an indirect way the same 

 purpose ; for on the one hand it is connected with the lateral nerves of 

 the central system, and on the other it forms the marginal terminus 

 of the superficial system. 



Lang ('81, p. 72) has described in Gunda a marginal nerve directly 

 connected with the lateral nerves given off from the main trunks, but 

 has been unable to find any other evidence of a plexus. In Rhyncho- 

 desmus, according to the same author ('81, p. 62), there are both dorsal 

 and ventral plexuses, which are in contact with the deep surfaces of the 

 longitudinal muscles, and are connected with the central system by 

 vertical branches from the main trunks, from the lateral nerves, and 

 from the transverse commissures, but there is no peripheral nerve. Lang 

 ('81, p. 57) also finds a plexus in connection with the deeper longitu- 

 dinal muscles in Planaria torva. Iijima ('84, p. 426) has likewise found a 

 dorsal plexus in a similar position in PI. polychroa and in D. lacteum, and 

 Loman ('87, p. 76) has found the same conditions in Eipalium suma- 

 trense and B. javanum. In Gunda ulvae and PI. abscissa there exists, 

 according to Iijima ('87, p. 349), a second, dorsal pair of longitudinal 

 stems, giving off" branches that break up into a plexus and unite with 

 the plexus from the lateral branches of the main trunks, the whole form- 

 ing a " Nervenschlauch." He says that, a "Randnerv" is present, but 

 he does not state what are its relations to the plexuses 



From this brief survey it is obvious that Gunda represents one 

 extreme, and Rhynchodesmus the other; since in the former there are 

 no superficial plexuses, and in the latter there is a superficial plexus on 

 both dorsal and ventral surfaces in addition to the parts found in Gunda, 



