148 COMPARATIVE ANATOMY CHAP. 



and are connected by means of a chief commissure (brain commissure) 

 at the foremost end in the scolex. There are generally several more 

 besides, often 8 longitudinal nerves, which, however, do not stretch 

 backwards beyond the scolex, and further other nerves, which proceed 

 to the suckers, the hooks, and in the Tetmrhynchidcc to the 4 muscular 

 proboscides, etc. Apart from the brain commissure, all these nerves 

 in the scolex are connected together in a complicated way by circular 

 square, polygonal, or cross-shaped commissures. In the segments 

 (proglottides) no commissures between the longitudinal trunks have yet 

 been observed. 



The brain (cerebral ganglion, cerebral commissure) of the Platodes is not to be 

 considered exclusively as a central nervous system, since more or less numerous 

 ganglion cells exist in the larger nerve trunks also, at those points where the nerves 

 and commissures branch off. Thus the ganglion cells in many Triclada are found 

 in the ventral longitudinal trunks of the ladder nervous system, principally at the 

 points of departure of the transverse commissures. We are thus led to conjecture 

 that the double ganglia of the ventral ganglionic chain of the Vermes have proceeded 

 from these points in a ladder nervous system. 



The nervous system of the Platodes is in our opinion of the greatest significance 

 from the point of view of comparative anatomy, because it joins on to that of the 

 Gnidaria on one hand, and on the other is, in many groups, modified in a direc- 

 tion which points to the nervous system of the Vermes, and also perhaps of the 

 lowest Molluscs. If we consider the matter without prejudice the following seems 

 characteristic of the nervous system of the Turbellaria and Trcmatoda. (1) The 

 arrangement of the nervous system in the form of a nerve plexus all over under the 

 skin, dorsally as well as veutrally, in close connection with the musculature which 

 has to be innervated. In this condition we recognise a great similarity to the higher 

 Cnidaria. (2) The development of a central organ (brain). We have seen in the 

 Cnidaria that nerve centres are formed in close connection with the sensory 

 organs, which is comprehensible a priori, since it is in the nerve centres that the 

 junction of the sensory nerves with the motor nerves or nerve fibres takes 

 place. Now the finer structure of the brain of the Platodes shows, most unmistak- 

 ably, that the brain is nothing more than a specially developed part of the nerve 

 plexus, in which motor and sensory nerves unite. The sensory organs become, in 

 connection with the development of bilateral symmetry, more and more localised in 

 the anterior end of the body, i.e. in the end which goes first in creeping ; and there- 

 fore the centralised nerve plexus (or in other words the brain), in which the motor 

 and the sensory fibres unite, must, following the sensory organs, shift more and more 

 towards the anterior end of the body. 



In Accela we find 2 cerebral centres ; in Cestoda the many often very complicated 

 commissures between the nerves in the scolex must collectively be considered 

 as the brain ; in certain land Triclada the only part which can be called the brain 

 is a not very sharply demarcated tract of the longitudinal trunks at the anterior end 

 of the body, in which the transverse commissures lie specially crowded together, and 

 into which also the sensory nerves enter. 



Bearing in mind the fact that in the Ctcnophora, in Cceloplana, and in Ctcnoplana 

 the sensory body is developed in the middle of the dorsal surface, we must consider 

 this as the original position of the brain. As a necessary result of the development 

 of this central portion, the peripheral nerves, the nearer they approach to this part, 

 unite into increasingly massive trunks, which finally enter the brain. In consequence 

 of the position of the brain at the anterior end of the body, the nerves proceeding 



