146 COMPARATIVE ANATOMY CHAP. 



Tetraphyllidce, or at the foremost end of the body, the apical cone, or 

 rostellum of many Twniadce. 



In the TetrarhynckidcB at the foremost end of the body there are 

 4 proboscides furnished with barbed hooks, which can be protruded 

 from special proboscis sheaths and withdrawn again by special muscular 

 retractors. 



VIII. The Nervous System. 



The nervous system in the Platodes is completely detached from 

 the body epithelium. Nearly all its elements lie in or close under the 

 dermal musculature. 



In the Polydada (Fig. 103, p. 141) it consists of a close network 

 of finer or coarser nerves, which is spread in or under the dermal 

 musculature over the whole body, and which, like the musculature, is 

 less developed on the dorsal than on the ventral side. In this plexus 

 specially strong nerves occur, which, converging from all sides and 

 thus growing thicker, unite in a nervous centre, the brain, which lies 

 deep in the parenchyma under the anterior median intestinal branch 

 between the middle and the front of the body. The more elongated 

 the Poly dado, are, the nearer the brain lies to the anterior end, and 

 the more conspicuously do the longitudinal nerve trunks stand out 

 among the nerves converging towards the brain. The most strongly 

 developed of these longitudinal nerves are, in order of importance, 

 2 inner nerves, which run on both sides of the median line, 2 

 lateral and 2 dorsal. From the brain special nerves proceed to the 

 sensory organs. 



The brain in all Polydada (with one single exception) lies in front 

 of the mouth. Only in Oligocladus (Ewoleptidce) does it lie behind the 

 mouth over the beginning of the pharyngeal pouch. The two inner 

 longitudinal trunks in this genus embrace the front end of the pharyngeal 

 pouch, and are connected only behind the same by means of a web of 

 commissures. The brain, the first part of the longitudinal trunks, and 

 the first transverse commissure between these, thus form together a 

 ring which surrounds the anterior portion of the pharyngeal pouch. 



In the Triclada (Fig. 104, p. 142) the brain always lies far 

 forward. The ventral inner longitudinal trunks are always specially 

 strongly developed, and are on the one side connected together by a 

 web of commissures, and on the other give off anastomosing branches 

 outwards. In Gunda both the branches which proceed outwards and 

 the commissures are very simple and regular, and correspond in number 

 with the successive pairs of lateral branches of the intestine. We thus 

 have here a typical ladder nervous system. 



In the RhabdoccelidcB (Fig. 105, p. 142) we generally meet with 

 the nervous system in a simpler form. It consists of the brain, which 

 lies in the anterior portion of the body, and, proceeding from it, of the 

 two ventral longitudinal nerves, and of several smaller nerves which 



