136 COMPARATIVE ANATOMY CHAP. 



the sexual organs, the musculature, and the nervous system. The 

 musculature is arranged under the body epithelium in a muscle layer 

 whose elements have transverse, longitudinal, and diagonal courses. 

 Dorso-ventral muscle fibres stretch between the ventral and dorsal 

 surfaces. The whole arrangement of the musculature seems adapted 

 for the creeping motion. The nervous system forms a plexus of nerves 

 lying in or on the muscle layer, which is more strongly developed 

 on the ventral than on the dorsal side. In this plexus single stronger 

 nerve trunks are more clearly developed, and they meet together from 

 various directions in one central point, the brain. In very many 

 Platodes this nerve plexus forms the so-called ladder nervous system, 

 in which we distinguish the following parts : (1) the brain, lying at 

 the anterior end of the body ; (2) the two principal longitudinal trunks 

 proceeding out of it and running on the ventral side ; (3) transverse 

 commissures which connect these latter. 



The so-called water-vascular system is very characteristic of the 

 Platodes. It is a system of fine canals, on the one hand ramifying in 

 the mesoderm, and on the other emerging"externally, which has certainly 

 an excretory, and perhaps at the same time a respiratory purpose. In 

 the Cnidaria no such system has been observed. 



The Platodes are hermaphrodite. Besides the sexual reproduction 

 by means of fertilised eggs, there is also (in Trematoda) parthenogenetic 

 reproduction and (in Turbellaria and Cestoda) asexual multiplication 

 by fission or gemmation. 



For the comprehension of the relation of the Platodes to the Cnidaria, the know- 

 ledge of two animal forms, which have been considered to be intermediate forms between 

 the Ctenophora and the Turbellaria (Polyclada), is necessary. Only one specimen 

 of each has till now been described. One of these forms is Cceloplatia Mecznikowi, 

 the other Ctenoplana Kowalevskii. Unfortunately our knowledge, especially of the 

 first form, is very insufficient. Their sexual organs and their development are 

 unknown, so that we cannot be sure whether we have to do with young stages or 

 with adult animals. But in any case both forms are of the greatest interest. 



Cceloplana is a little animal about J of an inch long and broad, whose 

 appearance quite coincides with that of a Polydad. The body is flatly compressed 

 and ciliated all over ; it creeps on the ventral surface. In the centre of the dorsal 

 surface lies a vesicle with a mass of otoliths. Near it on each side, right and left, is 

 a long tentacle feathered on one side, which can be withdrawn into a special sheath. 

 In the middle of the ventral side lies the mouth. The gastro-canal system consists 

 of the quadruply-lobed stomach and numerous anastomosing canals radiating from 

 it. From the stomach 2 canals rise towards the dorsal surface of the body, where 

 they apparently end blindly in front of and behind the otolith vesicle. 



Ctenoplana has in general the same body form as Cceloplana ; but besides the 

 general ciliation this animal also has on the dorsal surface eight short rows of stiff 

 plates arranged like a rosette ; these correspond with the ciliated or rowing plateis of 

 the Ctenophora, and lie in special grooves, out of which they can be protruded. The 

 arrangement of the gastro-canal apparatus is like that in the Cceloplana. In the 

 middle of the dorsal surface occurs a formation similar to the sensory body of the 

 Ctenophora. At the base of the depression containing the otoliths there is on each 

 side a nerve centre with nerves proceeding from it, and near these on each side a 



