ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 195 



both the mesenchyma gives rise to muscular, nervous, and connective 

 tissue. The nervous system of the Ctenophora consists of a nervous 

 plexus scattered through the mesenchyma, of an ectodermal plexus 

 with eight fibrous tracts, and of an ectodermal sensory body. In that 

 of the Polyclades we distinguish a nervous plexus closely connected 

 with the mesenchymatous musculature, which in all probability arose 

 in connection with the musculature from the cells of that layer ; a 

 system of nerve-trunks, placed in the mesenchyma, connected by com- 

 missures and anastomoses, and radiating from a single point ; of these 

 eight are specially noticeable. The third portion consists of sensory 

 organs (eyes), with sensory nerves, the prime origin of which appears 

 to have been from the ectoderm. The three parts are here in connec- 

 tion with one another, and this, in addition to such differences as are 

 due to adaptation, appear to be the only important points of distinc- 

 tion between the two groups. 



Starting from the Polyclades, we may note that differentiation has 

 proceeded in two directions — the one associated with the degeneration 

 of the parasitic, the other with the elevation of the free-living forms. 

 The brain is the point at which all the nerve-trunks meet ; it is, there- 

 fore, largest in those forms in which the nerve-trunks are the best 

 developed ; and its size, in the Polyclades, though they are the 

 most primitive of the Platyhelminthes, should not be any cause for 

 astonishment. Among the Trematoda, Tristomum most nearly 

 approaches them in habit and organization ; the brain, however, is 

 more simple. This simplicity is still more marked in Pleurocotyle and 

 Distomum nigroflavum, where the brain is merely a transverse com- 

 missure. In AmpJiilina and those Cestoda in which the scolex is but 

 feebly provided with muscles, the brain is so feebly developed as to be 

 barely distinguishable ; where, however, as in the Tetrarhynchi, the 

 musculature is more abundant, the transverse commissures are corre- 

 spondingly better developed. 



In the Triclades the brain is feeble in the fresh-water forms ; in 

 the terrestrial ones it is impossible to speak of it as a definite central 

 organ. In the marine forms, e. g. Gunda, it is highly developed, and 

 consists of a large posterior, motor, transverse commissure, and a large 

 anterior sensory commissure, the two being connected together by a 

 sensori-motor commissure. After dealing with its position, the 

 author passes to the Peripheral portion. The concentric arrangement 

 of this in the Polyclades has been already referred to. As before, 

 Tristomum presents the closest resemblance to that group, but certain 

 changes have been effected, in consequence of the development of the 

 ventral sucker, and there has been a reduction of the nerves at the 

 anterior end of the body. In Pleurocotyle and Distomum little but the 

 two longitudinal nerves have been preserved, and the commissural 

 system would seem to have completely disappeared. In Amphilina 

 the longitudinal trunks pass into one another ; in the Taeniadse the 

 ' branches for the suckers are still retained ; and the Tetrarhynchi 

 have special paired nerves, which pass to the proboscis ; no commis- 

 sural fibres have as yet been detected in the Cestoda. 



Along the other line of development the central nervous system 



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