398 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



centre are able to diffuse themselves over the whole 

 substance of the jelly-fish. 



We have, it is clear, to consider the nervous system 

 as at first forming a diffused network over the 

 whole body, and this truth must be constantly borne 

 in mind, for it applies not only to the Ccelenterata, 

 but also to the lowest worms. At the same time, 



Fig. 170. Diagrams to show the relative positions of the longitudinal 

 Nerve Cords in different genera of Nemerteans. The epidermal 

 tissues are left white, the muscles are darker, and the nerve cords 

 are darker still. A, Carinella ; B, Cerebr.itulus ; c, Langia ; D, Amphi- 

 porus; E, Drepanophorus. (After Hubrecht.) 



we have to note the tendency of the nerve cells and 

 fibres to seek a more sheltered position than that 

 which can be afforded them by the surface of the 

 body ; nowhere, perhaps, are the various stages of 

 modification better seen than among the TVemertean 

 worms, of which Carinella is one of the lowest and 

 simplest examples. 



It will be seen that in the figure (Fig. 170) Cari- 

 nella (A) has the longitudinal nerve cords just under- 

 lying the epidermal, and placed above the muscular 

 tissues. 



