174 THE SEA SHOEE 



of a nerve cord, and distributing nerves in pairs to various parts of 

 the body ; and it may be well to note here one very important 

 point of distinction between the general arrangement of the central 

 portion of the nervous system in the worms and higher invertebrates, 

 as compared with that of the corresponding structure in the verte- 

 brates : In the former the main axis of the system, consisting, as 

 we have seen, of a chain of ganglia connected by a nerve cord, is 

 invariably placed along the ventral portion of the body-cavity the 

 surface on which the animal crawls ; while in the vertebrates the 

 axis of the nervous system lies along the upper or dorsal part of the 

 body ; and, instead of lying in the general body-cavity, in company 

 with the organs of digestion and circulation, is enclosed in the bony 

 canal formed by the vertebral column. It will be seen from this 

 that when it is desired to examine the nervous system of the in- 

 vertebrate animal, the body-wall should be opened along the middle 

 of the ventral surface, while, in the vertebrate, the central axis 

 should be exposed from above. 



Many of the vermes are parasitic, either attaching themselves 

 to the exterior of other animals, and deriving nourishment by suck- 

 ing their blood, or they are internal parasites, living in the digestive 

 canal of their hosts and partaking of the digested food with which 

 they are almost perpetually surrounded, or burrowing into the 

 tissues and imbibing the nutritive fluids which they contain ; and it 

 is interesting to study even these degraded members of the group, 

 if only to observe how then* physical organisation degenerates in 

 accordance with their depraved mode of living. In them we find 

 no digestive system with the exception of the simplest sac from 

 which the fluids they swallow may be absorbed, for their food is 

 taken in a condition ready for direct assimilation ; and the food so 

 obtained being readily absorbed into all parts of their soft bodies, 

 and being sufficiently charged with oxygen gas by the respiration 

 of their hosts, they require no special organs for circulation or 

 respiration, nor, indeed, do we find any. Further, we find that the 

 nervous system is often undeveloped ; for since the parasites, and 

 especially the internal ones, are so plentifully surrounded with all 

 the necessaries of existence, their bodies are so simple in construc- 

 tion that no complex nervous system is required to promote or 

 control either locomotion or internal functions. Even the general 

 body-cavity often disappears in these degraded creatures, the 

 internal organisation being of such a low type that there is no 

 necessity for it ; and all the abundant nourishment absorbed over 



