Morphology of Organs. 149 



of the nervous system consists of separated ganglia,^ and is not 

 a hollow structure. 



There is, however, in spite of the important differences just 

 enumerated, this in common between the two types of nervous 

 system, that they both possess a mass of nerve-cells in the 

 anterior end of the body above the mouth. 



Thus the brain of the vertebrate, or at least a portion of it, 

 seems to correspond to the supra-oesophageal ganglia of the 

 invertebrate. That this point in common is important seems 

 to be shown by the following considerations. In the lower 

 worms, the Platyhelminthes, Nemertines, and even in a simple 

 type of Annelid, /€.olosoma, the central nervous system consists 

 of the cerebral (supra-oesophageal) gangUa alone. Here we 

 may possibly have the common starting-point of the vertebrate 

 and invertebrate central nervous system. On this view it will 

 be assumed that out of the nerves arising from the brain of 

 these simple animals two ventral ones, in the case of the inver- 

 tebrate, and two dorsal, in the case of the vertebrate, became 

 respectively approximated, and formed the rest of the central 

 nervous system. This is, of course, hypothesis; but, as a 

 matter of fact, the first stage in proof of such an hypothesis 

 has been discovered in the Nemertine worms ; in some of these 

 there is a dorsal, and in others a ventral, approximation of two 

 especially stout nerves, arising from the brain. There are, 

 however, other views attempting an explanation of the 

 differences between the invertebrate and the vertebrate nervous 

 systems. The matter does not at present admit of dogmatic 

 teaching. 



As to the invertebrate types, it is quite easy to see the 

 likeness in their nervous systems. In Lumbricus there is a 

 ganglionic swelling for every segment of the body, except the 

 first one or two. But it seems as if a slight growing together 

 of ganglia originally perfectly distinct had occurred in the first 

 of the subcesophageal ganglia. The crayfish presents us with 

 a further modification. In the first place, the ganglia are more 



' In the earthworm the distinction between the ganglia and the 

 connectives is not so marked as in the other types. There are some, but 

 not so many, nerve-celjs in the connectives. 



