ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 33 



aaother by partitionB developed from the sheatli of the nerve. The 

 fibres themselves are made up of fibrils which are inclosed in a 

 slightly refractive and feebly granular substance. The myenteric 

 plexus forms, along the digestive tube, a triple plexus, on the branches 

 of which ganglionic cells are irregularly scattered. The centre of 

 the ganglia is formed by a fibrillar substance and a slightly refractive 

 body, which is of the same nature as the peripheral matter of the 

 cells ; the central fibrils have no definite arrangement ; the nerves 

 arise from the centre of them. The envelope of the nervous system 

 is formed by a lamellar connective tissue, which is composed of fine 

 fibrils. Among the cells of the ganglia a peculiar kind of connective 

 cell was observed ; this was oval, and contained a large nucleus ; from 

 the two poles of the cell long fibrils are given off. 



In the Hirudinea all the ganglionic nerve-cells are unipolar ; those 

 of the gastro-intestinal system have the same essential structure 

 but are not invested in a proper membrane, the sheath that invests 

 them being part of that system which has been compared by Eanvier 

 to Henle's sheath in vertebrates. The fibres that make up the nerves 

 vary in size, and are separated from one another by thick partitions, 

 and are composed of fibrils inclosed in a slightly granular protoplasm. 

 The sympathetic system forms a double plexus along the digestive 

 tube, and on its branches are developed ganglionic cells. The con- 

 nective chain is formed by three nervous cylinders ; no nuclei are to 

 be seen either in the protoplasm of the connectives or of the nerves. 

 No multipolar nerve-cells are to be found in the centre of the ganglia, 

 as Walter and Hermann have imagined. The investment of the 

 nervous system is a continuous sheath which is only open near the 

 ends of the nerves. 



The last group dealt with is that of the Oligoch^ta, and in it we 

 find that the nerve-cells of the cerebral and ventral ganglia are mostly 

 unipolar, and are formed of a viscous slightly granular substance. 

 Near the homogeneous nucleus fatty granulations are to be found. 

 Bipolar and multipolar cells are also to be observed, but they do not 

 occupy any definite position. The nerve-fibres form the columns of 

 the chain, have no proper walls, but are simply bounded by the par- 

 titions of connective tissue ; these tubes are formed of a viscous and 

 almost homogeneous substance, which is only feebly coloured by 

 osmic acid ; these fibres anastomose with one another. 



The giant nerve-tubes are three in number, and extend along 

 almost the whole length of the chain. The central, which is the 

 largest, commences at the middle of the first ganglion, and the other 

 two at the second ; they end at the terminal ganglia. They appear 

 to have no relation to the nerve-fibres. 



The nerves have the same structures as the fibres of the columns. 



The whole system (with the exception of the cerebral ganglia) is 

 completely invested in three sheaths — epithelial, muscular, and struc- 

 tureless (of a cuticular character) ; the first and third are alone 

 formed on the cerebral ganglia. 



All the ventral ganglia give off three nerves on either side. The 

 first is very sharply distinguished into two halves. 



Ser. 2.— Vol. IV. D 



