VI. Nemertinea 



The first work of any importance which deals with the nervous 

 system of these worms is that of De Quatrefages in 1846. He de- 

 scribes the central nervous system as composed of two distinct 

 lateral lobes united below and above by commissures. From the 

 lateral lobes two more or less isolated longitudinal bands extend 

 themselves towards the posterior end of the animal. So far as the 

 figures are concerned this early work is even more detailed than 

 that of M'Intosh in 1873. The more recent information about this 

 interesting group has been furnished especially by Hubrecht in 

 numerous papers from 1875 to 1887. Although the cellular details 

 are not shown, the relative position of the central fibrous core is 

 given in relation to the surrounding nerve cells. He also clearly 

 distinguishes the dorsal median nerve springing from the slender 

 dorsal commissure. The dorsal and ventral lobes of the brain are 

 shown more clearly than in earlier writings. In Eupnlia a dorsal, 

 middle and ventral lobe are shown. 



Hubrecht in his two paoers of 1887 suggests the nemertineans 

 as a group of animals valuable in tracing the relationship of the 

 vertebrates and invertebrates. He bases his hypothesis largely 

 upon the arrangement of the parts of the nervous system. In the 

 group there is some variation in the extent and position of the 

 lateral nerve cords and in some, the mouth opens behind the brain 

 and in some in front of the brain. Such facts as these give sug- 

 gestions of an intermediate condition between annelids and arthro- 

 pods on the one side and vertebrates on the other. Other writers 

 have compared the large lateral nerves of nemertineans with the 

 central nerve cords of some round worms. 



Burger in a number of works from 1883 to 1895, has made a 

 considerable studv of the nervous system by various methods. He 

 has also studied the histological details of the nervous .system. His 

 oapers are the mo.st comorehensive and imoortant in this field. 

 Burger describes the nerve cells as all uninolar and uninclosed in 

 snecial membranes. He classifies nerve cells as follows: (1) The 

 smallest cells sensory in nature; (2) medium sized cells; (3) large 

 cells: (4) very large cells, the so-called "Neurocorde" cells. 



Montgomery, 1897, agrees with Biirger in many respects, such 

 as uninolar condition of the nerve fibers, but these are coinposed 

 of "a homogeneous unstaining axis cylinder which is probably fluid 

 and a fine spongioolasmic layer." 



In Cei-ebratulns, the large nerve fibers diff'er from the others in 

 size. They do not give oflF collaterals but divide dichotomously and 

 are arranged segmentally. The largest ganglion cells are present 

 in three pairs in the ventral brain lobes and are distributed irreg- 

 ularly along the lateral cords, but are absent in both ends. In the 



