50 bulletin: museum of comparative zoology. 



Three or four more or less distinct varieties of nerve cells can be 

 distinguished. Burger ('95), Montgomery ('9V''), and others have 

 described these different kinds of nerve cells in great detail. They 

 are : (a) unipolar, naked cells of small size, with deeply staining 

 oval nuclei ; (b) slender, pear-shaped, somewhat larger cells, often 

 arranged radially about the fibrous core ; (c) miich larger, flask- 

 shaped or retort-shaped cells in which the cell body stains almost 

 as deeply as the nucleus, and which are commonly situated more 

 peripherally than the clusters of the other two varieties ; each of 

 the cells of this last variety is enclosed in a delicate sheath of con- 

 nective tissue. All the nerve cells, but especially those of the first 

 two varieties are arranged in groups or masses, and their delicate 

 processes unite in bundles before entering the fibrous core. 



In addition to these three types of cells a fourth variety is found 

 in certain heteronemerteans and in a few hoplonemerteans. These 

 are very much larger than any of the other types of nerve cells and 

 their processes are so very large that they resemble the neurochords 

 of annelids. There are often one to three pairs of these neuro- 

 chord cells in the ventral ganglia, and many more pairs in the 

 lateral nerves. In such cases the processes of all usually unite 

 into a single l)undle. 



In some nemerteans a few delicate muscular fibers pass longitudi- 

 nally in the substance of the lateral nerves between the fibrous core 

 and the outer sheath of connective tissue. 



Peripheral nervous system. — In addition to the brain and pair 

 of lateral nerves nearly all species of nemerteans are provided with 

 dorsal, esophageal, jyrohosddial, and cej^halic nerves, together with 

 nerves to cerebral sense organs and numerous branches fi"om the 

 lateral nerves to adjacent muscles, sense organs of integument, etc. 

 Many paleonemerteans have also a ventral nerve, corresponding in 

 position to that in the dorso-median line, but situated ventrally. 



Nervous layers in, body vKdls. — All except the Ho])lonemertea 

 have one, or sometimes two, nervous layers in the body walls, formed 

 by anastomosing fibers from the lateral nerves. In the Iletero- 

 nemertea this plexus lies between the circular and outer longitudinal 

 muscular layers, and consists of a cylindrical nervous plexus extend- 

 ing nearly the whole length of the body. A secondary nervous 

 plexus may lie immediately internal to the circular muscles. This is 

 formed of branches passing through the circular muscles from the 

 outer nervous laver. 



