124 



WESLEY R. COE. 



a form which was taken from the bottom at a depth of 833 meters, 

 which is in some respects intermediate between the Pelagica and 

 the Reptantia. 



Brain. In Fig. I a general plan of the nervous system is shown. 

 The brain of the pelagic forms is essentially similar to that of the 

 littoral species, but as a rule the dorsal ganglia are considerably 

 smaller than the ventral. The two ganglia of each side are 

 closely fused together. The fibrous cores and the three types of 

 ganglion cells are also similar, but neurochord cells have not been 

 found in any species. 



dc 



FIG. i. Diagram of nervous system of Neuronemertes aurantiaca Coe, 

 showing relation of brain and lateral nerves (/) to peripheral nervous 

 system ; dg, dorsal, and vg, ventral ganglia of brain ; dc and vc, the corre- 

 sponding commissures; n, cephalic nerves; g, gastric nerve; pn, proboscidial 

 nerve; dn, dorsal nerve, with metameric ganglia (<?/) ; din, dorsolateral 

 nerve ; dp, Ip, vp, dorsal, lateral and ventral peripheral nerves, respectively ; 

 PC, posterior commissure ; en, caudal nerves. 



One of the most striking peculiarities of the pelagic forms is the 

 great amount of gelatinous tissue, or parenchyma, which sur- 

 rounds all the internal organs, giving the body a high degree of 

 permeability and a low specific gravity, enabling the worms to 

 float freely in the water at great depths with a minimum of mus- 

 cular exertion. This parenchyma surrounds the brain and us- 

 ually separates it widely from the thin musculature of the head. 



The probosis passes through the ring made by the ganglia and 

 their dorsal and ventral commissures, while the stomach lies 

 beneath the ventral commissure in most species. 



Three longitudinal nerve stems extend the entire length of the 

 body ; the pair of lateral nerves, situated in the body parenchyma 

 beneath the intestinal diverticula, and the median dorsal nerve, 

 which lies directly beneath the basement layer of the epidermis. 

 The former are direct continuations of the ventral brain lobes, 

 while the dorsal nerve has no direct connection with the brain 



