52 NERVOUS SYSTEM AND SENSE ORGANS 



by ventral and dorsal commissures passing about the proboscis. 

 The dorsal band is often more slender than the ventral and from it 

 a slender dorsal nerve runs the length of the body. Each lateral 

 brain lobe is often partly divided into a dorsal and ventral lobe. 

 From each lateral ganglion a large nerve trunk passes back and may 

 unite with its fellow of the opposite side just above the anus. 



Nerves are given off from the brain to the eyes when present, 

 and to anterior portions of the body. Two branches come off from 

 the dorsal commissure and run to the proboscis. The so-called 

 vagus nerves arise from the internal borders of the brain not far 

 from the origin of the lateral cords. They are sometimes united 

 by a commissure and then pass down the oesophagus. 



Eyes are usually present along the sides of the head, sometimes 

 a single pair, at other times one or more groups on each side. The 

 eyes in their simplest conditions are mere pigment spots, in others 

 there is a clear area filled with fluid which is supported by strands 

 from cells and held by a limiting membrane. Sensory cells are con- 

 nected with the brain by fibers and with pigment at the outer side. 

 The sensory area seems to be like rods in certain forms. 



In some cases otocysts have been found on the surface of the 

 brain. At the anterior tip of the head groups of cells bear long 

 bristles. In some, these areas are retractile. Taste has been sug- 

 gested as the function of these "frontal" organs. The so-called 

 "side" organs occur as a pair of epithelial patches on each side of 

 the body in the region of the excretory pore. These have an abun- 

 dant nerve supply but their function is unknown. 



In most forms a peculiar pair of organs is found in the head 

 region in close connection wtih the brain. Hubrecht suggests that 

 they may be respiratory. Burger thought that they might be organs 

 used for determining the condition of the water. They may be 

 shallow depressions, longitudinal or slit-like or the slit may be at 

 right angles to the body. In some, ciliated ducts pass inwards and 

 penetrate into special lobes called the cerebral orgmis. 



Thompson, 1908, in Cerebratulus lacteus finds six ventral com- 

 missures from the ventral lobes of the brain. Some of these come 

 from the fibrous core and some come from the cellular sheath of 

 the brain. Other commissures are found beyond the brain. 



Six pairs of "neurocord" cells and one unpaired cell are found 

 in the ventral lobes of the brain. There is probably individual 

 variation as to their number. 



The brain is complex but resembles in its form and commis- 

 sures that of the tubularian worms. 



Coe and Ball, 1920, in Nectonemertes, find both dorsal and 

 ventral commissures well developed. Cerebral and frontal organs 

 are lacking. 



