Notes on the Central Nervous Systems 

 of Holothurians 



WILLIAM A. HILTON 



A number of attempts were made to stain the central nervous system of Stichopus 

 with methylene blue, but without success. Dissections were most successfully made 

 when the animals were fixed in hot mercuric chloride. Specimens injected were filled 

 with the fluid and the whole body was later immersed. Specimens with the body 

 distended were obtained in this way and the nerve trunks were easily followed 

 after the removal of the longitudinal muscle bands. Parts of the nerve trunks were 

 removed and sectioned. 



Specimens of Leptosynapta were imbedded and sectioned whole and a few young 

 of Stichopus were sectioned whole. 



Very little difference was noted between radial and circumoral nerves at different 

 places. The general character of the nervous system was very much as found in other 

 echinoderms. The deep nerve bands were not well marked in many of the sections, but 

 groups of cells more or less joined with the superficial trunks were found without 

 difficulty. 



In adult Stichopus the cells are fairly numerous out in the fibrous area of the 

 superficial band. The radial nerves show a bilateral division as there is often a 

 depression on the inner side which approaches the appearance of a cavity. 



In my preparations the cell body is not very distinct from the nucleus. There 

 may be six or more layers of cells at the edge of the outer band. The nerve processes 

 consist of many fine strands running in through the fibrous area, but there seems to 

 be in addition to this a groundwork of material which may in large part be made 

 of fibers and fibrils whose identity could not be discovered. 



In the younger specimens of Stichopus the fibers were more prominent and the 

 cells in the fibrous area were markedly fewer in numbers. The fibers for the most 

 part seem to run the long way of the nerve trunk. Especially were long strands 

 found in the circumoral ring, although fibers could be seen crossing in considerable 

 numbers as well. In the radial nerves the crossing and interweaving of the nerve 

 fibers as they show in cross section is very intricate. 



Small nerve trunks were followed along the tubes of the water vascular system. 

 These were seen to have a similar but simpler structure than the radial nerves. 



In Leptosynapta a very similar sort of nervous system was found, but the nerve 

 cells were larger and on the whole fewer in number. The deep nerve trunks were 

 but poorly represented. 



The fibers from the basal cells show very clearly both in the superficial and the 

 deep nerves and the fibers cross in a complex manner at all levels. There are but few 

 cells scattered in the fibrous area and there is apparently but little ground substance. 

 The fibers stand out more clearly than in Stichopus. Possibly more fibers from the 

 outer margin cross near the inner. This may be due to the branching at the ends of 

 the fibers. 



Fibers and fibrils are all about the same size in Stichopus and Leptosynapta. The 

 circumoral nerve ring seems thicker than the radial, but this may in part be due to 

 contraction. The fibers in the circumoral ring seem smaller than in the radial. In 

 all parts there are many crossings of fibers in the fibrous portions at all angles. 



