Journal of Entomology and Zoology 67 



HOLOTHUROIDEA 



In sea-cucumbers the chief parts of the nervous system are 

 much as in other groups but the superficial and deep radial and 

 circum-oral systems are quite distinct from each other. 



The more superficial system is composed of five strands in 

 an epineural cavity under the longitudinal radial muscles but well 

 in from the surface of the body. The oral ring circles the peris- 

 tome; at the base of the tentacles between its radial branches 

 there are strands, one for each tentacle ; other branches go to the 

 pharynx and intestinal tract. The epineural cavity seems not 

 present in some forms, possibly due to contraction of the animal. 

 The radial nerves end at the anal end of the body but there is no 

 special terminal tentacle. The radial nerves give off branches 

 to the tube-feet and also to the skin ; two nerve plexuses have been 

 recognized, a supei'ficial just under the epithelium and a deeper 

 one in the body-wall. Both of these networks receive some 

 branches from the radial nerve. 



The deeper nerve ring or hyponeural divides into two strands 

 on the inside of each superficial radial nerve according to Hero- 

 uard, '87. This deeper system is chiefly motor while the super- 

 ficial system is sensory, a generalization which he extends to other 

 echinoderms. Branches from the deep system are said to supply 

 muscles of the body-wall and lantern region. 



Among the earlier works dealing with the nervous system of 

 holothurians was that of Krohn, 1841, where the radial nerves 

 were noticed but little detail given. Semon, 1883, and especially 

 Hamann, show the general form and histological structure of the 

 nervous system. Herouard, '87-'89, brings out some points, espe- 

 cially emphasizing the motor and sensory divisions of the nervous 

 system, as already noted. 



Gerould, '96, shows the nervous system in Caudina but little 

 is said about it. Clark in Synapta. 1898, shows the nervous system 

 in section. Red spots at the bases of the tentacles, the so-called 

 eyes, are figured. 



Five radial nerves are recognized and smaller branches to the 

 tentacles. Each radial nerve is divided longitudinally into an 

 outer and inner band as in other forms, but unlike others has no 

 vessel of any kind accompanying the nerves and no spaces or 

 lacunae. Each tentacle nerve sends off branches to the digits so 

 that almost the whole surface of the tentacle becomes sensory. On 

 the bases of the tentacles and in the ectoderm over the body are 

 sense buds or tactile papillae such as described by Hamann, '83. 

 Under each of these is a small ganglion. From the lower side of 

 the circum-oral ring, between every two tentacles, a broad nerve 



