SYNAPTA VIVIPARA. 77 



The nervous system consists as in all Synaptidae of the central circumoral ring with the 

 five radial branches and the smaller branches to each of the tentacles, but there are some 

 additional nerves and certain of the sense-organs which have not been figured hitherto. 

 Each of the radial nerves is divided longitudinally into an outer and an inner band as in 

 other Synaptas, but, unlike them, there are no canals or vessels of any kind accompanying 

 the nerves. There are, therefore, in the radii of S. vivipara, no spaces or lacunae in 

 connection with either the blood, water, or nervous systems, but they are marked simply 

 by the longitudinal muscles and nerves (Fig. 99). Each tentacle-nerve sends off branches 

 to the digits (Fig. 55), so that almost the whole surface of the tentacle becomes sensory. 

 On the base of the tentacles and in various parts of the ectoderm all over the body, there 

 are numerous sense-buds or " taste-papillae," (Fig. 84), such as were first described by 

 Hamann ('83). The structure of these organs has been well described by him and still 

 more recently by Cuenot ('91). My observations support the opinion of the latter, that 

 under each one of these sense papillae there lies a small ganglion. From the lower side 

 of the circumoral ring, there arises between every two tentacles a broad band-like nerve 

 (Figs. 75 and 76) which runs inwards towards the mouth, innervating the ectoderm of the 

 oral disc as well as the muscles of the oesophagus. Hamann ('83) describes a single 

 nerve to the oesophagus, and Semon ('88) speaks of it in S. digitata, but so far as I can 

 learn no other nerves from the inner side of the ring have been described in holothurians. 

 At the base of each tentacle, there are easily seen a pair of reddish brown spots, the so- 

 called eyes (Figs. 77 and 78). Similar spots are mentioned in various Synaptas by 

 Miiller ('50), Baur ('64), and Semper ('68), but Semon ('87) and Hamann ('84) seem 

 to doubt their visual function. There can be little doubt, however, that in Synapta 

 vivipara these eyes are actually of service as light-detecting organs. In position and 

 general structure they resemble those described by Ludwig and Barthels ('91) for Synapta 

 oittata. They consist of a distinct, rather horny mesodermal layer, of a light brown 

 color, containing scattered nuclei, overlying the swollen end of a large nerve which arises 

 on each side from the base of the tentacle-nerve (Fig. 77). The ends of these nerves 

 are made up of" large nerve-cells with large nuclei, which are somewhat swollen and 

 apparently vacuolated at their outer extremities. They are polygonal in outline, when 

 seen in cross-section (Fig. 79), and the inner ends taper off into fibers which run out into 

 the nerve (Fig. 80) . The mesodermal covering, which also has the appearance of being 

 vacuolated, is clearly a continuation of the thin mesoderm layer which surrounds all the 

 nerves. The eyes are about 60^ in diameter, the mesodermal covering being six or eight 

 mikrons thick. That this covering may be affected by light is probable, for its color is 

 due to the pigment it contains. The other noteworthy sense-organs are the otocysts 



