THE APODOUS HOLOTHUKIANS 47 



21-22) occur in a number of species of several distinct genera, but many species 

 are known which do not have them. They are small cup-shaped outgrowths on 

 the inner face of the tentacles, near the base, just large enough to be seen with 

 the naked eye, or even smaller. The number on each tentacle varies from 1 to 

 30 or even more in large animals. Each cup is lined with cilia and is connected 

 by a small branch nerve with the tentacle nerve. There seems to be little doubt 

 that these cups serve as organs of either taste or smell, although the evidence 

 is not conclusive. Tactile organs, or "touch-papillfe" (Plate VI, fig. 17), occur 

 in many and perhaps all Synaptids, not only in the tentacles, which are them- 

 selves such important organs of touch, but also scattered all over the surface 

 of the body. These "sense-buds" or "touch-papillae" are small groups of epi- 

 thelial cells connected at their inner ends with special ganglia situated at the 

 ends of small nerves, which arise as branches of either the radial or tentacle 

 nerves. 



Nervous system. — The complete nervous system is known in very few 

 Synaptids, but those species in which it is known agree so well that there is 

 little reason to expect anything strikingly different in other species. At the ex- 

 treme anterior end of the body lies the circumoral ring, a band of nerve tissue 

 surrounding the oesophagus and giving rise to the five radial nerves and the 

 tentacle nerves, the number of which corresponds to that of the tentacles. The 

 circumoral ring lies above or within the calcareous ring, close upon the outer 

 wall of the circular sinus in which the cesophagiis lies, and is a tenth of a milli- 

 meter^ more or less, in thickness. The outer layers contain most of the ganglion 

 cells, the interior being chiefly fibrous. In those Synaptids which have eyes 

 the optic nerves arise directly from the circumoral ring, and in some species 

 special nerves or nerve bands arise from the inner or lower side of the ring and 

 run to the oral disc and oesophagus. The radial nerves run outward, over or 

 through the radial pieces of the calcareous ring, and then, bending abruptly 

 baelavard, run with little diminution in size to the extreme posterior end of 

 the body. Each nerve consists of an inner and an outer band, the latter of 

 which contains most of the ganglion cells on its outer side. The two bands lie 

 close together in the connective tissue of the body-wall just outside of the circu- 

 lar muscles. Each radial nerve gives rise to the nerves supplying the posi- 

 tional organs, and to numerous small branches, which supply the muscles and 

 "sense-buds" of the body-wall. In many Synaptids there is an open space or 

 canal, between the radial nerve and the longitudinal muscle, known as the hg- 

 poneural canal; but it is not always present ; it is wanting in Synaptula hydri- 

 formis. The tentacle nerves arise from the outer, lower side of the circumoral 

 ring and run upward on the inner face of the tentacle, giving off branches to the 

 digits and to such sensory cups or papillie as may be present. 



