HOLOTHURIOIDEA. 551 



latter varies and may be increased with an increase in the number of the 

 tentacles in Apneumona. The pharynx of some Dendrochirotae is pro- 

 tected by a row of calcareous plates. 



The circumoral nerve-ring not only gives off the radial nerves but 

 other nerves to the tentacles. There appears to be a plexus with ganglion 

 cells in the integument, and the nerves to the tube-feet and tentacles are 

 in some instances connected with sense-cells in the ectoderm. The dorsal 

 processes of Elasipoda are especially rich in nerves, and are probably 

 tactile in function. Auditory vesicles, ten in number, have been found at 

 the origin of the radial nerves from the ring in Synapta. Similar struc- 

 tures occur in the Elpidiidae, and are either confined to the ring, or spread 

 also along the two lateral nerves of the trivium. They inclose numerous 

 otoliths. The five caeca first developed by the water-vascular ring in the 

 larva appear to correspond with the five primary tentacles, and are inter- 

 radial. A second set of five caeca develope into the radial water-vascular 

 vessels. The tentacles increase in number subsequently, and in the adult 

 vary from ten to thirty, but their interradial origin is not evident, and the 

 ring appears to give off five vessels from which the tentacular branches 

 and radial vessels are alike derived. In Thyonidium the ten tentacles are 

 alternately large and small, and in the Dendrochirota the ventral pair are 

 constantly of a smaller size than the remainder. They vary much in shape. 

 In Haplodactyla they are cylindrical, but they may be peltate (Aspido- 

 chirotae), arborescent (Dendrochirotae), and peltate, digitate or pinnate in 

 others. Tentacular suckers are found in some Synaptulae. The tentacles 

 are sometimes retractile, and this is especially the case in Dendrochirotae, 

 in which the pharynx has special retractors. There are ampullae to the 

 tentacles in Aspidochirotae, Molpadia, Chirodota, &c., but never in Elasi- 

 poda or Dendrochirotae. There is generally one Polian vesicle which is 

 ventral, and on the left side in Elasipoda, but in some forms the number 

 may be greatly increased. The radial water-vascular trunks are absent in 

 Apneumona ; present but devoid of feet in Apoda Pneumonophora ; and the 

 same is the case with the two vessels of the bivium in Psolus. The tube 

 feet are either partially or completely retractile, and furnished with a 

 terminal disc which is supported by a calcareous plate except in the 

 majority of Elasipoda, or they are conical papillae without discs. Both 

 kinds may occur in the same animal, e. g. Stichopus, or the first kind alone, 

 e. g. Cucumaria, or the second alone, e. g. many Holothuriae. Among the 

 Elasipoda the median ambulacrum of the trivium has rarely any feet, and 

 the lateral ambulacra carry cylindrical feet, as a rule few and paired from 

 side to side of the ventral surface. The papillae or conical processes are 

 usually dorsal. They are well developed in most Elasipoda, and are paired 

 from side to side. In the same group there is often a long dorsal appen- 

 dage with a broad base, which extends across the median dorsal inter- 



