THE APODOUS HOLOTHUEIANS 147 



and a few of these may accompany the vessel nearly to the circular canal, in 

 which region they would of course lie on the inner or axial side. The tentacle 

 canals are not peculiar histologically (see p. 137), but are provided with valves 

 consisting of connective tissue and radial muscle fibers covered with epithelial 

 cells on both sides. These valves are situated with the concave side toward the 

 tentacle, and thus prevent the passage of fluid from the tentacle canals. Gerould 

 ('96, p. 47) says the valve is in the "radial canal near its junction with a ten- 

 tacular vessel," which might mean that there is one valve or that there are 

 three valves in each radial canal; but in his explanation of Plate VI, fig. 77, he 

 says the section is through "a tentacular canal * * * showing the valve of 

 the tentacle," which indicates that there are 15 of these valves, as we would ex- 

 pect, placed one in each tentacle canal. In most Molpadids each tentacle canal 

 not only runs forward with the tentacle, but extends backward on to the outside 

 of the calcareous ring to which it is closely attached (see Plates X, fig. 1; XI, 

 fig. 3). Commonly this backward extension is prolonged considerably past the 

 posterior margin of the ring and hangs free in the body-cavity as a " tentacle- 

 ampulla." These ampuUne are not histologically peculiar, save that the longi- 

 tudinal muscles of the tentacles extend backward only into the outer anterior 

 wall. The occurrence of well-marked ambulacral appendages is confined to the 

 genera Himasthlephora (Plate XIIT, fig. 1) and Gephyrothuria, where each of the 

 two dorsal radial vessels gives rise to a very few (2-5) ambulacral canals, which 

 are provided with ampulhe (Plate XIII, figs. 3, 4) and connect with the remark- 

 able lash-like papillfe characteristic of those two genera. In Gephyrothuria and 

 all other Molpadids the radial canals run backward to the cloacal opening, 

 where they terminate in "anal papillfe." These papillae are often very insignifi- 

 cant, but may be very distinct ; their walls generally contain an umisual num- 

 ber of calcareous bodies, but may be wholly free from them. Their presence or 

 apparent absence in preserved specimens is dependent not only on the amount 

 of contraction, but on individual diversity and possibly to some extent on age. 

 Although commonly single, the anal papill;F may be accompanied by two, or 

 even more, somewhat smaller lateral ambulacral vessels, so that there is a 

 small group of papilli^ at the end of each radius, and even when the anal papilla? 

 are very small and apparently single, a pair of lateral vessels may be present be- 

 neath the skin, as Gerould has shown to be the case in Caudina arenata. In his- 

 tological structure these vessels and the anal papillae are similar to the pedicels 

 of other holothurians ; the wall consists of a thin, connective-tissue membrane, 

 a layer of longitudinal muscles and a lining epithelium; the anal papillfe have 

 an outside layer of connective tissue containing blood sinuses and often cal- 

 careous liodies and are covered with the ordinary pody-wall epithelium. In 

 Himasthlephora the group of pedicel-like outgrowths are at the base of the 

 caudal appendage, and rudimentary pedicel-like bodies also occur at the an- 



