THE APODOUS HOLOTHURIANS 145 



mentary canal and the body-wall. So far as known, the body-cavity is never in 

 normal communication with the outside, but it is connected through the madre- 

 pore body and stone-canal with the water-vascular system, and the fluid with 

 which it is filled is probably not essentially different from that in the water- 

 vessels. 



CALCAREOUS RING. The calcareous ring in the Molpadiidae is as a rule very 

 well developed and in the great majority of cases is remarkably wide, the radial 

 pieces (except in Eupyrgus (Plate XII, fig. 19), Himasthlephora (Plate XIII, 

 fig. 2) and Gephyrothuria) being provided with conspicuous bifurcate pos- 

 terior prolongations (Plate XII, fig. 3). So far as known, there are always five 

 radial and five interradial pieces which make up the ring, the latter being much 

 the smaller. The supposed exceptions in "Embolus pauper" and "Liosoma 

 arenicola" are obviously based on mutilated or imperfect specimens, or else on 

 hasty or careless examination. There is usually no appreciable asymmetry in 

 the ring, though occasionally the ventral half is somewhat better developed than 

 the dorsal. The interradial pieces are much smaller than the radial, are sym- 

 metrical, slightly concave behind, and have a single median point in front; on 

 each side of this point the plate is a little flattened or hollowed for the reception 

 of a tentacle ampulla or the attachment of the basal part of the tentacle canal. 

 The radial pieces are perfectly symmetrical only in Ceraplectana (Plate XIII, 

 fig. 7), where each has a single anterior median point to which the longitudinal 

 muscle is attached; no tentacles are associated with the radial pieces in this 

 genus. In all the other genera the radial pieces are much broader than the 

 interradial and are provided with two anterior projections, which, however, are 

 not just alike. One is broader than the other and serves for the attachment of 

 the longitudinal muscle (Plate IX, fig. 2) ; it is sometimes perforated for the 

 passage of the radial nerve (?). The lateral radial pieces of both sides are so 

 placed that the attachment of the muscle is on the lower or ventral projection, 

 while a tentacle ampulla or the base of a tentacle canal lies just dorsal to it. 

 The ventral radial piece has the muscle attached sometimes to the right, some- 

 times to the left projection, but the tentacle canal is always between the pro- 

 jections. No cartilaginous ring, posterior to the calcareous one, ever occurs in 

 the Molpadiidae. 



WATER-VASCULAR SYSTEM. The circular canal lies just posterior to the cal- 

 careous ring, with which it is often more or less united by strands of connect- 

 ive tissue. Its lumen is a millimeter, more or less, in diameter, while its wall is 

 exceedingly thin. The wall consists of an epithelial layer of ciliated cells, con- 

 tinuous with that elsewhere in the body-cavity; a layer of connective tissue con- 

 sisting chiefly of fibers running parallel to the direction of the canal and con- 

 taining numerous wandering cells; a thin, structureless membrane; a layer of 



muscle fibers which run around the canal (at right angles to its direction) and 



10 



