54 THE APODOUS HOLOTHIIRIANS 



condition may occur. The number of iuterradial pieces ranges from five up- 

 ward, but the most frequent number is seven, the two dorsal interradii each 

 having an extra piece. Where the number of interradial pieces is more than 

 seven, there is usually an agreement between the number of tentacles and the 

 total number of plates in the ring, but several species have been described 

 in which there is no such agreement. It should be borne in mind that new 

 pieces may form in the ring with the growth of the animal, and that such 

 unusual numbers as 16 and 18 plates very possibly are an indication of im- 

 maturity. There will probably prove to be a reasonably definite correlation be- 

 tween the number of tentacles and the number of pieces in the calcareous ring, 

 when all the facts are known. 



Cabtilaginous ring. — In the genera Synapta, Polyplectana and Synaptula 

 and sometimes in Opheodesoma, there is a noticeable ring of cartilage or some 

 similar connective tissue immediately iJosterior to the calcareous ring (Plate VI, 

 fig. 18). In bulk it may exceed the latter, with which it is in more or less close 

 contact. This ring is often without projections or perforations, but in some cases 

 it is provided with 15 openings, either anteriorly or posteriorly. The presence or 

 absence of this ring has been used as a generic character, but as its develop- 

 ment is apparently more or less variable, it is still a doubtful question as to how 

 great a taxonomic value it has. Its function has never been determined, but 

 it probably gives a more or less useful, though flexible, support to the calca- 

 reous ring. 



Water-vascitlar system. — Encircling the oesophagus at a greater or less 

 distance behind the calcareous ring or the cartilaginous ring, when the latter 

 is present, is a tube, which forms the central part of the water-vascular system 

 and is known as the circular ring. It is from a fifth to a half of a millimeter 

 in diameter, more or less, according to the size of the individual, and the wall 

 is a thin layer of connective tissue and circular muscles, covei'ed on the out- 

 side with the body-cavity epithelium and lined with a very similar layer of 

 cells. From this circular ring arise the tentacle canals, which run, one to 

 each tentacle, passing upward, on the inner side of the calcareous ring. Just 

 before entering the tentacle, each canal is provided with a valve which per- 

 mits fluid to pass into the tentacle, but prevents its outflow. The number of 

 tentacle canals may be fewer than the mnn1)er of tentacles, but such cases are 

 due to immaturity or arrested development. In many, if not all, Synaptids, 

 the anterior margin of the calcareous ring i)rojects forward so far (Plate VI, 

 fig. 18) that the outer half of the lower part of the cavity of each tentacle 

 forms a blind sac or rudimentary ampulla, separated by the calcareous ring 

 from the inner half, which is directly continuous with the tentacle canal. The 

 sacs are always very short and closely appressed to the outer side of the cal- 

 careous ring. Dependent from the circular canal are the polian vessels (Plate 



