THE HOLOTHURIOIDEA 235 



veloped in some species of Synapta and Chiridota. Very remarkable are 

 the funnels or ciliated urns (Fig. V. 4, c.u) situated on the mesentery, 

 and sometimes also on the inner surface of the body-wall. These cup- 

 shaped organs, the function of which is obscure, are attached by stalks to 

 the epithelium, and may be joined together into large bunches (Chiridota). 

 The genera Synapta, Anapta, Chiridota, and Trochodota are herma- 

 phrodite. The genital pore is situated behind the tentacles (Fig. V. 2). 



The absence of radial water- vascular canals, and of interruptions in the 

 circular musculature, as well as the presence of ciliated urns, distinguish 

 the Synaptidae from all the preceding familiea The first of these 

 characters has no doubt been secondarily acquired, since the canals are 

 present in the larva; correlated with this reduction is the method of 

 progression by means of the tentacles and of contractions of the body- 

 wall, accompanied by the entire disappearance of podia. On the other 

 hand, the shape of the tentacles, the spicules, the presence of retractors, 

 indicate a distant relationship to the Molpadiidae. 



Phylogeny of the Holothurioidea. It has been shown above 

 that the Actinopod Holothurians fall into two groups. In the first, 

 containing the Holothuriidae, Elpidiidae, and Pelagothuriidae, the 

 tentacles are more or less peltate ; the calcareous ring is radially 

 symmetrical and of simple structure, it may be reduced and even 

 absent; the stone-canal often retains its primitive opening to the 

 exterior ; the genital tubes are sometimes restricted to the left side ; 

 there are never retractor muscles. In the second group, containing 

 the Cucumariidae and Molpadiidae, the tentacles are simple or 

 branched, never peltate ; the calcareous ring is much developed, 

 with posterior prolongations, and often bilaterally symmetrical ; the 

 stone-canal always opens internally ; there are always right and left 

 tufts of genital tubes ; retractors are generally developed. The 

 two groups, then, probably represent two diverging stems, arising 

 from a common ancestor possessed Of respiratory trees. The Syn- 

 aptidae would appear to have come off far down the Cucumarian 

 stem, perhaps in common with the Molpadiidae. These relation- 

 ships may be expressed in the following diagram : 



Pelagothuriidae. 



Elpidiidae. 



olothuriidae. 



Holothurioidean. 



ancestor. . _ 



.Cucumariidae. 



Molpadiidae. 

 Synaptidae. 



