HOLOTHUROIDEA. 269 



Actinocucumis Ludwig ; Colochirus Troschel, feet in rows on ventral 

 surface ; Psolidium Ludwig ; Theelia Ludwig ; Psolus Oken with 10 

 tentacles, a ventral creeping sole, dorsal surface arched without ambula- 

 cral appendages, often with calcareous scales ; principally arctic and 

 antarctic. Rhopalodina J. E. Gray, mouth and anus close together at 

 the end of stalk-like process, the dorsal interradius much shortened ; 

 on account of the peculiar course of the radial canals there appear to be 

 ten radii. On the Congo coast. Ypsiloihuria E. Per. (Sphaeroihuria 

 Ludw.). 



British genera and species : 



Cucumaria : 



A. Tube-feet confined to ambulacra. 



I. Tube-feet rion retractile. 



Skin smooth . . . . . . C. hyndmani Forbes. 



Skin stiff C. lactea F and G. 



II. Tube-feet retractile. 



a Skin smooth. 



Attenuated at either end . . C. pentactes Mont. 



Body sac-like or elongated . . C. planci Gmel. 



b Skin rough ..... C. hispida Barr. 



B. Tube-feet scattered . . . (7. frondosa Gunn. 



Thyone : Body not curved on itself, T. fusus O. F. M. Body 

 curved on itself, T. raphanus D. and K. 



Psolus : Tube-feet in 3 complete rows, P. phantapus Struss., tube-feet 

 of median row few or none, P. jabricii D. and K. 



Phyllophorus : P. pellucidus D. and K., P. drummondi Thomp. 



Order 2. PARACTINOPODA. 



The oral tentacles arise from the circumoral vessel. Tube-feet and 

 ambulacral papillae, respiratory trees and cuvierian organs are not 

 present. 



Fam. Synaptidae. Radial canals absent in the adult. Mouth terminal. 

 Body cylindrical, worm-like. 10-27 (often 12) feathered tentacles ; 

 tentacular ampullae only indicated. Calcareous ring often with more 

 than 5 interradial pieces. Anchor-shaped spicules are present in Synapta 

 and its allies. Wheel-shaped spicules in most other genera. Stone-canal 

 sometimes multiple. Otocysts on the origin of the radial nerves. Longi- 

 tudinal muscles usually undivided ; retractor muscles sometimes present. 

 Respiratory trees absent. Ciliated organs on the wall of the body -cavity 

 present, cuvierian organs absent. Generative organs often hermaphrodite. 

 Synapta Eschscholtz, in sand and mud ; Euapta, Chondroclaea, and 

 Labidoplax Oestergren ; Anapta Semper ; Chiridota Eschsch. ; Trocho- 

 dota Ludwig ; Trochoderma Theel ; Myriotrocha Steenstrup ; Acantho- 

 trochus D. and K. British species : Synapta inhaerens O. F. Mull. ; 

 S. buski Maclnt., rare, S. digitata Montagu, British and French coasts, 

 etc. 



