HOLOTHURIOIDEA 359 



4. Anal papillae, tentacle ampullae/ and respiratory trees present ; 

 body thick, sausage-shaped, ending in a thin caudal pro- 

 longation ...... IV. Molpadonia 



Anal papillae, tentacle ampullae, and respiratory trees absent. 

 Body worm-shaped, without a caudal prolongation V. Apoda 



I. Order. Elasipoda Theel. 



Tube-feet usually present ; tentacles shield-shaped (in Pelago- 

 thuria more dendritic) ; retractor muscles and respiratory trees 

 absent. Mesentery of the posterior intestinal loop attached in 

 the right dorsal interradius, usually near the right dorsal longi- 

 tudinal muscle. Among the calcareous bodies are pointed rods 

 and such as are derived from pointed rods. Shape of body 

 usually bilateral-symmetrical. 



All Elasipods are deep-sea forms, living on mud bottom. 

 They fill their intestinal canal mth mud, subsisting thus on the 

 organic substances contained in the mud. Some of them, like 

 Psychropotes and Scotoanassa, are probably active swimmers in 

 the deeper layers of the sea. Development entirely unknown. 



To this order, which otherwise comprises only deep-sea forms, 

 the pelagic form Pelagothuria also is usually referred, as an 

 aberrant Elasipod. The order is divided into five families, of 

 w^hich only one, the Lcetmogonidce, is known to be represented in 

 the British seas. There can, however, hardly be any doubt 

 that they will all prove to be represented there. 



Key to the families of the Elasipoda known from or likely to be 

 represented in the British seas. 



1. Body ovoid, without tube -feet, but with a circular disk round 



the anterior end, supported by long rays. Calcareous bodies 

 absent ...... Pelagothuriidce 



Body bilaterally symmetrical ; tube-feet well developed ; no 

 circular disk round the anterior end. Calcareous deposits 

 present ......... 2 



2. Lateral papillae present, non-retractile ; the calcareous deposits 



of the papillae arranged parallel to the longitudinal axis of the 

 latter. Skin usually thin, parchment-like or brittle 



DeimatidcB 



No non-retractile lateral papillae, but more or less well developed 



lateral tube -feet ; calcareous deposits of the papillae not 



arranged parallel to the longitudinal axis of the latter. Skin 



usually thick, gelatinous ...... 3 



^ Tentacle ampullae appear to be lacking in the small Arctic Molpadid, 

 Eupyrgus scaber Ltk. 



