HOLOTHURIOIDEA 377 



Another pelagic Holothurian, Planktothuria diaj^Jiana Gil- 

 christ/ agrees with Pelagothuria in the peculiar shape of the 

 tentacles, and therefore may perhaps also belong to this family, 

 in spite of the conspicuous differences from Pelagothuria (no free 

 circular disk, presence of rudimentary tube-feet, etc.). It is 

 known only from off South Africa, but as a deep-sea pelagic 

 organism may well be expected to have a wide distribution, and 

 thus perhaps to occur also in the N. Atlantic deep sea. 



II. Order. Aspidcchirota Grube. 



Tube-feet present ; tentacles shield-shaped. No retractor 

 muscles. Respiratory trees present. Mesentery of the posterior 

 intestinal loop attached in the right ventral interradius. Cal- 

 careous deposits simple, pointed rods, tables, buttons, etc. Body 

 cylindrical or flattened, bilaterally symmetrical. 



All feeding on bottom material, with which they fill their 

 alimentary canal, shovelling it into the mouth by means of their 

 peltate tentacles. 



This order comprises three families, all represented in the 

 British seas. 



Key to the families of the Aspidochirota. 



1. Tentacle ampullae absent . . . . .1. SynaUactidce 



„ ,, present ...... 2 



2. Gonads on both sides of the dorsal mesentery II. Stichopodidce 

 Gonads only on the left side of the dorsal mesentery 



III. Holothuriidce 



1. Family Synallactid^ 



Tentacle ampullae absent. Gonads variously developed. 

 Respiratory trees usually not connected with the alimentary 

 canal through a rete mirabile. Stone canal usually in connection 

 with the body wall, sometimes opening outwards through the 

 body wall. No Cuvierian organs. Deposits tables ; C-shaped 

 bodies may be present and, very rarely, buttons. 



Mainly deep-sea forms of rather large size. Some of them 

 {e.g. Pseudostichopus) have the habit of agglutinating foreign 

 bodies — sponge spicules, Pteropod shells, Globigerinas, etc. — to 

 their skin ; the tube-feet of these forms are generally much 

 reduced, and the calcareous bodies are also very much reduced 

 or totally absent. 



Two genera, Mesothuria Ludw. and Bathyplotes Ostergren, 



1 J. D. F. Gilchrist, " Planktothuria diaphana, g. and sp. n.", Quart, 

 Journ. Micr. Sc, vol. 64, 1920. 



