358 ECHINODERMS OF THE BRITISH ISLES 



from Lower Cambrian (N. America) as Holothurians is, at least, 

 disputable. 



The preservation of Holothurians generally offers some 

 difficulties, because they — especially the Dendrochirotes — retract 

 their tentacles or by the strong muscular contraction expel 

 their intestinal organs. Generally they are rather easily 

 narcotised by adding sulphate of magnesium to the sea- 

 water, and thus they may be preserved with the tentacles 

 expanded. They should always be preserved in alcohol, not in 

 formaline, in which the calcareous bodies are mostly dissolved, 

 identification thereby being made uncertain. Preservation in 

 formaline-alcohol (the formaline neutralised) is often recommend- 

 able, but the specimens should then be transferred to alcohol. 



The presence or absence of tube -feet and respiratory trees, and 

 the shape of the tentacles, are mainly used as a foundation for classi- 

 fication. The issue of the tentacle canals from the radial canals 

 or from the watervascular ring has also been made the foundation 

 for a primary subdivision, but it appears from more recent 

 researches that this character is not of such fundamental import- 

 ance as it was formerly supposed to be. The more natural 

 classification appears to be the one dividing the Holothurians 

 into the five orders : Elasipods, Aspidochirotes, Molpadonians, 

 Dendrochirotes, and Apodous Holothurians,^ all of which are, or 

 (the Molpadonians) will probably be found to be, represented in 

 the British seas. 



Key to the orders oj the Holothurioidea. 



1. Tube-feet well developed ; tentacles shield- or bush-shaped . 2 

 Tube-feet either completely lacking or present only in the shape 



of small papillae round the anal opening. Tentacles feather - 

 shaped or digitate ....... 4 



2. Tentacles shield-shaped ; no retractor muscles ... 3 



,, bush-shaped ; retractor muscles present 



III. Dendrochirota 



3. Respiratory trees absent ; mesentery of the posterior loop of the 



intestine attached in the right dorsal interradius 



I. Elasipoda 

 Respiratory trees present ; mesentery of the posterior loop of 

 the intestine attached in the right ventral interradius 



11. Aspidochirota 



^ The classification here adopted differs to some degree — especially in 

 the position of the Synallactids — from that given in the author's work on 

 the echinoderms in Dantnark''s Fauna ; the rearrangement is due to the 

 important arguments produced quite recently by Sven Ekman (" Syste- 

 matish-phylogenetische Studien iiber Elasipoden u. Aspidochiroten ", 

 Zool. Jahrb., Abt. f. Anat, ti. Ontogenie, 47, 1925). 



