570 ECHINODERMATA IIOLOTHUROIDEA chap. 



All authors recognise six divisions, and the only dispute is as to 

 whether they are to be regarded as families or orders. Ludwig ^ 

 divides the group into two orders, Paractinopoda and Actino- 

 poda, but the first includes only those forms which have lost the 

 radial canals, and this is only one step farther in a degeneration, 

 intermediate stages of which can be traced in the other divisions. 

 There is really no ground for placing the Paractinopoda in contrast 

 to all the other divisions, and the only alternative is to regard 

 the six main divisions as orders, since a class must be divided into 

 orders. In the case of only one, however, is a further division 

 into families practicable, and therefore each of the others will 

 contain a single family. 



Order I. Aspidochirota. 



Holothuroidea with shield -shaped feelers provided with 

 ampullae ; with radial canals and numerous podia and with re- 

 spiratory trees. Ee tractor muscles absent. Nearly a third (158) 

 of the species of Holothuroidea belong to this order, but there are 

 only six genera, and of these Holothuria includes no less than 

 109 species. The Aspidochirota seem for the most part to live 

 on somewhat firm ground, the surface of which they are con- 

 tinually sweeping with their shield-shaped feelers, which brush 

 the adherent organisms into the capacious mouth. Four species 

 of Holothuria — viz. H. intestinalis, H. tremula, H. aspera, and 

 II. nigra are recorded from British waters. The first-named is 

 a northern form, distinguished by the fact that all its podia have 

 suckers ; it is found in the north of Scotland. H. tremula is 

 intermediate in structm-e between H. intestinalis and H. nigra, 

 and is found in deep water off our western coasts. H. aspera, 

 remarkable for the radiating spines growing out from its ossicles, 

 has been recorded only once from deep water. Of the other 

 genera it is only necessary to mention Stichopus, remarkable for 

 the square outline of its transverse section, and for the restriction 

 of the ventral tube-feet to the radii ; there is also a well-marked 

 tapering of the anterior end, so that this genus may be said to 

 have a neck. Stichopus is almost entirely confined to tropical 

 waters, and some of its species, as also species of the ubiquitous 

 genus Holothuria, as well as many other undetermined species, 

 constitute the valuable " Trepang," which is a delicacy much prized 



•^ Bronn's Thier-Beich, vol. ii. Abt. 3, Buoh i. " Die Seewalzen," 1891, X'P- 327 etseq. 



