Proceedings of 

 the United States 

 National Museum 



SMITHSONIAN INSTITUTION • WASHINGTON, D.C. 



Volume 122 1967 Number 3592 



THE PSOLID HOLOTHURIAN GENUS LISSOTHURIA 



By David L. Pawson 



Curator, Division of Echinoderms 



The holothurians of the family PsoUdae are unique in that they 

 have adopted an essentially sedentary mode of existence. The body 

 is more or less flattened and strongly bilaterally symmetrical. While 

 the dorsal surface of the body is generally invested in an armor of 

 imbricating calcareous scales, the ventral surface is a soft thin sole 

 surrounded by tube feet that enable the animal to clmg to a hard 

 substrate. 



These holothurians may be regarded as "pelmatozoans," while 

 the majority of the other holothurian groups are essentially "eleu- 

 therozoan" or free-living. As in other groups of sedentary marine 

 animals, the mouth and anus are on the same side of the body, in this 

 case the dorsal surface, and the gut describes a U-shaped course. 

 Among other echinoderms the U-shaped, pelmatozoan-type gut is 

 found in many Paleozoic groups (including crinoids, blastoids, cystoids, 

 and edrioasteroids) ; crmoids are the only survivmg members of these 

 groups. While the modifications resultmg from the adoption by 

 psolids of a sedentary existence show some similarities to those in 

 the extmct edrioasteroids (see Fell, 1965), it is believed that the 

 psohds themselves are not an ancient group of holothurians. Although 

 they are strongly bilaterally symmetrical, their internal anatomy 

 reflects a former radial symmetry, and it seems probable that psolids 



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