276 allan hancock pacific expeditions vol. 11 



Systematic Arrangement 



I had hoped to include a complete revision of all the tropical shallow 

 water aspidochirotes, of which the Museum of Comparative Zoology 

 possesses an unequalled series, but this was not found practical. Like 

 every worker who studies these forms, I am greatly indebted to Tan- 

 ning's revision of the genus Holothuria. But this magnificent work 

 suffers from his dependence in too many cases on the accounts of earlier 

 writers; hence many errors have been perpetuated and related forms 

 have been placed far apart. I would have preferred to follow his ar- 

 rangement, but this has not always been possible. Where his groups have 

 been well defined, as in his concept of Brandt's Microthele, I have used 

 them. In other cases, as in his use of the name Sporadipus, I have had to 

 take another course, and the best solution seemed to be to split up the 

 old genus Holothuria into a series of new genera. As far as possible I 

 have used the old ending -othuria with a suitable prefix, resulting in 

 names like Selenkothuria, Theelothuria, Mertensiothuria, etc., which 

 should not be too difficult to apply. 



Many attempts have been made to arrange the numerous species in 

 the old genus Holothuria in a reasonable systematic order, and the key 

 which is given here for the genera known from the Panamic region 

 should make it clear how the lines have been drawn. 



The members of this group are essentially adapted to live either 

 free on the bottom of lagoons, or buried in sand or mud, or clinging to 

 rocks, often exposed to the surf. Within each habitat are groups in 

 different stages of development, which can be separated by means of 

 their spicules. Most primitive are undoubtedly those with numerous 

 regular tables and regular smooth buttons, somewhat reminiscent of 

 certain synallactidlike members of the Stichopodidae. A more advanced 

 stage is indicated by the presence of irregular buttons, or the develop- 

 ment of rosettes, or the reduction of the inner layer of spicules, while 

 the tables have become variously modified. In the surf-loving forms, one 

 group has a well developed layer of tables, another section has them re- 

 duced almost completely, and in some species there are only a few rods 

 in the inner layer. Among the burrowing forms, some are more or less 

 cylindrical in body form while others are flattened, and the spicules vary 

 from regular tables and buttons, to hemispherical tables and regularly 

 knobbed buttons, to reduced tables and irregularly knobbed, deformed 

 buttons, etc. There is nothing essentially new in the present arrange- 



