INTRODUCTION. 



Early in the spring of 1900 the collection of apodous holothurians of the,- 

 United States National Museum was placed in my hands for identification. This 

 collection consisted of nearly 1,000 specimens from various localities along 

 the American coast, both east and west, from the Banks of Newfoundland and 

 the Aleutian Islands to the Strait of Magellan. It was rich in Molpadiida?, and 

 furnished an excellent basis for the revision of that family, but it was poor 

 in Synaptidae, though many of the American species were represented by nu- 

 merous specimens. Owing, however, to the great need of a revision of the 

 Synaptids, it was thought best to take the opportunity of making it, and the 

 present paper was intended to be a complete account of all the footless holo- 

 thurians known to science. After the manuscript was completed, in 1906, a 

 supplementary collection from the National Museum, containing over 1,200 

 specimens, was placed in my hands, and the examination of this additional ma- 

 terial has proved of great value, not only by enabling me to test and correct 

 the artificial keys, but by throwing much additional light on the validity of 

 a number of doubtful forms. This report, as it now stands, is intended to 

 include all species described prior to January 1, 1907, and to furnish a means 

 for their ready identification. The synonymy of each species has received care- 

 ful attention, but no attempt has been made to include every writer who has 

 mentioned the form, unless he has in some way modified or qualified the name 

 in use. The bibliography is thought to include every paper which contains any 

 matter of importance relating to the two families, but a number of papers are 

 omitted which simply contain casual references to, or familiar facts concerning, 

 some of the well-known species. 



As a matter of convenience, the classification of the two families is dis- 

 cussed first, and this is followed by an annotated list of the species in the col- 

 lections of the National Museum, including descriptions of new genera and 

 species. The remaining space is occupied by an account of the morphology, 

 embryology, physiology, ecology, and taxology of the two f amilies, with arti- 

 ficial keys and an account of each species, especial attention being given to 

 the geographical distribution. The figures are intended to illustrate not only 

 the new forms described, but also previously known species that have not been 

 figured, and some others, figures of which will be of service to the student 

 of these animals. A synonymic index is also given, to aid in finding any species 

 referred to by previous writers. A sincere effort has been made to place the 

 nomenclature on as firm a basis as possible, by the use of the now very gen- 

 erally accepted principles laid down in the International Code. This has in- 



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