ADVERTISEMENT. 



The present memoir by Dr. Hubert Lyman Clark, of the IMusenra of 

 Comparative Zoology, in Cambridge, Massachusetts, on "The Apodous Holo- 

 thurians," sea-cuenmbers or trepangs, forms part of Volimie XXXV of the 

 Smithsonian Contributions to Knowledge. 



Ill this memoir the attempt has been made to give a complete summary of 

 our present knowledge of the two families of sea-cucumbers which lack tube- 

 feet. As a system of classification is the essential foundation upon which any 

 discussion of structure, habits or relationships must be built, the first section 

 is devoted to a discussion of the history of the classification of the two families 

 (Synaptidffi and Molpadiida?), with comments on the principles involved and a 

 final sinnmary of the system adopted. Most of the original investigations hav- 

 ing 1)een liased on material in the collections of the United States National 

 Museum, the second section is an annotated catalogue of the Apodous Holo- 

 thurians of that institution. The third section deals with the family Synap- 

 tida^ and takes up in regular sequence, so far as our present knowledge per- 

 mits, the structure, physiology, development, habits and classification of these 

 animals. In the last subdivision of this section, each species, recognized as 

 valid, is treated separately in some detail. In the fourth section, the family 

 MolpadiidiB is treated in the same way. 



The most important feature of the work is the recognition of the changes 

 taking place in the maturing and sensscence of individual holothurians, par- 

 ticularly in the family Molpadiidte. As a result of this, radical changes in 

 nomenclature have been necessary, but every effort has been made to have the 

 system adopted accord with the most widely accepted codes, and thus be as 

 stable as possible. Special attention has been given to geographical distri- 

 bution, hilt the work in this line is chiefly of value as a summary of our present 

 very inadequate knowledge. Artificial keys to genera and species have been 

 freely used with the intention of making the work as useful as possible to all 

 subsequent investigators, and the numerous figures, most of which are copied 

 from other writers, are given with the same end in view. 



In accordance with the rule adopted by the Institution, the work has been 



submitted to a commission consisting of Prof. A. E. Verrill, of Yale University, 



and Prof. W. K. Brooks, of the Johns Hopkins University, who recommended 



its publication in the present series. 



Chas. D. Walcott, 



Secretary. 



Smithsonian Institution, 



AVashington, J«»e, 1907. •' 



