TROPICAL PACIFIC HOLOTHURIOIDEA. 123 



over the amount of their muscular contraction, and the particular form assumed, 

 are subject to extraordinary diversity, as a result of which two very different 

 specimens may prove to have the same essential structure and the same calca- 

 reous particles. In many cases the specimens are so distended with fine mud, it 

 is hopeless to make anything out of the internal structure, and not infrequently 

 the outer surface of the body-wall is so largely rubbed away, no pedicels are left 

 and it is difficult to find any calcareous particles. In view of these conditions, 

 calcareous particles prove exceptionally useful and faith in their genetic signifi- 

 cance and taxonomic importance is greatly strengthened. It is very doubtful 

 whether holothurians with essentially different spicules should be placed in 

 the same genus and it is almost universally true that essential similarity in 

 spicules indicates close relationship. At present our classification contains 

 some very large and unnatural groups treated as genera (Cucumaria, Thyone, 

 and Holothuria for example) which can probably be broken up into a number 

 of small, natural genera by a proper valuation of their calcareous particles. 



Of the new species described, all except the Benthodytes and the Euphron- 

 ides, are particularly characterized by their spicules. The new Benthodytes 

 is easily recognized, when well preserved by the body-form and arrangement of 

 pedicels, and the Euphronides by the size and position of the dorsal appendage, 

 but whether badly damaged or contracted specimens could be correctly identi- 

 fied is very doubtful. Of all the new forms, the two remarkable species of Myrio- 

 trochus are unquestionably the most interesting, as the genus has hitherto been 

 considered characteristic of the subarctic zone. It is very perplexing to find 

 the two closely allied species at one and the same station and nowhere else, 

 for they are unquestionably quite distinct. The new Chiridota is interesting 

 because it shows a well-marked spicule-character, unusual in this homogeneous 

 genus. The same is true of the new Molpadia, since that is a genus in which 

 the calcareous tables often lack any real distinctiveness. 



The collections add little to our knowledge of the geographical distribution 

 of holothurians, except in the notable extension of the range of Myriotrochus, 

 to which reference has already been made. But it is of some interest to find 

 Chiridota rigida as far southeast as Easter Island, Benthodytes glutinosa in the 

 eastern Pacific and Ceraplectana trachyderma so far to the southward of the 

 type-locality, the only place whence it was hitherto known. 



The colored figures (Plates 1-3) were prepared by Mr. Agassiz and Mr. 

 Westergren during the voyage of 1904-1905, but unfortunately the specimens 

 from which they were made were not preserved separately or otherwise ade- 



