i895. RESULTS OF "CHALLENGER'' EXPEDITION. 49 



Holothurians are derived from two sources. Those that have com- 

 paratively recently migrated from saallower waters, even to depths of 

 2,900 fathoms, and that, like the Cucumarice, still resemble their 

 littoral ancestors, are few both in species and individuals. The great 

 majority are Elasipoda, which must have originated long ago from a 

 type very different to the present shallow-water fauna. From the 

 three Elasipoda previously known, and all from the N. Atlantic and 

 the Arctic Sea, the " Challenger " raised the number to fifty-two 

 species, divided into nineteen genera, which were nearly all found at 

 depths greater than 1,000 fathoms, and had a universal distribution, 

 some species ranging almost from pole to pole. The answer to the 

 question how our knowledge of the Holothurians was increased by 

 the "Challenger," will, therefore, largely consist of a summary of 

 elasipodan peculiarities. 



The differentiation of an upper and lower surface, producing a 

 bilateral symmetry, which may be seen in some of the ordinary 

 Dendrochirotae and Aspidochirotae, is in the Elasipoda carried to a 

 strange extreme, and is accompanied by an unusual symmetry in the 

 arrangement of the pedicels and processes (Fig. 7). The ambulacral 



Fig 7. — Peniagone ivyvillei, Theel ; 2,425 fms. ; nat. size. 



appendages of the under surface alone are intended for locomotion, 

 these being particularly large in the typical Elasipoda, and arranged 

 in a single row on each side of the body, forming distinct pairs of legs. 

 They tend to appear in fixed places and in a fixed number, often a 

 small one, in every species of the more typical Elasipoda. They 

 usually lack a terminal plate, sometimes even a sucking disc ; but 

 they are stumpy and often have a firm external skeleton. They are 

 probably used, not as the tube-feet of other echinoderms, but as the 

 limbs of more highly organised animals, to move and to dig in the 

 soft bottom-ooze. The appendages of the upper surfaces also tend to 

 be fixed in position and definite in number ; and their large nerve-supply 

 indicates that they serve as organs of touch. The tentacles form a 

 disc around the mouth, and as the animals move along the bottom of 

 the ocean, help to fill the alimentary canal with the ooze, from which 

 such nutriment as it contains is extracted. There is no trace of 

 respiratory trees. Auditory organs, in the shape of small sacs 



