11G THE VOYAGE OF H.M.S. CHALLENGER. 



sometimes attain a considerable size as in Colochirus quadrangular is, Less., Stichopus 

 naso, Senip., Holothuria armata, SeL, &c. The pedicels, which seem to be indefinite 

 in number can with few exceptions be entirely retracted within the body-wall ; they are 

 either irregularly scattered all over the body as in Thy one, Okeri, &c, or disposed in 

 rows all along the five ambulacra as in Cucumaria, Blainv., &c, or they are found only 

 on the ventral surface, where they are generally arranged in three rows as in Colochirus, 

 Troschel, Psoitis, Oken, &c. The ambulacra! papillae, which are present in the genera 

 Stichopus, Brandt, Miilleria, Jager, Colochirus, Troschel, and in most species of the 

 genus Holothuria, L., are sometimes disposed in more or less plainly marked rows, 

 but are ordinarily scattered, and show no traces of any regular arrangement ; in some 

 cases they are found not only on the dorsal surface, but dispersed all over the body, 

 as in several forms of the group Sporadipus, Grube. The form and the position of 

 the pedicels and processes as well as their number being usually well-defined are highly 

 remarkable in the Elasipoda, and give the external configuration of the body its charac- 

 teristic appearance. Consequently, it is of the greatest importance to point out as 

 plainly as possible in what respects these organs differ from those in the Pedata. I never 

 found any pedicels fully corresponding to what Semper terms " ambulacral pedicels," and 

 it would be more correct to class all the pedicels of the Elasipoda under the head of 

 " ambulacral papillae." Thus, when describing the order in question, I prefer to give the 

 name "pedicels" to the ambulacral appendages on the ventral surface, these being 

 exclusively adapted to purposes of locomotion, contrary to those on the dorsal surface, 

 which may be called " processes." Just as the ventral is in most cases obviously distinct 

 from the more or less highly convex dorsal surface, so a generally striking difference may 

 be observed between the ventral pedicels and the dorsal processes, this difference being 

 not only conspicuous in the external shape, in the size and in the number, but also in 

 the corresponding ambulacral cavities or ampullae, which will be described more in 

 detad under the rubric " water- vascular system." Only a few examples in the 

 Psyckropotidee, viz., Benthodytes sanguinolenta and Benthodytes typica, exist, which 

 correspond with several forms in the Pedata in carrying on the dorsal surface a number 

 of appendages which by their narrow cyHndrical shape, their minute size, and their 

 capabdity of being entirely retracted within the integument, bear the strongest resem- 

 blance to true pedicels. My intention is to give a summary account of these appendages 

 a little further on after having first described the pedicels more in detail. 



Two kinds of pedicels are distinguishable in the Elasipoda ; the first, small, and often 

 resembling rounded protuberances, are generally strengthened by a small number of 

 spicula, their rounded or slightly flattened ends being without any calcareous terminal 

 plate ; the second, large, generally cylindrical or conical, more or less stiff, not retractile, 

 but to a certain degree contractfle, and with their tops either large, sole-like, or discoidal, 

 as in Orphnurgus, Lcetmogone, &c, or more or less tapered as in Scotoplanes globosa, &c. 



