568 ECHINODERMATA — HOLOTHUROIDEA chap. 



species, and even in the same species, and hence are probably not 

 of physiological importance. We shall therefore confine our 

 attention mainly to those differences in structure which are 

 correlated with differences in habits, and therefore of systematic 

 importance. We shall consider in order (1) the feelers ; (2) the 

 method of protecting these ; (3) the rest of the water-vascular 

 system; (4) the gills; and (5) the skeleton. 



Feelers.- — These organs have been made the basis of the 

 division of the Holothuroidea into orders, and as they are the 

 means by which food is obtained, and are thus of first-class 

 physiological importance, this procedure is fully justified. In 

 three orders they have the shield-shaped ends described in the 

 case of Holothuria nigra, but in another large order (Dendro- 

 chirota) they are much branched, and end in a mass of delicate 

 twigs. In another order (Synaptida) they are feather-shaped, 

 with two rows only of branches, whilst finally in Molpadiida 

 they are simple finger-shaped processes with one or two lateral 

 branches. The number of the feelers varies from ten to thirty. 



In the Dendrochirota the entire anterior portion of the body 

 can be introverted into the interior, so that in this way the 

 crown of feelers can be effectively protected. The retractor 

 muscles are modified portions of the longitudinal muscles of 

 the body-wall, which traverse the body-cavity, and are inserted 

 into the radial pieces of the calcareous ring. Similar muscles 

 are found in the genus Molpadia and in many of the Synaptida. 

 In Aspidochirota and Pelagothuria they are totally wanting, and 

 here the feelers possess long ampullae which allow of the 

 tentacles being individually contracted to very small dimen- 

 sions. These ampullae seem to be present in nearly all cases in 

 Molpadiida, and in Synaptida, although in the last-named order 

 they are very feebly developed, and must be looked on as vestigial. 

 In Dendrochirota, owing to the strongly developed retractors, they 

 would be useless, and so are absent. 



Water- Vascular System. — In Synaptida the radial canals 

 are totally absent in the adult, and the only podia are the 

 feelers, which spring directly from the ring-canal. The radial 

 canals are present in Pelagothuria, but the feelers are still the 

 only podia ; in the Molpadiida there are only five small terminal 

 tentacles round the anus in addition to the feelers. In the 

 Elasipoda all the podia have pointed ends, but the dorsal podia 



