THE APODOUS HOLOTHUBIANS 155 



fifths of the specimens of Caudina arenata ^yhich have been collected have been 

 the victims of such gales. The food of the Molpadids seems to be exclusively 

 the organic matter which they digest out of the sand and mud which passes into 

 tlie mouth with every forward movement. Whether this mud is sucked in or 

 l)ushed in by the tentacles is not known. Apparently the Molpadids are the 

 least active of all holothurians, and it would seem to be the case that they 

 often lie unmoved for hours, if not days, at a time. Sluiter ( '88) says that his 

 specimens of Aphelodactyla in captivity were more active at night than during 

 the day, as they then showed the oral disc and tentacles above the mud; it is 

 probable, however, that this change of position was due to the condition of the 

 water (amount of oxygen, etc.) rather than to absence of light. The shallow- 

 water species of Caudina and Aphelodactyla seem to thrive well in aquaria, with 

 proper care, and it is to be hoped that we may soon have more information in 

 regard to their habits and physiology. The Molpadids are often eaten by bot- 

 tom-feeding fishes, but are not known to have other enemies. No parasites 

 have yet been described of which they are hosts. The Molpadiida^ are of no 

 value whatever to man in any direct way, though as an article of food for fishes 

 they may play a slight part in benefiting him. Nothing whatever is known of 

 their geological history. 



Taxology. 



The classification of the Molpadiida^ herein adopted and the reasons there- 

 for are fully discussed on pages 17-21, and the principles which have governed 

 the acceptance or the rejection of species are essentially the same as those 

 which were used in connection with the Synaptida?. (See antea, pp. 68-70.) Here 

 as there, the form and distribution of the calcareous particles is the most im- 

 portant character for distinguishing species, but in the Molpadiidae the phos- 

 phatic deposits are also to be taken into account. Color, size, and body-form 

 are usable characters in some cases, and the texture of the body- wall is also of 

 importance at times. Of characters for distinguishing genera the number and 

 form of the tentacles and digits, the presence or absence of ampulhB and pos- 

 terior projections on the radial pieces of the calcareous ring, and the presence 

 or absence of a caudal appendage are the best; but size and the development 

 of phosphatic deposits are sometimes to be considered. Most of the genera 

 here recognized are quite distinct groups, but it must be acknowledged that 

 Caudina and Molpadia intergrade, while it is extremely likely that Himasthle- 

 phora is a synonym of Gephyrothuria. The situation among the species is much 

 worse, for both Molpadia and Caudina contain badly confused species and 

 groups of species and Aphelodactyla is little better off. It is sincerely hoped, 

 however, that the attempt here made to bring some kind of order out of this 

 chaos may prove a useful foundation for the future work, which shall satis- 

 factorily solve all of the perplexing riddles which the taxology of this family 

 presents. 



