154 THE APODOUS HOLOTHUKIANS 



ExcEETiON.— That the respiratory trees function to some extent as excretory 

 organs seems very probable, not only because they are the most obvious organs 

 to which such a fiuietion could be assigned, but because the same organs in 

 pedate holothurians are known to play that part. Excreta is probably gathered 

 not only from the fluid of the body-cavity, but from the haemal system also. 

 The presence of wandering cells in large numbers throughout the body is evi- 

 dence that excretion by means of those peculiar cells is continually going on. 

 "Whether the change of calcareous deposits into phosphatic bodies is associated 

 with excretion is still to be demonstrated. 



Sensation. — Aside from the sense of touch, Molpadids are not known to 

 possess any capacity for sensation. No observations have been recorded that 

 show reaction to any other than mechanical stimuli, except Sluiter's ('88) state- 

 ment that Aphelodactyla shows its oral end above the mud only at night. If 

 this is really the case, it might indicate a reaction to light, which would be very 

 interesting, as no light-detecting organs are known to occur in the family. It 

 is probable that the tentacles and the cloacal papilla^ are the most important 

 seats of the tactile sense. 



Regeneration. — No observations have l^een recorded as to the possibility of 

 regeneration among the Molpadiid^T. 



Reproduction. — The method of reproduction is exclusively sexual and the 

 sexes are always separate. No viviparous species are known nor any which 

 care for the young in any way. Nothing whatever is known as to the extrusion, 

 fertilization, or segmentation of the eggs, nor as to the time, place, and condi- 

 tions of breeding. In Caudina arenata, Gerould ( '96) found that the spermato- 

 zoa were mature and extruded in February. March, and April, but he failed to 

 secure mature ova. 



Ecology. 



The Molpadiida; are all marine animals and have a bathymetrical range 

 from a little below low-water mark to at least 3,900 m. The great majority, 

 however, occur only at depths of more than 100 m., and consequently 

 little is known of their habits. Although occurring in all parts of the 

 world, they seem to be most abundant in the Indo-Pacific region, while com- 

 paratively few are known from the tropical and subtropical Atlantic. They 

 are common in both the Arctic and Antarctic oceans, though the number of 

 species in those regions is not large. So far as is known, the Molpadids show 

 great similarity of habitat and habits and are found ouly on soft sandy or, more 

 commonly, muddy bottoms. Here they lead an almost exclusively subterranean 

 life, only disturbed by the rare appearance of a dredge or trawl, or the occa- 

 sional intrusion of some fish in search of a meal. Those which live in shallow 

 water near shore are occasionally distui-bed by heavy gales, which ultimately 

 succeed in digging them out and washing them up on shore; probably four- 



