THE APODOTJS HOLOTHUBIANS 153 



material finally passes into the cloaca and is thence washed out by the currents 

 of water from the respiratory trees. 



RESPIRATION. According to the observations of Gerould ('96), the tenta- 

 cles seem to play an important part in obtaining the all-essential oxygen for the 

 use of the body. The fluid contained in the water-vascular system contains 

 numerous colored corpuscles which appear to react toward oxygen like true 

 blood-corpuscles, and there is a continuous circulation of the fluid, at least in 

 the tentacles. The flow is forward on the inner side to the tip of the terminal 

 pair of digits, and then back and outward to the tip of the other pair; then 

 backward into the ampulla. "When aeration becomes poor, the tentacles and 

 buccal region become distended with the water-vascular fluids and the posterior 

 part of the body becomes pale and contracted." Elsewhere (page 10) Ger- 

 ould says: "The color, which depends upon the state of aeration of the blood, 

 varies from pink to a purplish hue." Apparently the word "blood" is used 

 here to include all the fluids of the body which contain blood-corpuscles (water- 

 vascular, body-cavity, and ha?mal). But respiration is also greatly facilitated 

 by the so-called respiratory trees, and it is probably chiefly from these that the 

 body-cavity and haemal fluids get their oxygen. Indeed, when we consider the 

 habit of most Molpadids of lying buried in mud and sand, with only the tip of 

 the tail above the surface, it seems probable that the tentacles play an impor- 

 tant part in respiration only under unusual conditions. Both Sluiter ('88) and 

 Gerould ('96) are agreed that water is forced out of the respiratory trees and 

 drawn into them through the cloacal opening "by the alternate contraction and 

 relaxation of these organs and of the wall of the body. The latter, by reason of 

 its natural rigidity, resumes its normal shape when its circular muscles are re- 

 laxed, and so increases the capacity of the body-cavity, thus bringing about an 

 influx of water. These movements are accompanied by the correlated opening 

 and closing of the cloacal opening through the alternating contractions of the 

 radial and sphincter muscles." Sluiter says that in Aphelodactyla there were 

 two or three respiratory movements per minute, and Gerould found that in Cau- 

 dina the same is true; he says further that the cloacal opening was "generally 

 kept open 18-20 seconds, and then closed for 13-17 seconds," and that while 

 there is some irregularity in the length of the periods, that "of dilation always 

 slightly exceeds that of closure." Aside from the fact that such regular 

 respiratory movements would not be likely to occur if the function of the 

 "trees" was purely excretory, the intimate relation between that organ and 

 the intestinal haemal vessels, which contain numerous blood-corpuscles, would 

 lead us to believe that the bringing in of oxygen was one of its important func- 

 tions. Very possibly oxygen may also be obtained through the skin by cor- 

 puscles in the radial water-vascular canals and particularly near their posterior 

 termination. 



