64 THE APODOTJS HOLOTHURIANS 



RESPIRATION. The providing of oxygen for the tissues of the body is not 

 delegated to any particular system, so far as we know, but the needed gas is 

 taken from the water directly through the skin, probably of all parts of the 

 body, but particularly of the tentacles. Probably some oxygen passes entirely 

 through the skin into the body-cavity fluid, whence the alimentary canal and 

 reproductive system may be supplied. Naturally the tentacles and anterior 

 part of the body require more oxygen than other parts, and there is some rea- 

 son for believing that the oral disc and tentacles are particularly active in 

 respiration. There is no reason for attaching any respiratory function to the 

 blood system, but we as yet know nothing of the actual movements of gases to, 

 from, or in the tissues. 



EXCRETION. The getting rid of waste matter from the tissues takes place in 

 Synaptids as in other echinoderms, chiefly by means of the wandering cells 

 already referred to (p. 48), but assisted in a large measure by the peculiar 

 ciliated funnels (p. 57) so characteristic of the family. Whether the water- 

 vascular system, the blood system, or the alimentary canal play any part in 

 excretion we do not know. Excreta passes into the body-cavity fluid from all 

 those tissues which it touches, and much or all of this waste matter, certainly 

 if it be in solid particles, is sooner or later swept into the ciliated funnels. 

 It collects there and seems to be seized on by the "wandering cells," which 

 carry it into the connective tissue of the body-wall. Here these cells may re- 

 main indefinitely, but probably most of them ultimately pass" on through the 

 skin to the water outside. Fluid and gaseous excreta probably pass directly 

 through the skin into the water. As a matter of fact, we really have almost 

 no evidence on the subject of excretion, except observations made on the action 

 of the ciliated funnels, and while collecting solid particles from the body- 

 cavity fluid is undoubtedly an important function of these organs, it is by no 

 means proven that they are exclusively excretory. As for the wandering cells, 

 most statements regarding them are hypothetical, but there is some evidence in 

 support of the view that as the animal grows older, these cells, with their 

 loads of excreta, collect more and more thickly in the skin and cause the darker 

 or brighter colors which large Synaptids show as compared with their young. 

 What the essential difference is between such cells and the normal pigment 

 cells has yet to be shown, but there can be little doubt that there is a definite 

 correlation between color and excretion. Whether the calcareous particles are 

 to be regarded as the product of excretion is an open question. 



SENSATION. The senses of a Synaptid are apparently very simple. Touch 

 is the most widely distributed, almost any part of the body responding to a 

 mechanical stimulus, but it is specially keen in the digits of the tentacles, which 

 are exceedingly sensitive. So far as we can interpret the actions of a Synap- 

 tid, the sensations caused by a mechanical stimulus is the same as in all other 



