THE APODOUS HOLOTHURIANS 63 



trous movements, however, probably do not occur under ijornial conditions. 

 Subterranean species draw back into their burrows when attacked, and may 

 even retreat into them for some distance. Few, if any, Syna^itids swim, except 

 in the larval state, but young ones up to 2 em. in length are sometimes found 

 floating, if not swimming, in the water. They seem to move partly by the aid 

 of the tentacles and partly by undulatory movements of the body. Many 

 species crawl about with more or less activity, by means of the tentacles, and 

 a few forms can climb the glass walls of aquaria, using the glandular sensory 

 epithelium of the outside of the tentacle tips as adhesive organs. In rare cases 

 the calcareous particles of the body-wall assist in climbing about in seaweeds. 

 As for the rate of movement, it of course varies greatly with the individual and 

 the external conditions, but a healthy Synaptid of moderate size can move 

 from 20 to 30 mm. per minute; larger specimens may move more rapidly. 



Digestion and absorption. — So far as is known, these processes show no 

 special peculiarities, taking place as they do in a remarkably simple alimen- 

 tary system. The food pushed into the mouth by the tentacles is carried by 

 the short and narrow oesophagus into the stomach, where it becomes mixed 

 with the digestive secretion of the gastric glands. Digestion begins here, but 

 probably takes place also in the intestine, while absorption, which may take 

 place to a certain extent in the stomach wall, is mainly accomi')lished by the 

 wall of the intestine, the absorbed food material being received in the two 

 main trunks of blood system through an extensive system of lacun;e in the in- 

 testinal wall. The rectum receives and ultimately rejects the excrement. It 

 is possible that the "larval glandular organs" of Synaptula hydriformis 

 (see page 61) and the "contractile rosettes" of Leptosynapta minuta (see 

 Becher, : 06) are absorptive organs, but it is by no means certain what their 

 function is. 



Circulation and nutrition. — There is no true circulation in the blood-sys- 

 tem of the Synaptidje, and the so-called blood plays no part in the carrying of 

 oxygen to the tissues or in removing excreta from them. We are to look 

 upon the movements of the fluid as normally away from the digestive system 

 to the various parts of the body, and as carrying simply the absorbed food to 

 its destination. The system thus is essential to normal and healthful nutrition, 

 but cannot be considered as in any true sense a circulatory system. There is 

 no heart or other vascular organ for propelling the fluid, but contractions of the 

 muscles in the wall of the alimentary canal serve to force the fluid products of 

 digestion into the lacunas and thence into the larger vessels, and from them on 

 and away, while contractions of the nniscles in the body-wall and elsewhere 

 may serve to open up the minute lacunar spaces that drink in the nutritious 

 fluid. No proper investigations of the fluid or its movements in a Synaptid 

 have yet been made, and whether the corpuscles play any important part is 

 unknown. 



