GENERAL ZOOLOGY 



Tentacles 



Madreporiti 



Genital 



Stomach 



Gonad 



Ampullae 



Longitudinal 

 muscle band 



Cloacal 

 muscles 



Intestine 



Cloaca 



"Water 

 lungs" 



Fig. 16.13. Internal anatomy 

 of a sea cucumber, Thyone 

 hriareus. One of the "water 

 luns;s," or respiratory trees, 

 has been removed, and the 

 mesenteries supporting the di- 

 gestive tract are not shown. 

 Note that the madreporite 

 opens into the perivisceral 

 coelom, not externally. (Re- 

 drawn from W. M. Reid in 

 F. A. Brown, Jr., et al.. Selected 

 Invertebrate Types, copyright 

 1950 by John Wiley and Sons, 

 Inc., printed bv permission.) 



Anus 



radial ambulacral canals as in other echinoderms. The animal usually lies 

 on one side, and this "ventral" surface has larger and more numerous tube 

 feet than the "dorsal" or uppermost side. In addition, the distance from 

 mouth to anus is greater along the ventral midline than along the dorsal; 

 and to this extent Thyone presents a bilateral appearance, with dorsoventral 

 as well as anteroposterior differentiation. 



Thyone lives partly embedded in the ooze of muddy bottoms just below 

 low-tide level, with its anterior and posterior ends exposed. The anterior end 

 is directed diagonally upward, and the tentacles are free to move over the 

 surface of the surrounding mud. If the animal is disturbed, the tentacles are 

 withdrawn by inversion of the entire oral end of the body; excess water is 

 discharged from the anus, and the body becomes tense and turgid. In feed- 

 ing, the tentacles are moved about until well covered with silt from the 

 bottom and are then thrust, one at a time, deep within the mouth and re- 

 lieved of their burden as they are again withdrawn. The animal "breathes" 

 sea water, pumping it in and out through the anus. In locomotion, the 

 organism moves by extending its tube feet, attaching their terminal suckers. 



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