PHYLUM ECHINODERMATA 



207 



Behavior. — The 



tube-feet, when present, 

 are organs of locomotion. 

 They pull the animal 

 along on its ventral, 

 flattened surface. 

 Waves of muscular con- 

 traction which travel 

 from one end of the 

 body to the other are 

 important in locomo- 

 tion, and the tentacles 

 may also assist. 



The common sea- 

 cucumbers, Thyone 

 briareus, are sensitive 

 to contact with solid 

 objects, and many of 

 them burrow in the 

 sand or mud. They 

 are extremely sensitive 

 to a decrease in the 

 light intensity and will 

 contract the body if an 

 object passes between 

 them and the source of 

 light. They are also 

 negatively phototropic, 

 since they move away 

 from the light. The 

 following has been 

 written concerning this 

 species: "Passing most 

 of its life buried in the 

 mud, Thyone probably 





Fig. 147. — Internal anatomy of a sea- 

 cucumber, one of the Aspidochirotce. 1, radial 

 vessel; 2, water-vascular ring; 3, blood-vascular 

 ring; 4, Polian vesicle; 5, oesophagus; 6, ventral 

 blood-vessel of intestine; 7, connecting blood- 

 vessel; 8, second part of intestine; g, 10, radial 

 longitudinal muscles; //, left respiratory tree; 

 iZf dorsal-blood vessel of intestine; 13, circular 

 muscles of body-wall; 14, Cuvierian organs; 

 '5, cloaca; 16, anus; 77, radial muscles of 

 cloaca; 18, cut edge of body- wall; iq, right 

 respiratory tree; 20, posterior edge of dorsal 

 mesentery; 21, median ventral longitudinal 

 muscles ; 22, third part of intestine ; 23, first 

 part of intestine; 24, gonad; 25, internal madre- 

 porites of two stone-canals; 26, dorsal mesen- 

 tery; 27, genital duct; 28, interradial; 2g, radial 

 piece of calcareous ring ; 30, genital opening. 

 (From Sedgwick, after Leuxkart.) 



