210 ELEMENTARY ZOOLOGY. 



opening on the aboral surface completes the tube. Trace these 

 structures in Fig. 63. 



In the sea-urchin the mouth apparatus is much more complex ; and 

 the tube is much longer, as is also the case in the sea-cucumbers, 

 but it is a good deal more simple and uniform. 



230. The Water -vascular System and its Functions. This system 

 is a device found in no other group of animals. It is the most 

 distinctive feature of the echinoderms. It is a series of vessels 



-d. b. 



FIG. 64. Transverse section of arm of a starfish, near the disc. Lettering as in 

 preceding figure, a.r., ambulacral rafter; ov, ovary, containing ova. 



Questions on Figures 63 and 64. What are the chief systems 

 of organs that have parts in the disc of the starfish? Which of 

 these have radial portions going into the arms? Follow carefully 

 the ectodermal, entodermal, and mesodermal boundaries. Locate 

 and identify the various structures lettered and determine as far 

 as possible whether the essential part of each is furnished by ento- 

 derm, ectoderm, or mesoderm. Is there a ccelom? Your evidences? 



communicating (usually) with the outside world, by means of the 

 madreporic body found on the aboral surface in the starfish. The 

 vessels contain sea-water, except in the sea-cucumbers, in which 

 they contain the same kind of fluid that is found in the body cavity. 

 In the starfish a tube runs from the madreporic body toward the 

 mouth (stone canal, Fig. 65, s). It communicates with a circular 

 tube running around the mouth (Fig. 65, r. c). From the circular 



