ECHINODERMATA 



215 



tion of the body cavity. In some cases these two cavities are in 

 communication in the adult. The water vascular system consists 

 essentially of a ring-vessel about the mouth from which pass 

 radial tubes, one in each arm. From the radial tubes arise 

 lateral channels which communicate, directly or through bladder 

 like ampullcB, with distensible feet which reach the exterior by 

 pores in the skeleton (Figs. 99, 100). The tip of the foot may 

 be provided with a sucking-disc, serving as a means of attach- 

 ment and of locomotion. Frequently the walls of these feet are 

 thin and apparently serve for respiration, and the terminal 



Fig. 99. 



Fig. pg. Transverse section of the arm of a Stariish near the disc.^ Diagrammatic Lettering 

 as in preceding figure, a.r., ambulacral rafter (ossicle); op,, ovary, containing ova. 



Questions on figures 98 and 99. — What are the principal sets of organs repre- 

 sented 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 essenti'dl part of each is furnished by ectoderm, entoderm or mesoderm. 

 Is there a ccelom?;' Your evidences? What is the relation of the water-vascular 

 cavity to the coelom, in origin? 



"foot" at the end of each radius may be highly modified to 

 form a sense organ {tentacle). The feet, the ampullae, and even 

 the radial vessels may be wanting. The ring-canal, in typical 

 forms, communicates with the surrounding sea-water by means 

 of a tube {stone canal) which terminates in a sieve-like plate, 

 the madreporic body, through which the water enters the water- 



