214 



ZOOLOGY 



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

 in communication in the adult. It 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 ampullae, 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 attachment and of locomo- 

 tion. Frequently the walls of these feet are thin and apparently 

 serve for respiration, and the terminal "foot" at the end of each 



FIG. 99. 



d. b. 



r.p 



FIG. 99. Transverse section of the arm of a Starfish near the disc. Diagrammatic Lettering 

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



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

 sented^ 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 ectoderm, entoderm or mesoderm. 

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

 cavity to the ccelom, in origin? 



radius may be highly modified to form a sense organ (tentacle.) 

 The feet, the ampullae, and even the radial vessels may be want- 

 ing. 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- vascular system. In the 



