ECHINODERMATA 211 



Starfish? Evidences? Find the madreporic body. Make a plot of all the os- 

 sicles in this region, noting the differences. Find the genital pores. 



3. "Aristotle's lantern" (the mouth apparatus). 



Examine the structure as a whole. How related to the body? Study the 

 parts in their relation to each other. Number, and method of action? 



Descriptive Text 



241. The Echinoderma (spiny-skinned) form a very distinct 

 group of animals, which in the adult condition at least show a 

 decided radial symmetry. They possess a more or less extensive 

 calcareous exo-skeleton with outwardly directed spines. The 

 starfishes, sea-urchins, brittle stars, sea-lilies, and sea-cu- 

 cumbers are representatives. They are marine in habit and 

 may be either fixed or slow-moving. They agree with the 

 Coelenterates in having radial symmetry, and in the absence 

 of a well-marked brain and other signs of cephalization. There 

 is considerable ground for believing that this is an outcome of 

 their sluggish habit, since the larval free-swimming condition is 

 bilaterally symmetrical, and radial symmetry is clearly adapted 

 to a passive life. It is difficult to determine the relationships 

 of the Echinoderms; yet it seems probable that their ancestors 

 were bilateral forms. 



242. General Characters. 



1 . Larvas bilaterally symmetrical ; adults with a more or less 

 complete radial arrangement of equivalent parts, usually on the 

 plan of five. In this radial plan all the principal sets of organs 

 share : as the nervous, digestive, reproductive, etc. 



2. Complete differentiation of digestive tract and body 

 cavity. The latter is spacious. 



3. Blood-vascular system partially differentiated from the' 

 body cavity, but communicating with it. 



4. Exo-skeleton calcareous, derived from the mesoderm; 

 composed of isolated spicules or united plates; usually bearing 

 spines. 



5. A water-vascular system present, consisting of a series of 

 tubes (closed except at one point), muscular sacs (ampullae) 

 and distensible feet, and serving a locomotor arid respiratory 

 function. 



