21O ZOOLOGY 



3. "Aristotle's lantern 1 ' (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 Echinoderms (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 

 Ccelenterates 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. Perhaps they should be considered as 

 connected with the worms rather than with the Ccelenterates. 



242. General Characters. 



1. Larvae are bilaterally symmetrical; in the adult there is 

 a more or less complete radial arrangement of equivalent parts, 

 usually on the plan of five. In this radial plan all the princi- 

 pal sets of organs share: as the nervous, digestive, reproduc- 

 tive, etc. 



2. There is a complete differentiation of digestive tract and 

 body cavity. The latter is large. 



3. The blood- vascular system is partially differentiated 

 from the body cavity, but communicates with it. 



4. A calcareous exo-skeleton occurs, derived from the meso- 

 derm. It may consist of isolated spicules or united plates. 

 Associated with these are usually spines, from which the group 

 is named. 



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

 (closed except at one point), muscular sacs (ampullae) and dis- 

 tensible feet, serves a locomotor and respiratory function. 



