228 



ECHINODERMA. 



mesodermic tissues (mesenchyme), and in consequence there 

 is frequently a very complete skeleton. From the primi- 

 tive gut of the larva, pouches grow out to form the 

 usually spacious ccelofue and the characteristic water vas- 

 cular systein. The branches of this system, together with 

 the nerves, exhibit in most cases a typical five-rayed 

 arra7igement. In development there is a marked disti?ic- 

 tion between mesoblast derived from gut pouches, and mesen- 

 chyme produced by immigrant amc&boid cells. There is 

 usually a very striking circuitousness or indirectness in 

 development. 



The Echinoderms are all marine. By reason of their 



P"lG. 74. — Pluteus larva with rudiment of adult. 

 (After Johannes Muller.) 



du7-able skeletons, they are extremely well represented as fossils, 

 yet this does not alter the fact that the group is well-defned, 

 and shows no close relation to any other, whether in its livitig 

 or extinct representatives. 



The average habit is sluggish, and this may be correlated 

 with the constant development of lime in the tissues. This 

 poiver of forming skeletal substance is indeed so deep-seated 

 that lime may appear in almost any of the organs of the 

 body. The diet is vegetarian (most sea urchins), carnivorous 

 (starfishes ), or consists of the organic particles found in sand 



