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 ccelome and the characteristic water vas- 

 cular system. The branches of this system, together with 

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

 arrangement. In development there is a marked distinc- 

 tion between mesoblast derived from gut pouches, and mesen- 

 chyme produced by immigrant amoeboid cells. There is 

 usually a very striking circuitousness or indirectness in 

 development. 



The Echinoderms are all marine. By reason of their 



FIG. 74. Pluteus larva with rudiment of adult. 

 (After JOHANNES MULLER.) 



durable skeletons, they are extremely well represented as fossils, 

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

 and shoivs no close relation to any other, whether in its living 

 or extinct representatives. 



The average habit is sluggish, and this may be correlated 

 with the constant development of lime in the tissues. This 

 power of Jorming skeletal substance is indeed so deep-seated 

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

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

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



