PHYLUM VIII, ECHINODERMATA 



Typically radially symmetrical marine animals with a skele- 

 ton of calcareous plates or spicules embedded in the skin. The 

 arrangement of the skeletal plates and of the internal organs is 

 usually pentamerous, the numeral five being thus the governing 

 number of the echinoderms. They are likewise characterized 

 by the presence of a water vascular system Avhich functions in 

 respiration and movement. 



They represent an advance upon the Coelenterata in the pres- 

 ence of a digestive tube distinct from the body cavity (coelome), 

 in their more highly developed nervous system, in the possession 

 of a blood vascular system and in an almost exclusively sexual 

 mode of reproduction. 



The seven classes of the Echinodermata form a more or less 

 closely related group. If the comparison is made with consid- 

 erable latitude, the arms of the starfish and crinoid are homolo- 

 gous to the ambulacra of the cystoid, blastoid and echinoid ; 

 moreover, if the starfish be placed with mouth uppermost, the 

 lower side of the central disk corresponds to the base of the cys- 

 toid, blastoid, and crinoid, while the upper side with its central 

 mouth and radiating ambulacra is similar to the upper side of 

 these classes, and internally the position of the ambulacral, 

 blood vascular, and nervous systems is then similar ; in echi- 

 noids, a like orientation with the starfish may be made by bend- 

 ing the arms of the latter until they almost meet dorsally. 



The three classes, — cystoids, blastoids, and crinoids, — the 

 individuals of which are usually fixed to some foreign object, 

 are of all the Echinodermata the most intimately related. 

 The blastoids are nearest to the cystoids, the hydrospire prob- 

 ably corresponding to the pore-rhombs, while the position of 



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