CHAPTER XI 



PHYLUM ECHINODERMA 



The Echinoderma are radially symmetrical, coelomate animals 

 with usually a subdermal skeleton of calcareous plates and a 

 vascular system known as the ambulacral system, used chiefly in 

 locomotion. All representatives of this phylum are marine in 

 habits. Here are included the starfishes, serpert stars, sea 

 urchins, sand dollars, sea cucumbers, sea lilies, and some groups 

 which are known only from fossil remains. Of the echinoderms 

 living at the present time, five distinct types of organization 

 are represented, hence there are five recent classes in the phylum. 



A B 



Fig. 82. — Diagrams to represent the formation of the rudiments of the coelom 

 and of the water-vascular system in an echinoderm as viewed from the dorsal 

 surface. Left hydrocoel rudiment in solid black; right stippled. A, constriction 

 of hydrocoel rudiments from coelom; B, left hydrocoel acquires communication 

 with exterior through stone canal; C, left hydrocoel encircles stomodaeum and buds 

 off rudiments of five radial canals, coelomic pouches increase in size and surround 

 the stomach. (Orig.). 



The radial symmetry is not perfect, for some structures are 

 distinctly bilateral in their arrangement. In the classification 

 of Cuvier, the echinoderms were included along with the coelen- 

 terates within the group Radiata. The superficial resemblance 

 in arrangement of parts seems to be an adaptation to the sessile 

 habit, developed independently in these two groups, and does 

 not indicate any phylogenetic relationship. Radial symmetry of 

 Coelenterata is primitive, while that of the Echinoderma is 

 only secondarily derived from a bilateral condition, as is evi- 

 denced by the marked bilaterality of the larvae. Details of the 



165 



