PHYLUM 7. 

 ECHINODERMATA. 



Marine animals with a radially symmetrical, five-rayed body, with cal- 

 careous spicules or plates in the body wall, and a system of tubes extend- 

 ing throughout the body known as the ambulacral or water-vascular 

 system. 



External Form.— The larval echinoderm is typically a minute, bilat- 

 erally symmetrical animal (Fig. 964) which swims about actively in the 



surface waters of the sea; it goes 

 through a complicated metamor- 

 phosis and is finally transformed 

 into the radially symmetrical adult, 

 which lives on or near the bottom, 

 either creeping, or in some cases 

 swimming, slowly about, burrowing 

 in the sand or in the rocks, or 

 permanently attached by a stalk to 

 one place. The body of the adult 

 is made up of ten principal di- 

 visions, which radiate from the 

 main body axis like the spokes of a 

 wheel (Fig. 965). These divisions 

 are the five radii or rays (1) and 

 the five interradii or interrays (2), 

 which alternate with them. The main axis of the body passes between 

 its two most important surfaces. These are the oral or under surface, 

 in the center of which is usually the mouth (3), and the aboral or 

 upper surface, which is opposite to it, and in most cases contains the 

 anus (4). The relative length of its main axis determines the gen- 



* See "Invertebrate Animals of Vineyard Sound," by A. E. Verrill, Rep. of U. S. 

 Fish Com. for 1871. "Echinodcrmata," by W. N, Lockington, Stand. Nat. Hist., 

 Vol. 1, 1888. "Echinodcrmen," by II. Ludwig and O. Hamann, Bronn's Klasseu u. 

 Ord., 2 Bd., 3d Abt., 1889-1904. "Distribution of the Echinoderms of Northeastern 

 America," by A. E. Verrill, Am. Jour. Sci., Vol. 49, p. 127, 1895. "Echinoderms of 

 the Woods Hole Region," by II. L. Clark, Bull. U. S. Fish. Com., Vol. 22, p. 543, 1902. 

 "Les Echinodermes," by Delage et H6rouard, Trait6 de Zool. Coner, 1903. "Echino- 

 dermata," by E. W. MacBride, Camb. Nat. Hist., 1906. "Echinoderms of Connec- 

 ticut," by W. R. Coe, Bull. No. 19, St. Gool. and Nat. Hist. Sur., 1912. "A Biological 

 Survey of Woods Hole and Vicinity," by F. B. Sumner and others, Bull. Bur. Fisheries, 

 Vol. 31, 1911. 



614 



Fig. 964 — Larvae of echinoderms (after 

 J. Miiller). A, youngest stage, common 

 to all the classes ; B and C, two stages of 

 the auricularia (Holothurioidea) ; D and 

 E, two stages of the bipinnaria (Asteroi- 

 dea) ; F, a pluteus {Echinoidea). 1, 

 mouth : 2, anus. 



