PHYLUM ECHINODERMA 169 



ment which is a prominent morphological character in these 

 groups. Recognition of this character is expressed in the estab- 

 lishing of two subphyla: the Eleutherozoa, which lack a stalk, and 

 the Pelmatozoa, which characteristically carry a stalk. 



Economic Importance. — Starfishes are among the worst 

 enemies of the oysters (Fig. 83) and cause great losses in that 

 industry. IXIost of the other echinoderms have little direct 

 economic importance. They feed chiefly on seaweeds, and in 

 turn some species serve as food for fishes. 



SUBPHYLUM ELEUTHEROZOA 



The echinoderms which are devoid of a stalk and are con- 

 sequently capable of free locomotion have been assembled 

 within the subphylum Eleutherozoa. The common starfishes 

 (Asteroidea), the brittle stars and serpent stars (Ophiuroidea), 

 the sea urchins and sand dollars (Echinoidea), and the sea 

 cucumbers (Holothuroidea) are the classes included within this 

 subphylum. 



Class Asteroidea 



The Asteroidea are the common starfishes. In these, the body 

 is composed of a central disc from which usually five arms radiate. 

 Arms may occur in multiples of five, though a few species fail to 

 adhere to the pentagonal form. In the species which have more 

 than five rays the extra rays are commonly added after the larva 

 has metamorphosed to a five-rayed condition. New rays continue 

 to bud off between the older ones until more than twenty rays are 

 found in full-grown individuals of a number of species. A sharp 

 differentiation of arms and disc is wanting in the cushion stars. 

 The coclomic cavity of the disc is continued into the rays and 

 many of the viscera thus extend into the arms. The mouth occurs 

 on the ventral surface, in the center of a membrane called the 

 peristome. Along the oral surface of each ray extend the tube 

 feet, which are confined to a depression called the ambulacra! 

 groove. 



Starfishes are found in all the oceans and at all depths. They 

 dwell on the bottom, though they frequently crawl onto sub- 

 merged piles and wharves. Many species live wholly in deep 

 water, while others are readily observed in the tide pools. 



Arrangement of Plates. — Each skeletal plate of the ambulacra! 

 grooves is provided with notches in the margins which articulate 



