PHYLUM ECHINODERMATA 



309 



OTHER ECHINODERMATA 



Starfishes 



The principal characteristics of echino- 

 derms have been described in our account 

 of the starfish. Three orders and about 20 

 famihes of starfishes are recognized. About 

 1200 species are known. Usually 5 or multi- 

 ples of 5 rays are present. These rays may be 

 long or short, sometimes so short that the 

 body resembles a 5-sided pad. Starfishes are 

 common marine animals all over the world 

 and may be found in both shallow and deep 

 water. 



Brittle stars and basket stars 



Brittle stars (Fig. 190) and basket stars 

 have slender or branched flexible rays; the 

 tube feet have largely lost their locomotor 

 function and serve as sensory and respiratory 

 organs. Food consists of minute organisms 

 and decaying organic matter lying on the sea 

 bottom. Locomotion is comparatively rapid. 

 The rays bend like a whip and enable ani- 

 mals belonging to certain species to "run," 

 cling, and probably swim. The term "brittle 



star" is derived from the fact that these ani- 

 mals break off their arms when they become 

 injured. 



Sea urchins 



A common type of sea urchin (Fig. 190) 

 is Arbacia punctulata, a purple-colored spe- 

 cies that lives in both shallow and deep 

 water from Cape Cod to southern Mexico. 

 It is somewhat globular in shape. The test 

 (shell) (see head piece, p. 303) is made up 

 of calcareous plates which bear movable 

 spines about 25 mm. long. There is a system 

 of plates in the ray, 5 pairs of columns of 

 ambulacra] plates, so called because they 

 are penetrated by tube feet and 5 pairs of 

 columns of interambulacral plates (see 

 headpiece). These correspond to the same 

 regions on the starfish, assuming that the 

 rays are folded back on its aboral surface. 

 Most of the skeletal plates bear spines which 

 are attached by muscles and move freely on 

 little knoblike elevations called tubercles. 

 The food consists of plant and animal mat- 

 ter which falls to the sea bottom and is 

 ingested by means of a complicated struc- 

 ture known as Aristotle's lantern (Fig. 195). 



Genital pore- 



Rectum 

 Spine 



Esophagus 

 Radio 



Ring can^ ^ 



Coelom — -, - - — 

 Tube feet 



Stomach 



Ampullae 

 Tooth 



Anus-1 



Madreporife 



— Genital pore 

 Gonad 



Stone canal 

 Stomach 



Mesentery 

 Aristotle's 

 lantern 



Pediceilaria 



Figure 195. Sea urchin, Arbacia, showing both external and internal structure. 



