Phylum Echinodermata and Related Groups 551 



The sea urchins Hve on rocky shores, in sandy areas, and occa- 

 sionally on mud-bottomed areas; often they occur in tremendous num- 

 bers. Although they appear to be comparatively sessile animals, they 

 are able to move about by the combined action of the spines and the tube 

 feet. The spines are arranged on ball and socket joints and thus have 

 great freedom of movement. If the sea urchin is irritated, all the spines 

 of the region point toward the source of irritation. 



The heart urchin (Fig 183,;) has become nearly bilaterally sym- 

 metrical by elongation of the body. The mouth is at the "head end" 

 and the anus at the opposite, and the animal moves usually in only one 

 direction. The spines all project backward. 



The sand dollar, Echinarachnius (Fig. 183,c), is a flattened, 

 cakelike animal with minute spines. . 



With but few exceptions, the spawning of the sea urchins and sand 

 dollars occurs simultaneously in any one colony or "herd." The reac- 

 tion is apparently triggered ofif by the release of eggs or sperm of a single 

 individual ; this causes the release of the gametes from all the others in 

 the immediate area. In such a way, fertilization is assured. 



Economically these forms are of but small importance. In many 

 countries, the eggs of the sea urchins are valued as food items. For 

 the biologist, the eggs are of great interest, for they are used extensively 

 to demonstrate and study rates of development and experimental 

 procedures. 



THE CLASS HOLOTHURIOIDEA 



Unlike the forms so far considered, the members of this class have 

 soft leathery bodies which contain only microscopic calcareous plates. 

 The elongated, lifeless-looking sea cucumbers are often seen lying along 

 the sea bottom. At the anterior end is a mouth surrounded by tentacles. 

 On the outer surface are the usual five ambulacral grooves ; however, 

 usually only three possess tube feet and some species lack tube feet 

 entirely. 



The digestive tract is quite slender and elongated and connects to a 

 cloaca at the posterior end. From the cloaca, two branched series of 

 tubes, the respiratory trees, extend forward. Water is pumped in 

 through the anus into these tubes. Respiratory exchange takes place 

 through the thin walls. 



Like the members of the other classes, the water-vascular system 

 consists of a madreporite connected with a ring canal around the esoph- 



