THE INVERTEBRATE LEGIONS 127 



thev move. These are little tubes that have suction discs at their ends and 

 w^hich operate by means of an internal bulb that pumps water into the muscular 

 foot, causing it to expand, and that sucks water from the foot, causing it to 

 contract. The strength of several hundred feet working together is considerable. 

 There are five groups of these spiny-skinned animals. 



Starfishes: Class Asterida — Figure 42 



As the name implies, these are star-shaped animals. They are especially 

 diverse on the west coast of North America where the sun star, with a diameter 

 of up to 2^2 feet, is the largest starfish known. All starfishes are carnivorous on 

 bivalve molluscs. They setde over a bivalve and exert a steady pull on the 

 bivalve's shells. Eventuallv, the bivalve tires, and the starfish then extends its 

 stomach into the bivalve and digests the bivalve right in its shell. Starfishes are 

 not very variable in form. (The very common large West Indian star, Oreaster, 

 is shown in figure 43.) Some starfishes are blue, and others are red, brown, or 

 even bright yellow. All can regenerate arms if these are lost. 



Brittle Stars: Class Ophiurida — Figure 42 



These are the most acti\'e of the echinoderms, able to move quite rapidly 

 with their long serpentlike arms. In fact, they are often called "serpent stars." 

 They are fragile and may even purposefully break arms off when they are 

 handled, a defense mechanism that occupies the predator's attention while the 

 brittle star makes its escape. In the middle of the underside of the body is a 

 mouth with serrate, toothlike edges. These are used to sift out and chew the 

 small pieces of organic matter on which these animals feed. Most species are 

 nocturnal, and some are very common in deep water. (A typical brittle star 

 is shown in Color Plate 10.) 



The basket star or Gorgon's head, Gorgoncephalus, is an extraordinary and 

 very sluggish brittle star that has finely branched arms. It is common in the 

 tropics and reaches a width of 2 feet, but because of its sedentary habits and 

 protective seaweedlike shape, it often goes unnoticed. It may be found adhering 

 to corals or clumped in the branches of sea plume gorgonians, where it catches 

 small, planktonic animals with its tentaclelike arms. 



Sea Urchins and Sand Dollars: Class Echinida — Figure 42 



The bodies of these animals are rounded and spinv in the case of sea urchins 

 and flattened and smooth in the case of sand dollars. There is a shell formed 

 of calcareous plates and on the underside is a five-toothed chewing apparatus 

 called "Aristotle's lantern" with which these animals scrape their algae food 

 from hard surfaces or chew organic matter. These animals move slowly by 

 means of tube feet and depend on their spines or hard shells for protection, for 

 their meat is tasty, forming an excellent chum for fishes. Sea urchins and sand 

 dollars range in all waters of the world over all types of bottom. The long spines 

 of the needle-spined sea urchins, Diadeina, common in all warm seas, are very 

 sharp and brittle and easily pierce flesh, gloves, or even shoes. The spines break 



