STAR-FISH. 



657 



curious kinds of spines with branched heads, known as paxillae (Px). 

 Between the plates there pass out thin-walled processes (Rsp), which serve 

 for respiration by bringing the fluid of the body-cavity into the proximity of 

 the outer water. Besides the paxillae, there occur on the outer surface of a 

 star-fish, as well as on that of a sea-urchin, small pincer-like organs called 

 pedicellariae. These resemble little pincers supported at the end of a 

 movable stalk, and they have probably been evolved from the smaller spines 

 that cover the teat of these animals. They move with extreme rapidity, and 

 catch hold of any minute object that is brought into contact with their inner 

 surfaces. They are variously supposed to hold small particles of food, to 

 clean the test of dirt, and to catch hold of fronds of sea-weed for the steady- 

 ing of the animal ; while the fact that in the sea-urchin they are sometimes 

 provided with poison-glands seems to show that they serve also as weapons 

 of ofience. At the extremity of each arm is a single tube-foot, with no 



FiQ. 5. — A Bhittle-stah (^OphiopTiolis aculeata). Two-thirds natural size. 



sucker at the end : this is always stretched straight out, and is known as the 

 unpaired tentacle. Immediately above it is a small eye coloured with red 

 pigment, and protected by small tentacles. 



Star-fish are sluggish animals, rarely moving of themselves, and staying for 

 days in the same position. They can, however, travel at a fair pace when 

 disturbed. When turned over on their backs, they are able to right them- 

 selves, but some of them, which have very stiff skeletons, take much longer 

 over the process than those that are able to twist their arms about in 

 acrobat fashion. They are, like most echinodernis, a sociable class, living 

 usually in swarms. Some species of shallow-water forms have been observed 

 to pair during the breeding season. They are voracious animals, eating all 

 kinds of food, and taking plenty at a time. The stomach can, in most cases, 

 be extruded through the mouth, and can enfold such articles of food as an 

 oyster ; star-fish are, in fact, dangerous enemies to oysters, and also attack 

 43 



