PHYSICAL SUPPORT AND MOVEMENT 



89 



which they have secreted. Most skeletons serving other functions are 

 either flexible or jointed. 



Function of Hard Parts in Movement. — Only occasionally are the 

 hard parts of much service in movement among the simpler animals. 

 One of the best examples of such use 

 is the earthworm, which is provided 

 with a number of spines, or setae, 

 projecting from the body in each 

 segment except a few at the ends. 

 These setae are operated by muscles 

 attached to their inner ends and 

 sloping off in different directions 

 (Fig. 75), like the ribbons of a May- 

 pole, to the body wall. When the 

 worm crawls forward, the outer end 

 of the seta is tilted backward, so 

 as to catch the soil, and in crawling 

 backward or holding fast in the worm's burrow the seta points forward. 



Sea urchins also have movable hard parts, which, however, are not 

 precisely a part of the locomotor equipment. The fleshy parts are 

 enclosed in a round shell, or test, the surface of which is studded, porcu- 

 pinelike, with a host of spines (Fig. 



76). These spines are capable of cr-lki M^iB—:-f 



movement in any direction and, when ^^?Q|/ Vf^l 



the animal is thrust over on its side 



Fig. 75. — Seta and muscles in the 

 earthworm, drawn from a longitudinal 

 section anterior to the clitellum; cm, 

 circular muscles, and Im, longitudinal 

 mucles. 



.A B 



Fig. 76. Fig. 77. 



Fig. 76. — A sea urchin, covered with a test and spines. {From Haupt ," F iindamentals 

 of Biology.") 



Fig. 77. — Relation of muscle to hard parts in appendages of insect and man. A, leg 

 of insect; B, leg of man; /, femur; fs, skeleton of foot; i, insertion of muscle; m, nmscle; 

 o, origin of muscle; ta, tendo-Achilles; ti, tibia. {A after Berlese; B after Hesse and Dofiein.) 



or back, may give it an irregular motion that helps it right itself. 

 But the main movement is effected by fleshy tubes ending in suckers. 



The fullest use of skeletal parts for movement is found in the insects 

 and their allies and in the vertebrate animals. In both groups the hard 

 parts are joined by curved surfaces, which permit free movement of one 



