SKELETAL SYSTEMS AND MOVEMENT 



499 



ables such organisms to survive under condi- 

 tions that would destroy the naked types 

 such as amoebas; but as far as one can ob- 

 serve these naked protozoans are just as 

 widespread and abundant as those with 

 shells. Many other protozoans secrete inter- 

 nal skeletons of calcium carbonate or silicon, 

 which serve mainly to support the soft 

 bodies. Calcium carbonate shells make up 

 the globigerina ooze, and silicon shells make 

 up the radiolarian ooze that covers so much 

 of the sea bottom. The coral polyp secretes 

 a supporting skeleton of calcium carbonate, 

 into which it can contract and thereby se- 

 cure protection. The spicules and spongin 

 of sponges effectively prevent the collapsing 

 of a body that would otherwise be nothing 

 but a jellylike mass; in fact, massive sponges 

 could never have evolved without the sup- 

 port of skeletal structures. 



Exoskeletons 



Exoskeletons are characteristic of the 

 arthropods, but many lower invertebrates 

 secrete a cuticle which similarly serves to 

 protect the softer parts beneath. In many 

 Protozoa the pellicle also aids in maintain- 

 ing the shape of the body. The soft body 

 of many colonial coelenterates such as 

 Obelia is supported and protected by a 

 chitinous tube called the perisarc. The exo- 

 skeleton of an arthropod consists of a sub- 

 stance called chitin. Chitin is secreted by 

 the outer layer of the body wall and is made 

 hard by nonchitinous substances such as cal- 

 cium carbonate in the crayfish. What 

 amounts to an exoskeleton is present in cer- 

 tain vertebrates, such as the bony carapace 

 and plastron of the turtle covered by epider- 

 mal plates, the bony shell of the armadillo, 

 the epidermal scales of the scaly anteater, 

 the scales of fish and reptiles, the feathers of 

 birds, and the hairs of mammals. 



The chitinous exoskeleton of an arthro- 

 pod, such as that of the crayfish and of the 

 grasshopper, no doubt protects the animal 

 from certain enemies, but many amphibians, 



reptiles, birds, and mammals feed on prac- 

 tically no other types of animals. Such a 

 skeleton, however, serves for the attachment 

 of muscles and makes locomotion possible. 

 The arthropod skeleton covers the entire 

 body, but it is very thin at certain points, 

 thus enabling the animal to move parts of 

 the body and the appendages. 



Movements without 

 a skeleton 



Movement in the lower invertebrates is 

 usually accomplished without the assistance 

 of a skeleton. Thus amoebas use pseudo- 

 podia for this purpose; the flagellates, 

 flagella; and the ciliates, cilia. Many proto- 

 zoans possess contractile fibrils called 

 myonemes, which are of particular value in 

 contracting the body; the nature of the con- 

 traction is probably similar to that of true 

 muscles. The larvae of sponges are flagel- 

 lated and free-swimming, but the adults are 

 attached and incapable of moving from 

 place to place. Openings, however, such as 

 the osculum and certain pores can be closed 

 by muscle cells, the myocytes, which form a 

 ring around them. The epitheliomuscular 

 cells of the coelenterates are generally con- 

 sidered to be the most primitive type of 

 muscle cells. Also we encounter in coelen- 

 terates, for the first time, what may be con- 

 sidered a muscular system. In sponges the 

 myocytes are localized groups of cells; 

 whereas in the coelenterates, cells in the 

 epidermis contain longitudinal muscle fibers; 

 and those in the gastrodermis contain cir- 

 cular muscle fibers. The longitudinal fibers 

 bring about contraction of the entire body 

 and the bending of the body and tentacles; 

 the circular fibers by their contractions ex- 

 tend the body or tentacles and are responsi- 

 ble for the peristaltic waves that occur when 

 food is swallowed. 



Three sets of muscle fibers are present in 

 planarians and many other flatworms: (1) 

 an outer circular layer, (2) an internal longi- 

 tudinal layer, and (3) dorsoventral fibers 



