CHAPTER 8 

 PHYSICAL SUPPORT AND MOVEMENT 



In many animals the characteristic activities could be performed only 

 in the presence of hard parts which may collectively be termed the 

 skeleton. A skeleton is any more or less firm framework on or within 

 which the softer fleshy parts of the body are placed. The services per- 

 formed by the skeleton are chiefly of three types: (1) it provides support 

 for soft organs whose relations to one another could not otherwise be 

 maintained; (2) it protects delicate structures; and (3) it furnishes a 

 mechanism through which different types of movement may be executed. 

 Skeletons are widespread, from the protozoa to the largest mammals. 

 Such prevalence is testimony to their usefulness; yet some large groups 

 of animals (fiatworms, roundworms) and some members of other groups 

 (jellyfishes) get along without them. 



Support Furnished by Skeleton. — It is not practicable to separate 

 mere mechanical support from protection in many cases, though an 

 attempt will be made to choose examples 

 where this may be done at least in principle. 

 Sponges of all kinds possess narrow channels, 

 lined in places by collared cells (Fig. 33, 

 page 52) which take in food. Currents of 

 water are constantly maintained in these 

 channels by the flagella of the collared cells, Fig. 7.3. — e i e m e n t s of 



and it is essential that the passages be XTLttZX'o.S'T,^. 

 prevented from collapsing. While conceiva- spicules of different types. 

 bly the canals might be kept open by cells of ^^''"^ "^"^ ^^"''' '"^''^ Hertwig.) 

 firm consistency, they actually are kept open by means of a skeleton. 

 In the so-called bath sponges, this skeleton is a network of horny material; 

 in other kinds the skeleton is made of numerous limy or siliceous rods or 

 variously shaped objects called spicules (Fig. 73). 



Fresh-water mussels and marine clams bear on the outside of their 

 bodies a bivalve shell, consisting of two saucerlike pieces hinged together 

 at one edge and opening like a book. Between the edges of these pieces, 

 at certain places, water must enter and leave by fixed routes in order to 

 bring the animal its food and oxygen and remove its wastes. The actual 

 channels for the water are formed by the fleshy parts of the mussel, but 

 these fleshy parts must be kept in their proper positions. In many of 



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