112 



MR. H. R. A. MALLOCK ON 



attachment of the jointed parts is made by a very short, flexible 

 ligament which joins processes on each, and may be compared 

 with two tubes connected as shown in text-fig. 1, the tubes 

 representing the skeleton of the part. Such a joint has only one 

 degree of freedom, and the constraint is very close *. 



The close constraint imposed by these connections makes the 

 joints rather liable to damage from accidental forces, and it 

 seems probable that the limit to the size to which articulate 

 animals can be developed is in a great measure determined by the 

 brittleness which necessarily follows this type of construction. 



Text-figure 1. 



Diagram of typical Arthropod joint. 



When a moving body is suddenly stopped, as when striking 

 the ground after a fall, the forces called into play are propor- 

 tional to the mass of the body, and if the force is so applied as to 

 tend to turn a limb in any direction except that in which it is 

 properly intended to move, the reaction at the joint, i. e., the 

 force tending to break the ligament, will be, for similar structure, 

 in the ratio of the mass to the square of the linear dimension — 

 or, in other words, the chance of breakage decreases directly with 

 the size. 



The great difierence between the size of the largest articulate 

 and largest vertebrate rather bears out this view. 



The heaviest land f arthropod — a beetle — may weigh something 

 over an ounce, and an elephant 4 or 5 tons. Some of the aquatic 

 articulates — crayfish, I believe — weigh 30 pounds, and of the 

 aquatic vertebrates, whales approach 100 tons. 



Where more than one degree of freedom is required in the limb 

 of an articulate, this object is generally attained by placing two 

 or more joints of one degree of freedom in close succession, and 

 in nearly all cases the legs are thus constructed, the joints of 

 the coxje, trochanter, and femur being close together with their 

 axes mutually set at right angles. Text-fig. 2 shows diagram- 



* I am informed by Dr. Caiman that in the leg-joints of some crustaceans one 

 of the connections sketched in text-iig. 1 is absent, so that in these cases the joint 

 allows partial freedom of two degrees. 



t Excluding Land-Crabs. 



