AMERICAN NATURALIST 
VoL. XX VIL. February, 1893. 314 
JOINT FORMATION AMONG THE INVERTEBRATA. 
By BENJAMIN SHARP. 
z The following observations were prompted by a discussion 
= on the subject of joints, at a meeting of the Academy of 
Natural Sciences of Philadelphia held’ on November 92nd, 
1892, when Prof. Cope considered some facts relating to his 
theory of joint formation as seen among the vertebrata. 
i As his theory has been known for some time, and an 
extended consideration of it has recently been published’ I 
merely quote as a thesis, a sentence published in 1878’; 
“change of structure is seen to take place in accordance with 
"the mechanical effect of three forms of motion, viz, by friction, 
Pressure and strain.” 
: It occurred to me that if his theory had a general application, 
= Some additional proofs could be shown to exist among the 
-= Invertebrates, where we have the action of muscular force 
upon hard and resisting parts of the skeleton. Those which 
“present the best study for this purpose, appeat to be the 
a crustaceans, where we find an immense variety of articulations 
_ im the body and in the limbs; highly complicated locked 
joints, others allowing motion in but one plane, as well as 
: __ ‘The Mechanical Causes of the development ‘of the Hard Parts of the 
_ Mammalia, by E. D. Cope. Jour. of Morph. Vol. 1I, 1889, p- 137-290. 
*The relation of animal motion to animal evolution, by E. D. Cope, Amer- 
“e ican Naturalist, Vol. XII. 1878, p- 44- 
i 
