8 Dr. Mac Culloch on the'Means 



the commencement and the termination of the process. It, is 

 also easy to see that a collateral provision is made* for this 

 purpose ; as the suture is not only thinner but more feeble at this 

 part ; being easily separattd by insinuating a knife into it there, 

 while it is scarcely possible to enter the point or edge at the 

 opposite side. 



Those of your readers, who may find a difficulty in comprehend- 

 ing the nature of this process, from the preceding description, 

 may easily satisfy themselves respecting it, by examining the 

 structure of the parts, in an animal so common. This may be 

 done, even after boiling ; when the structure and disposition of 

 the muscles are, indeed, even more easily understood than^h the 

 living animal. But I may, I believe, add, that your London, 

 readers at least need not be surprised, if their trials of the 

 powers of the living animals, in this respect, should fail ; as they 

 are generally too much exhausted in that market, to enable therh 

 to display this extraordinary faculty. 

 Shetland, July, 1820. 



EXPLANATION OF THE PLATES I. AND II. 



Plate I, Fig. 1. Is a whole leg of the spider crab, intended to convey a 

 general view of the articulations and motions of the legs of this tribe. 

 The extreme, or first phalanx, has a direct motion forwards, but cannot be 

 thrown backwards even into a straight line with the second, being checked 

 by the form of the articulation. These motions are effected by two 

 muscles which occupy the entire length of the second phalanx. 



The second phalanx has no direct motions at all, but its lateral motions 

 on each side are tolerably extensive, and are performed by two muscles 

 similarly occupying the cavity of the third phalanx. 



The motion of the third phalanx on the fourth is direct ; but like the 

 first articulation, this third one is so constructed that the limb can scarcely 

 be thrown back, even into a straight line. But the construction of the 

 joint allows a very extensive motion forwards, and the connecting ligament 

 is therefore lax and broad. The muscles of > flexion and extension occupy 

 the whole cavity of the fourth phalanx. 



The fourth articulation is. very peculiar, the ligament being so narrow 

 as to be scarcely visible ; and, in consequence of the form of the shell 

 and the shortness of the muscles, the motiori is very confined. It is lateral 



