1893.] Joint Formation Among the Invertebrata. 95 
will tend to draw this terminal segment backward (into) on 
the penultimate which enlarges with the increase of bulk of 
muscle, so that a well developed chelæ, as in the lobster, is 
found where the ultimate segment is pulled backward to 
about the middle of the penultimate segment. 
A true ball and socket joint rarely if ever obtains in the 
crustacea, and for very obvious reasons. The nature of the 
impassive connection of the joints would not allow of the 
motion and the active element, the muscle acts upon the 
inside of the joint or through its center, thus making it almost 
impossible for muscles to act so as to give the free movement 
of a ball and socket joint. The same end is obtained by a 
series of ginglymoid joints working in different planes so that 
in a series of three or four joints the end of the limb, for 
example, can be made to describe a circle, and I am inclined 
to believe that in order to obtain this free motion, the first two 
or three joints of crustacean limbs are as a rule very short | 
and work in different planes. 
Several objections may be easily found to the statement 
that posterior segments are always drawn into the anterior 
ones. Take for example, the shrimps, where the second 
abdominal ring overlaps the first so that the ring ahead of it 
as well as the others behind it is drawn within its anterior 
opening. I am inclined to regard this exception as more or 
less proving the rule, and to consider the second segment the 
fixed point into which the other two rings are drawn. We 
see a foreshadowing of this plan in the lobster group, where, 
as in Astacus, to use the words of Huxley, “the plure of the 
second somite are much larger than any of the others, and, 
their front edges overlap the small pleure of the first abdom- 
inal somite, and when the abdomen is much flexed these 
plure even ride over the posterior edges of the branchioste- 
gites. ”* | 
- An exception much more difficult to explain is that found 
in some of the anomurous forms as Galathea, where the fourth, — 
fifth, sixth and seventh segments follow the rule, and the first 
The Crayfish, an introduction to the study of zoology, by T. H. Huxley, 
New York and London, 1880, p. 98 and 99. 
