1916-17.] Experiments and Observations on Crustacea. 
249 
The Legs. 
Within the family Idoteidge, Miers (1883) distinguishes two subfamilies, 
Glyptonoteinse and Idoteinse. A distinguishing mark of the first is : “ The 
three anterior pairs of legs with the penultimate joint or propus dilated 
and forming, with the reflexible dactylus, a prehensile hand.” Under 
subfamily Idoteinse we read: “Legs all ambulatory; the three anterior 
pairs with the penultimate joint not dilated.” We may conveniently call 
the three anterior pairs of legs in Glyptonotus the gnathopods, and the 
four posterior pairs the perceopods. Before considering the special features 
of these two groups of limbs, one might comment upon certain features 
common to both and to the limbs of Ligia. 
Common Features. 
The taxis is isopodan, the flexion-complex uniplanar and tri-alternate. 
To cover a complete specification in serial order of the flexures of these 
limbs we may use the term “ full flexion-complex,” as distinct from 
“ uniplanar flexion-complex,” in which the limb is considered as moving 
in one “ principal ” plane. 
The Full Flexion-complex. — Two new movements are involved in the 
full flexion-complex. Assuming the principal plane to be vertical and 
transverse, the axis about which movement occurs at the mero-carpal 
articulation is vertical, not antero-posterior as in the case of most of the 
joints. The carpus articulates with the merum somewhat in the same 
way as the human wrist articulates with the forearm, forming in the rest 
position one straight line with the merum. The articulation being a simple 
hinge, from this position the carpus may be bent backwards or forwards 
like a patent door. 
The coxo-basal articulation is not a simple hinge. When the basi- 
podite is fully flexed on the ventral surface of the body it can still be 
made to execute an angular movement, of 30 to 70 degrees according to 
the particular limb in question, about its long axis. When fully extended 
(by convention extension occurs about an antero-posterior axis) it can be 
made to bend forwards about a transverse axis, and from here again in the 
medial direction it can be made to describe a cone about a vertical axis. 
In the extended position rotation about the long axis of the basipodite is 
minimal, and probably does not occur in the living animal ; contrariwise, 
in the fully flexed position there is no possibility of bending out of the 
principal plane. In either position the additional movements allow of 
a backward and forward swing of the (flexed) limb as a whole, as in 
