257 
1916— 17.] Experiments and Observations on Crustacea. 
into the posterior condyle, but into the free edge of the basipodite in its 
medial part. Reference to fig. 5, in which the point of insertion of each 
muscle is shown relatively to the intercondylar groove, will make plain 
the way in which these four muscles combine to produce any particular 
movement at the joint (forward bending, backward bending, extension, 
flexion, rotation of the basipodite about its long axis, etc.), for the fixed 
point, the intercondylar groove, lies in the centre of the four outlying 
points of muscular attachment. 
Fig. 5. — Proximal end of basipodite of second left peneopod of 
Glyptonotus, seen from antero-medial aspect. x 4. Same 
basipodite as in the previous figure. 
a, tendon of posterior extensor muscle ; b, posterior 
condyle ; c, intercondylar groove ; d, tendon of anterior 
extensor muscle ; e, anterior condyle ; /, tendon of anterior 
flexor muscle ; g, tendon of posterior flexor muscle ; s, 
sesamoid calcification. 
Within each of the tendons is developed a small “sesamoid” calcifica- 
tion, each being laid down in relation to the articular membrane, which in 
these regions (see preceding table) is loose and lies in contact with the 
tendons. These structures are of no little interest ; for, if we could deter- 
mine the conditions that give rise to calcification here, we should be in a 
better position to understand the principles that underlie formation of the 
crustacean skeleton as a whole. * Apart from this wider question, their 
* A full discussion of the various possibilities that arise in examination of the sesamoid 
calcifications from this point of view would occupy undue space, and I will state simply 
that repeated compression, as opposed to tension, in the long axis of the tendon appears to 
be one of the most probable out of many possible factors involved in producing the calcifi- 
cation. Quite apart from the conditions that produce and maintain the special structure, 
is one result achieved by the presence of the calcifications, viz. protection of the loose 
VOL. XXXVII. 17 
