Summary. 
335 
The action of these muscle fibres is antagonized by the several 
ligaments; these contain a considerable number of elastic fibres, 
and are put on the stretch by the contraction of the heart. As 
soon, therefore, as the action of the muscle fibres cease, the liga¬ 
ments pull the heart back again to its original position and form. 
When the heart contracts, the cavity is in like proportion made 
smaller, the force of the contraction closing the valvular aper¬ 
tures of the heart and forcing the blood out through the arteries 
into the general circulation. The contraction being ended, the 
ligaments, by their elasticity, cause the heart to again dilate; 
the backward pressure in the arteries causes their valves to 
close. Now the valves of the apertures open, and by a decided 
suction the cavity is filled with blood from the pericardial cham¬ 
ber, to be again expelled by the contraction of the heart. This 
pumping action can be easily demonstrated by opening the 
pericardial chamber and using normal salt solution as indicated 
earlier in the paper. 
As has been stated, the heart which was modeled was in a 
state of contraction; in proof of this statement it can be asserted 
that only during the stage of contraction can the prominent 
ridge, Fig. 1, PI. V, be seen; it entirely disappears during the 
diastole of the heart. Moreover, those sections which pass 
through the valvular apertures show the folds of the valves 
closely applied to each other, (Figs. 5, 6, PI. VI.) while the 
arterial valves are widely open. 
In regard to the rapidity of the heart’s action, no method 
was found by which it could be observed under perfectly normal 
conditions. Such observations as were made gave an average 
of seventy-two beats per minutes. 
SUMMARY. 
The heart of our common Crayfish, Cambarus, differs from 
the accepted description of Astacus in the following particulars. 
(1) While both forms have the same number of valvular 
apertures (six), their arrangement is different. In Astacus 
there are two dorsal, two ventral and two lateral; in Cambarus 
there are two dorsal and four lateral, the ventral apertures of 
Astacus being absent. In Astacus the dorsal apertures are 
