952 ANIMAL PHYSIOLOGY. 
Owing to the limitations of our space, the references to lower 
forms must be brief. 
We recommend the student, however, to push the subject 
further, and especially to carry out some of the experiments to 
which attention will be directed very shortly. 
In the lowest organisms (Infusorians) represented by Amoe- 
ba, Vorticella, etc., there are, of course, no circulatory organs, 
_ unless the pulsating vacuoles of some forms mark the crude 
beginnings of a heart. It will be borne in mind, however, that 
there is a constant streaming of the protoplasm itself within 
the organism. 
Among Ceelenterates (Figs. 254, 255) the digestive system, as 
_ yet but imperfectly developed, seems to embody in itself a sort 
of combination of the functions of the preparation and distribu- 
* tion of elaborated food; and it is worth while to note that even 
in the highest animals the digestive tract remains in close con- 
nection with the circulatory system. 
The heart is first represented, as in worms, by a pulsatile 
‘ tube, which may, as in the earth-worm, extend throughout the 
greater part of the length of the animal, and has usually dorsal 
and ventral and transverse connections. 
The dilatations of the transverse portions in one division 
(metamere) of the animal seem to foreshadow the appearance of 
auricles. 
The pulsation of the dorsal vessel in a large earth-worm is 
easy of observation. 
In the mollusks the heart consists of a ventricle and one or 
more auricles, and these chambers give off and receive large 
vessels (Fig. 227). 
These hearts may be observed pulsating with the naked eye 
or a lens in the clam, oyster, or snail, and are to be looked for 
in the first two on the side of the animal toward the hinge of 
the shell. 
It is worthy of note that in cephalopod mollusks (Cuttle- 
fish, Poulpe) there are branchial hearts, which may be re- 
garded in the light of pulsatile venous expansions, a remnant, 
perhaps, of conditions found in lower forms, in which we have 
seen that the rhythmically contracting tube plays a prominent 
role. 
In amphioxus, which is often instanced as the lowest verte- 
brate, the blood-vessels, including the portal vein, are pulsatile, 
while there is no distinct and separate heart; but, in connection 
with the above observations in cephalopods, it is to be re- 
