MINIMAL SIZE IN PLANARIANS 991 
cross sections of the diverticula appear much as in the larger 
worms except for the reduction in the number of cells. The size 
of the brain and the diameter of the nerve cords bear about the 
same ratio to the rest of the body as in large individuals. Ina 
few cases there was but one eye instead of two and this was not 
a median one as sometimes occurs in small individuals but was 
in the position of one of the lateral eyes. The eye is of normal 
size, in relation to other parts and it has essentially the usual 
structure, but there is a great reduction of the number of the 
retinal cells. 
The pharynx, which bears about the same relative proportion 
to the body as in larger planarians, is composed of the same epi- 
thelial and muscular layers. The relative proportion of the par- 
enchyma and digestive organs is little altered in the smaller indi- 
viduals. The relative thickness of the outer epithelium is how- 
ever much greater, since it is composed of but one layer of cells 
which have the same size in the large and the small planarians. 
Pigment cells occur sparsely scattered over the dorsal surface 
and appear of enormous size in relation to the rest of the body. 
On the whole, the small individuals are strikingly like the larger 
ones in general form and the relative proportions of the systems 
of organs. | | 
Observations were made on the movements and reactions of 
these minute forms. Methods of locomotion, exploring movements 
of the head, reactions to light and contact, responses to mechan- 
ical stimuli, righting movements, and various other activities, even 
down to the most delicate details, were carried out in practi- 
cally the same way as In individuals of normal size. These facts 
indicate how effectively the functional unity of the organism is 
maintained notwithstanding the enormous reduction in the num- 
ber of its cells. 
One factor which probably determines the mininal size which 
may be attained is the fact that the size of the cells cannot be 
reduced and there must be a certain number of kinds of cells to 
preserve the physiological unity of the organism. There must be 
nerve cells, muscle cells, parenchyma cells, epithelium, etc., if the 
planarian is to be a planarian. The work of the organism, like 
