The effect of animal parasites upon the morbidity and mortality of wild beasts and birds in captivity is by no means clear, and Doctor Weidman and I are inclined to be sceptical, with certain reservations of course, of their great importance in the death rate. Doctor Weidman has kindly agreed to contribute a chapter upon the general distribution of protozoal and metazoal parasites with a summary of their probable pathogenic importance. The groups known to have a decided pathological power might be divided into the toxic, the tumor formers and the mechanically obstructive; certain parasites have properties placing them in two of these classes. The first group comprises the hemosporidia and hemogregarines, the uncinaria and some of the cestodes, forms which produce hemolysis and hemorrhages with varying grades of anemia. The importance of this group is shown chiefly among the Aves, in which high grades of anemia are occasionally met from malarial infections, but cats and dogs or even herbivores also frequently suffer from hookworm. The tumor-producers are chiefly echinococcus worms, the cysts of which may grow large enough to occupy nearly the entire abdomen. A certain grade of anemia and general ill health accompany this hydatid disease, partly the result of a toxin and partly by damage to important viscera. Those parasites which obstruct mechanically do so by their own bulk or by an accompanying inflammation, incited by them as foreign bodies or by bacteria which have gained entrance at the irritated point. This is exemplified by the enormous collection of nematodes sometimes found in reptiles (a pailful was removed from a python) and by the tightly coiled or tangled thread and tape worms frequently found in birds. The effect of swelling by the mucous membrane under the influence of worms is illustrated by the infestation of the proventricle in parrots. Here spiroptera penetrate into and under the glandular layer which swells and pours out mucus, the total mass of nematodes, mucus and tissue obstructing the passage.