In dealing with this class of hosts, exclusive of man, I shall notice the entozoa of the various orders successively, taking the arrangement which I employed many years since when writing the Mammalian Division of a popular treatise called the ‘Museum of Natural History.’ The internal parasites of those orders which happen to embrace important domesticated animals will necessarily receive more attention than the others; some notice of the ectozoa being likewise added. PART I.--QUADRUMANA (Monkeys). Monkeys are less afflicted with flukes than most animals. The species known to me are _Distoma laciniatum_, found by Brongniart in the pancreas of _Simia maimon_; _D. orbiculare_ and _Amphistoma emarginatum_, from the intestines of _Cebus trivirgatus_; and _Bilharzia hæmatobia_, a single specimen of which I obtained from _Cercopithecus fuliginosus_. The monkeys of the Old World rarely harbor full-grown tapeworms, but Cysticerci are abundant (_Cyst. tenuicollis_, _C. cellulosa_, _C. pileatus_, _C. crispus_). The common hydatid (_Echinococcus polymorphus_) has been found in many of the Simiæ, and by myself in a Madagascar lemur (_L. macaco_). Dr Leidy also obtained three hydatid cysts from a large monkey. On the last day of the year 1857 I obtained some polycephalous hydatids (_Cœnurus lemuris_) from a ring-tailed lemur. They infested the liver, being more abundant in the lungs. They occupied both sides of the chest. Loose and detached specimens also existed in the cavity of the right pleura. Most of those occupying the chest were connected to the pleura, forming vesicular, semi-transparent masses, varying in size from a filbert to a large walnut, many being united in bundles of much larger size. One of these masses is here drawn (1, fig. 54). It consists of four large Cœnuri, their combined pedicles forming a single stalk. A fifth hour-glass-shaped rudimentary Cœnurus is also visible. Every Cœnurus supports a variable number of lobules, each lobe supporting one or more papillæ. Here and there the papillæ resemble chains of beads. No trace of tapeworm heads could be seen, but under a half-inch objective glass, I found some flat papillæ presenting oval depressions at the surface (2, fig. 54). On examining some of the larger papillæ they were found to consist of membranous layers folded one within the other. These were carefully dissected and examined with the aid of needles, when each one showed in the centre a well-formed tapeworm head with four characteristic suckers, and a prominent rostellum supporting a double coronet of hooks, thirty-two of the latter in all (3, fig. 54). The hooks displayed a marked disparity of size and form. Each hook showed conspicuous anterior and posterior root-processes, the larger set of hooks individually measuring about 1/225″ and the smaller 1/330″ in length (4, fig. 54). There were numerous calcareous corpuscles. The interior of each vesicle was filled with a fluid, in which there were no free scolices.