Toronto. University. Dept. of '
Biology

Synopsis of animal parasitology

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QL

757

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ANIMAL PARASITOLOGY

ISSUED BY

THE DEPARTMENT OF BIOLOGY

UNIVERSITY OF TORONTO

THE UNIVERSITY OP TORONTO PRESS

Ql.
75-7
/ 6,7

SYNOPSIS OF ANIMAL PARASITOLOGY

INTRODUCTION.

Parasitism is a condition of widespread occurrence in both
plant and animal kingdoms, though much more characteristic of
certain groups than of others. Its basis is to be sought chiefly in
a nutritive relation in which one organism, the parasite, obtains
its food-supply directly from the living tissues of another organism
which in this way becomes the host. In this condition it is purely
a matter of the degree of parasitism whether the infesting organ-
ism, naturally a smaller species, occupies a superficial position on
the body of the host, as an ectoparasite, or as an endoparasite,
penetrates into its interior.

This parasitic association is one of the results of competition
among organisms, and is accompanied by structural changes
analogous to those taking place in other directions in free-living
organisms by adaptation and specialization. All organisms tend
to utilize the available resources of food-supply, but a fundamental
difference exists between those possessed of carbohydrate-building
power, after the plan of the green plants, and those which, lacking
this property, are dependent for their supply of organic food on
preformed material. It is the latter group, comprising all animals
and many plants, to which the developments of parasitism especi-
ally apply; since while the majority of species, the free-living
animals, so called, as well as the saprophytes among plants,
obtain their food-supply from dead organic material, whatever its
original source, many others as parasites form a direct association
with the tissues of a living host. The utilization of this source of
food-supply and the adaptations produced must accordingly be
interpreted in the same manner as for free-living organisms. The
evolutionary development of all organisms has its beginnings in
free-living habit, and accordingly the classification of organisms
into natural groups is based mainly on the characters of free-living
forms. All parasites belong to these same major groups and this fact
facilitates their classification, especially when degenerative changes,
which conmonly take place in portions of their organization, have
obscured their relationships. Many natural groups contain
parasitic members, but the occurrence of greater proportions of
parasitic forms in such groups as the Platyhelminthes and Nemat-

helminthes among animals and the Fungi among plants is an
indication that parasitic adaptations have been much easier of
accomplishment in these groups than in others.

The direct structural adaptations of parasites to their hosts
are exceedingly various. Fungus parasites among plants commonly
exhibit absorptive or haustorial filaments by which they set up a
more intimate association with the cellular materials of the host,
while animal parasites frequently exhibit special organs of attach-
ment and a protecting cuticle for the surface of the body. Such
adaptations, however, are of minor significance in comparison with
others which ensure the production of a large number of young and
the transfer of the parasite in one stage or another to its proper
host.

To all organisms the risks of life, or constructively the mechan-
ism of survival, is of prime importance. But it is especially sig-
nificant in parasites where there is added to ordinary risks the
isolation of the parasite when established in the body of the host.
Access on the part of the offspring to the body of a new host must
in this case become more than a matter of mere chance. Parasites
accordingly exhibit various adaptations for increasing the number
of young, and make use of peculiar associations on the part of the
host to ensure continuity from one generation to another. It is
also on this account that parasites commonly exhibit an intricate
life-history, involving considerable differences in successive
generations, and alternate primary and secondary hosts.

Parasites usually also give indications in their structure of
degenerative changes when compared with free-living members
of the groups to which they belong. These changes are adapta-
tions in the sense that they make way for other adaptations of
more positive nature, of importance in the economy of the organism.
Such modifications include among animals the reduction of the
nervous system, especially the organs of special sense, and of the
muscular and digestive systems; among fungous plants degenera-
tion of the normal thallus form.

There is doubtless a more fundamental relation existing be-
tween parasites and their hosts than is implied by adaptations of
a structural or mechanical nature. The fact that certain parasites
attach themselves to particular hosts, and to no others, or the
specificity of the organisms, has in all probability a chemical
basis in the composition of their respective tissues. This kind of
relation has been more thoroughly studied in the case of the
pathogenic bacteria and is becoming more and more important in
practical methods of serum therapy.

Parasitic animal organisms are distributed through various
natural phyla, the chief of which are:

(1) Protozoa, or Unicellular animals;

(2) Platyhelminthes, or Flatworms;

(3) Nemathelminthes, or Roundworms;

(4) Annulata, or Segmented worms;

(5) Arthropoda, or Joint-footed animals.

PROTOZOA.

The Protozoa are the lowest of animal organisms in the sense
that the functions and specializations of the individual are not
expressed by aggregation of mutually dependent cells. Four
natural classes are distinguished, viz.,

(a) Rhizopoda. Organisms with protoplasmic extensions of
the cell, pseudopodia.

(b) Mastigophora. Organisms with one or two large flagella
for locomotion.

(c) Infusoria. Organisms with the cell surface occupied
uniformly or in part by cilia, the latter sometimes (Tentaculifera)
present only in the embryo.

(d) Sporozoa. Organisms specialized as parasites and having
partition cells or spores, produced in large numbers.

Among the Rhizopoda are forms resembling ordinary amoebae,
but living in the human alimentary canal, especially the large
intestine. One of these, Endamoeba coli, lives as a commensal
in the first portion of the large intestine while E. histolytica {Amoeba
dysenteriae) is the causative agent of amoebic dysentery, character-
istic of the warmer zones, associated with inflammatory conditions,
haemorrhage and ulceration of the intestinal wall from penetration
and destruction of the mucosa. Both organisms have sporulation
stages suggesting those of Sporozoa. Other species of amoebae are
sometimes found on the oral, pulmonary and urino-genital surfaces.

The parasitic Mastigophora are commonly differentiated as
Trypanosomes and Herpetomonads, and the Spirochaetes
and Treponemata are cometimes included.

Trypanosomes are elongated spindle-shaped organisms, with a
longitudinal undulating membrane and an accessory parabasal
body giving origin to a flagellum which is closely associated with
the undulating membrane. They are blood-parasites occurring in
a great variety of vertebrates. Many appear to be harmless to the
host, the latter in all probability having become immune to their
presence. There is usually a secondary invertebrate host.

Trypanosoma lewisi from the blood of the rat has been thoroughly

studied and is commonly taken as the type. It is non-pathogenic
and is transmitted from one animal to another through the bite of
the rat-flea {Ceratophyllus fasciatus) or dog-flea {Ctenocephalus
canis).

The most important human trypanosome disease is sleeping
sickness, distributed through equatorial Africa from Uganda west-
wards. The organism, T. gambiense, is transmitted through the
bite of a tsetse fly, Glossina palpalis. The incubation period lasts
from ten days to three weeks, and the disease may be carried for
some months. It is characterized by nervous changes, producing
muscular fatigue and torpor, especially marked during the day.

A similar disease affecting African domesticated animals, and
carried in the blood of native animals is known as nagana. It is
transmitted through the bite of another species of tsetse fly, G.
morsitans, the direct causative agent being T. brucei. The carrier,
G. morsitans, has also been shown to be capable or transmitting
both T. gambiense and T. rhodesiense, the causative organism
of the "Rhodesian" type of sleeping sickness. Dourine, a try-
panosome disease of horses, the causative agent of which is T.
equiperdum, is significant because of its presence in Western
Canada, following introduction into North America in imported
Percheron stock.

The herpetomonads form a composite group consisting of
elongated cells with parabasal body and terminal free fiagellum
(Herpetomonas), similar cells with a free undulating membrane
attached to the flagellum (Crithidia), and simple cells without
fiagellum or membrane, occurring as intracellular parasites, but
having flagellated phases as in Herpetomonas. The cell forms are
significant for their resemblances to the trypanosome and ordinary
flagellate (Euglena) forms. Herpetomonas and Crithidia are
intestinal parasites of arthropods. The intracellular form, known
as Leishmania, includes L. donovani, the organism of Indian
kala-azar, and L. tropica, the organism of Delhi boil.

In kala-azar the parasites are found inside the endothelial cells
of the capillaries, for the most part, of visceral organs. The para-
sitic cell (Leishman-Donovan body) is rounded or oval in shape,
and contains two chromatin bodies, of which one, much smaller
than the other, is a parabasal body. When grown on a culture
medium the organism assumes a flagellated form like Herpetomonas,
indicating that the intracellular form is an adaptively fixed or
"resting" stage.

Delhi boil is a cutaneous disease contributed chiefly in India
and the Orient, but also more or less generally in the tropics. The
boils occur commonly about the face. The parasite is found in the

endothelial and lymph cells. Both Delhi boil and kala-azar are
thought to be transmitted by insects. Infection of vertebrate
hosts by Herpetomonas and Crithidia yields pathogenic results for
mammals. The organisms assume both flagellated and non-
flagellated forms resembling Leishmania.

Spirochaetes are slender wavy cells, with lateral non-undu-
lating membrane. The nucleus is represented by a series of
chromatin bodies, and there is a characteristic formation of internal
spores, indicating that the cells may be bacterial and not protozoan.
Several species have been studied in freshwater and marine molluscs
and in birds and mammals. Spirochaeta duttoni is the causative
agent of African tick fever, an important human disease distributed
through the Congo State and elsewhere in Africa. The organism
is found in the blood and is transmitted through the medium of a
tick, Ornithodorus moubata. In the body cells of the latter the
spirochaete breaks up, releasing a large number of minute "coc-
coid" bodies which pass out with the excretions of the tick and
appear to reach the human host in this way. These bodies are
also present in the eggs of the intermediate host.

Recurrent or Relapsing fever of European countries is also a
spirochaete disease, the organism being 5. recurrentis . It is pre-
sumably transmitted from the bodies of lice which are crushed on
an excoriated surface rather than through the bites of the insects
themselves.

Two forms of spirochaetes, S. buccalis and S. den Hum, are
known to occur in the human mouth, neither one being, however,
pathogenic.

The treponemata are slender wavy or spiral threads without
membrane. The chief form is Treponema pallidum. It is the
causative agent of the loathsome but widespread disease of the
urinogenital organs, syphilis. T. pertenue is the agent in the human
disease of warm countries known as framboesia or yaws, charac-
terized by raspberry-like excrescences of the face and other parts
of the body.

The parasitic Infusoria may for convenience be considered in
two divisions: (a) The natural group Tentaculifera, as distin-
guished from ordinary Infusoria or Ciliata, contains peculiarly
specialized tentacle-bearing forms {Acineta, etc.), the embryos of
which are ciliated and become parasites in the cells of other pro-
tozoa, (b) Ciliates parasitic in various higher animals, including
man. The chief human parasite is Balantidium coli. It is an oval
ciliated organism, with an oral depression at the anterior end of
the cell, meganucleus and micronucleus, and two contractile
vacuoles. It is the chief causative agent in ciliate (balantidian)

dysentery, an intestinal disease of man, characterized by alternate
diarrhoea and constipation, intestinal catarrh, and sometimes
ulceration of the wall. The organism occurs in the pig, with similar
forms in amphibia and annulates. Opalina ranarum, parasitic in
the rectum of frogs, resembles Balantidium, but is multinucleate.

The Sporozoa are entirely parasitic. Their chief feature is
reduction of motile structures and, in many cases at least, develop-
ment of a more resistant cell membrane. Of prime significance,
however, is the adaptation of the life cycle to rapid reproduction,
attained by means of multiple cell division or "sporulation".

The group contains a number of early recognized forms known
as Gregarinida, sometimes as "gregarine worms", of which some
are simple spindle-shaped or elongated organisms living in the body-
cavities of invertebrates, e.g., Monocystis in the seminal vesicles of
the earthworm, while others show a transverse division of the cell,
differentiating a head lobe or epimerite which serves for attach-
ment and may be lacking in some stages, e.g., gregarines of the
digestive tract of insects, notably Gregarina blattarum of the intes-
tine of the cockroach.

Coccidium types or Coccidia are intracellular parasites of
the epithelial cells of various animals. Eimeria stiedae ( = Coccidium
cuniculi) of the liver of the rabbit was one of the first studied.
Eimeria schubergi from the intestinal epithelium of the centipede
LitJwbius is commonly taken as the type. The organism penetrates
the epithelial cell in the sporozoite stage. It reaches maturity
at the expense of the host cell. There are two main phases or cycles
in the life history, one adapted for distribution of the parasite in
cells of the host, the other for transmission to another host animal
of the same species. The processes are as follows:

(a) Re-infection of host. The adult cell undergoes multiple
division into merozoites, each of which gains access to a new
epithelial cell. The process is purely asexual and is known as
schizogony.

(b) Preliminary stages to transmission. After the period of
schizogony the adult cells become differentiated into sperm and
egg mother cells. The latter becomes an oocyte or macrogamete
by extrusion of a certain amount of the chromatin. The former
undergoes multiple division resulting in the formation of a large
number of bi-fiagellate sperms or microgametes. The fertiliza-
tion is followed by membrane consolidation around the egg which
thus becomes an oocyst. There is then an internal division into
four spores each enclosed by a thick encystment membrane. The
protoplasm of each spore undergoes division into two sickle-shaped
sporozoites. The development is now finished unless the liberated

6

spores or the oocyst reach the intestine of another animal, in which
case the cyst membranes are dissolved by the digestive juices and
the sporozoites are set free to infect the host cells.

The Haemosporidia are sporozoa parasitic in the blood of
various hosts. They are of several types, the most important being
that of the Plasmodium group of malaria organisms.

Malaria as a disease is of three principal kinds, known as
tertian, quartan, and quotidian or aestivo-autumnal fever. The
organism of tertian fever is Plasmodium vivax. The fever phase
in malaria is periodic, occurring every 48, 72 or 24 hours (third day,
fourth day, daily) according to the type of infecting organism, or
in the quotidian kind resulting from multiple infection. Trans-
mission takes place through the bites of mosquitoes of the genus
Anopheles, ordinary mosquitoes of the genus Culex not carrying
the organism.

The life history resembles that of Coccidium in most respects.
The two phases are as follows :

(a) Re-infection of the host. The organism penetrates the red
blood cells. Multiple division or schizogony follows with produc-
tion of merozoites. These are set free by destruction of the host
corpuscle and each is able to penetrate a new cell. The formation
of the merozoites is coincident with the fever phase of the disease,
and it is in this phase that the use of quinine as a specific is effective.

(b) Transmission stages. Macrogametes and microgametes are
formed, the latter being thread-like cells given off from the side of
the parent cell which, after the sperms are formed, becomes a
residual body. Fertilization takes place only in the body of the
mosquito host. The fertilized eggs are motile vermicules, ookinetes,
which penetrate the wall of the intestine, and taking a position
behind the epithelium give rise to large oocysts. These contain, as
a result of repeated division, hundreds or even thousands of pointed
sporozoites. They escape from the cyst and are found distributed
through the tissues of the mosquito host, including those of the
salivary glands. Salivary secretion thrown from the glands into
the oral tube of the mosquito is the medium by which a second
human host is infected.

The association of malaria with low-lying regions is determined
by the breeding habits of the Anopheles host, the larvae of which
are aquatic. Removal of malaria infection is made possible by
drainage, use of kerosene, which prevents the larvae from breathing
the surface air on which they depend for respiration, and intro-
duction of small fishes which feed on the larvae.

Febrile conditions are periodic because of uniform times of

sporulation. The quotidian form, however, may be the result of
double or triple infections having different times of sporulation.

PLATYHELMINTHES.

Organisms with leaf-like or ribbon-like form of body, associated
with low organization of the mesoderm, which consists of primitive
mesenchyme and inter-cellular spaces but no continuous
coelom. A high grade of specialization of the reproductive organs
associated in most cases with hermaphroditism. The group
shows a range of specialization extending from free-living to highly
modified parasitic types, the latter showing progressive develop-
ment of life history adaptations to the hosts, including structural
devices, sucking discs and hooks, for attachment. There is corre-
sponding degeneration of nervous system, sense organs, and in-
testine. Three classes are distinguished:

Turbellaria. For the most part free-living flatworms, with
ciliated epithelium forming the surface layer of the body.
Simple or sacculated intestine with oral but no anal aperture.
Organisms of marine and fresh water, sometimes, but seldom,
parasitic. Type Planaria.

Trematoda. Flatworms with cuticle and one, two (distom)
or more sucking discs. Digestive tube somewhat as in turbel-
larians, but often reduced. Organisms ectoparasitic or endo-
parasitic, i.e., living on the surface or in the interior of various
vertebrate hosts. Ciliated only in larval stage. Alternation of
generations with change of host. Type Fasciola hepatica;
Primary host, Sheep; secondary host, Pond-snail.

Cestoda. Flatworms without alimentary canal, usually seg-
mented into proglottides with anterior scolex. Tapeworms.
An intermediate stage in the life-history and change of hosts.
Type Taenia solium; primary host, Man; intermediate host, Pig.
Convenient for laboratory study is Dipylidium caninum from in-
testine of dog; intermediate host, dog-flea (Ctenocephalus canis).

Trematoda. The ectoparasitic forms are less specialized than
the endoparasitic ones, and have also a direct development (mono-
genetic). The endoparasitic or digenetic forms present in this
way a more advanced phase of the parasitic development.

Fasciola hepatica is a lanceolate worm of 20 to 30 mm. in length,
living in the smaller bile-ducts of the sheep. The body is covered
by a resistant cuticular membrane of uncertain derivation. There
are two suckers, one anterior, encircling the mouth, the other
central. Older individuals show the coils of the uterus gorged
with fertilized eggs. The latter develop only in water into which

8

they have been discharged after passing through the bile-ducts
and intestine of the host. The egg produces a ciliated embryo
(miracidium) with eye-spots, apparently reflecting the free-living
ancestral stage, but with a boring organ at the enterior end of the
body. The embryo enters a pond snail (Lymnaea truncatula,
Europe; L. humilis, North America), giving rise to sporocyst,
redia, and cercaria stages, asexual or multiplication phases
through which the permanent specialization is gradually attained
and the number of young greatly increased. The tailed cercariae
become encysted on blades of grass, from which position they reach
the stomach of the sheep. The organism is accountable for the
disease known as "liver-rot" which has destroyed vast numbers of
animals in Europe, notably France.

Many species of "flukes" and "distoms", as the trematodes are
commonly called, are parasitic on the epithelial surfaces of the
digestive tube and its appendages, lungs, and urinogenital organs in
vertebrates, a few occurring in man. Fasciolopsis buski, the largest
species, is very like the liver-fluke of the sheep both in appearance
and development. It inhabits the small intestine of man and the
pig in China, India and other parts of the Orient. Paragonimus
westermani is a common lung parasite of China and Japan. Clon-
orchis sinensis, also Oriental, occurs in the liver ducts, producing
profound modifications of the liver substance. Schistosoma haema-
tobium and 5. japonicum are two remarkable blood-parasites of man
and mammals, the former widely distributed in Africa, the latter in
China, Japan and the Phillipines. 5. haematobium is unisexual,
the male, an organism of 12 to 14 mm. in length, having the body
greatly flattened and the sides rolled in ventrally so as to form
a tube. The female, much more slender and 20 mm. in length, is
found more or less completely enrolled in the tube formed by the
body of the male. The eggs appear in the urinary bladder. They
develop in the water into embryos, which require certain species of
water snails for further growth and reproduction. From the snail
cercariae escape into water through which they are transmitted to
man. The structure and history of the Japanese form are similar.

Cestoda. Taenia solium is one of two chief species having the
adult stage in man, the other being T. saginata. They differ in
their intermediate hosts, that of the former being the pig, and of the
latter the ox. T. solium is about 3 m. in length in average cases,
and lives on the epithelial wall of the human intestine. The
rostellum has a doubtlet circlet of hooks. There are four muscular
suckers. Passing backwards from the head or scolex there is a
gradual differentiation of proglottides, and of the formed prog-
lottids every stage is discernible from the appearance of the gonads

and ducts to the completion of the reproductive processes, ending
in the production of ripe proglottides containing myriads of fer-
tilized eggs and embryos. The embryos, enclosed by the egg
membrane, reach the alimentary canal of the pig and are set free.
The characteristic development takes place in the muscles where
the cysticercus is formed. The latter is transmitted to the
human intestine in uncooked or insufficiently cooked pork. The
cysticercus is of considerable size, being 6 to 20 mm. in length.

The various genera of tapeworms are distinguished by differences
in the position of the genital pores. In Taenia these alternate
on the lateral margins of the proglottides. In Dipylidium they are
paired. In Diphyllobothrium they occupy the middle points of the
ventral surface.

Echinococcus granulosus, a small tapeworm of the intestine of
the dog is significant on account of the large cysts produced in
man through the development of the cysticercus stage. The adult
form in the dog is a minute worm of 2.5 to 6 mm. in length, and
composed of a scolex and three segments. The cyst phase develops
in human beings, for the most part in the liver, and is distributed
among those who have habitual association with dogs, especially
Icelanders. The cysts contain innumerable scolices and reach a
gross size of 4 inches or even more in diameter.

Hymenolepis nana is a slender tapeworm of 10 to 45 mm. in
length reported from various European countries and North and
South America. The parasites when present at all are present in
large numbers, and the host effects are correspondingly severe.

Diphyllobothrium latum is a large tapeworm of 2 to 9 m. in
length, significant as a human parasite and on account of its aquatic
larval stage. The adult is found in the small intestine of dog, fox,
cat and man. The eggs develop in water, giving rise to spherical
ciliated embryos. These pass through a minute crustacean host,
but eventually reach the intestine of a fish, commonly the pike,
where they bore into the muscles and are found as small yellowish
encysted bodies known as plerocercoids. These are transmitted
to the human intestine through the medium of insufficiently
cooked infected fish.

NEMATHELMINTHES.

The Roundworms are cylindrical, unsegmented worms, hav-
ing a continuous body-cavity, the latter lacking typically a lin-
ing membrane. The surface is covered by a smooth cuticle. There
are three main divisions of the phylum, Nematoda, Gordiacea,
and Acanthocephala, the first-named division being the character-

10

istic one. While most forms possess a digestive tube there are
marked indications of its reduction, and in the Acanthocephala the
tube is lacking. Some roundworms are free-living, others parasitic,
the latter including forms parasitic only in larval or adult stage of
the life history, and also facultative parasites. Most parasitic
forms are intestinal, but important species occur in the muscles
and connective tissues of the host. There is frequently some
device for retaining position, such as thread-like or lance-like form
or development of hooks.

Nematoda, typical round or thread worms, include a striking
variety of forms and life histories, some of the more significant of
which are described below. One of the oddities of the group is
Anguillula aceti, the vinegar "eel", the female of which is 2.5 mm.
in length, the male smaller. The organism lives in ordinary
vinegar, subsisting on the fungus sometimes present. Similar
forms live in soil or decaying matter.

The type of the group is commonly taken from Ascaris, the
species of which are parasitic in the intestinal tube of various verte-
brates, A. megalocephala occurring in the horse, A. suum in the pig,
and A. lumbricoides in man. Ascaris lumbricoides is a yellowish
cylindrical worm, the male 15 to 17 cm., the female 20 to 25 cm.
in length. There are four longitudinal lines, respectively dorsal,
ventral and lateral, the former two containing nerves and the latter
excretory vessels. The mouth at the anterior end is enclosed by a
trilobed lip. The genital aperture in the female is ventral, about
one-third of the distance back from the head; in the male posterior
and terminal, the end of the more slender body being curved
ventrally and the aperture marked by the penial setae. There is a
complete digestive tract. The muscles of the body wall are of a
peculiar and in some respects primitive type, being transitional
between protoplasmic cells and fibres or showing the features of
both. The ovaries are paired and communicate with the outside,
in the position indicated, through an unpaired vagina. The male
gonad is a single thread opening posteriorly through a seminal
vesicle into the posterior end of the intestine. Development is
direct but there is a migration of the young from the intestinal
cavity, by way of the portal system, to the heart and lungs, fol-
lowed by a return through the respiratory tubes and oesophagus
to the intestine. Infection takes place through the water from the
soil. Occurs most commonly in young children.

Oxyuris vermicularis, a minute pin-worm parasitic in the
human intestine, is typical of a group of worms having a pair of
lateral flange-like extensions of the cuticle, which in the female

11

continue posteriorly into a lance-like point. The male is 3 to 5
mm., the female 10 mm. in length. The development is direct.

Trichuris trichiura (Trichocephalus tridiiurus) lives parasitically
in the human intestine. The male is 40 to 45 mm. in length, the
female somewhat longer. The anterior part of the body is narrowed
into a thin whip-like structure, from the presence of which the
term whip-worm as applied to this species is derived.

Strongyloides stercoralis is a minute nematode occurring in two
generations. A parasitic form in the human intestine, sometimes
regarded as a female reproducing parthenogenetically, is of about
2.2 mm. in length, the eggs of which develop in the lumen of the
intestine or mucosa of the host. A free-living form, differentiated
as males and females, the latter of about 1 mm. in length, develops
with higher temperatures outside the body, the females producing
about 40 eggs which develop into larvae. The latter give rise to
the parasitic phase.

The parasites known as hookworms include two important
forms which occur commonly in the southern United States, by
reason of having a general distribution in the warmer zones and
doubtless introduced largely through the importation of negroes
from Africa. The forms are Ancylostoma duodenale and Necator
americanus. They are scarcely distinguishable externally but
differ in small details of structure. A. duodenale is about 9 mm. in
length in the male and 12 mm. in the female. There is an open
buccal capsule, the aperture of which is directed dorsally, the
interior containing small recurved hooks. The posterior end of the
body in the male is modified into an expanded bursa supported
by a number of rays. In the female the somewhat round end of the
body is supplied with a short sharp spike formed from the cuticle.
The organism lives in the human intestine, infection taking place
through water, contaminated food or through wounds on the skin.

Many nematode parasites are adapted for living in the tissues.
The disease, primarily of the muscles, known as "Trichinosis", is
caused by a minute worm Trichinella {Trichina) spiralis. The
male is about 1.5 mm., the female 3 to 4 mm. in length. They begin
their parasitic phase in the intestine of the infected animal, the
latter being one of the lower mammals and commonly the pig, from
which the infection passes to man through insufficiently cooked
pork. The male remains in the intestine of the host but the female
burrows through the intestinal wall and produces live young to
a number of about 1,000. The embryos pass around through the
medium of the lymph spaces and ultimately find their way into the
muscles where they become encysted. They may remain in this

12

condition for many years, but become active and sexually mature
when liberated in the digestive tract of a new host.

The guinea worm, Dracuncidus medinensis, is a remarkable
tissue parasite occurring in warm countries, especially Arabia to
Hindustan. The male, an organism of about 20 mm. in length, is
seldom found. The female is a very slender thread-like organism
of 50 to 80 cm. It is found associated with ulcerated patches on
the body surface. This is the stage of discharging the young and
takes place about a year after the primary infection. The ulcers
are commonly found about the feet or ankles, the centre in each
case being occupied by a projecting portion of the worm. The
native method of extracting the worm is to catch the end between
the edges of a split stick, exercising a slight pull by rolling the
stick around. The process is continued for several days. If the
worm is broken, however, the direct spread of the larvae may
greatly increase the infection. Normally the larvae live in the
water and pass through a crustacean host {Cyclops). They gain
access to the human body through the ingestion of Cyclops with
drinking water.

The filariae are somewhat similar forms which are widely dis-
tributed through various vertebrate hosts, occurring for the most
part in the blood and lymph. Filaria bancrofti (F. sanguinis
hominis) is a human parasite of the lymphatic glands and other
organs, generally distributed through the warmer zones. The
female is about 50 mm. in length, the male being scarcely half as
long. The larvae are most common in the capillaries of the lungs,
and usually appear in the surface capillaries at night, or when the
patient sleeps. Infection takes place through species of mosquitos,
the latter playing the role of secondary hosts.

Acanthocephala, "thorn-headed worms", include a small
number of species in which the anterior end of the body is narrowed
and rounded, and possesses an armature of curved hooks. The
chief species is Macracantharhynchus hirudinaceus , of which the
male is 10 to 15 cm., the female 30 to 50 cm. in length. It is found
usually in the intestine of the pig but may occur in other mammals
and in man.

Gordiacea, "hair worms", are fairly common as adults in
fresh water, whiFe their larvae are parasitic in insects or molluscs.
Occasionally they occur in the human intestine. The common
field cricket is often parasitized by the larvae of Paragordius varius.

ANNULATA.

Two characteristic invertebrate phyla, the Annulata and
Arthropoda, are readily distinguishable from all others by the

13

segmented or metameric condition of the body. In the Annulata
of which the beach-worm, Nereis, the earthworm, Lumbricus, and
the leech, Hirudo, may be taken as examples, the segmentation is
homonomous, each metamere being similar to the others, with
the exception of an imperfectly developed prostomium, enclosing
the brain and overlapping the mouth, a peristomium, surrounding
the mouth, and a terminal metamere containing the anal aperture.
Each metamere bears primitively a pair of flattened appendages, the
parapodia; there is a coelomic cavity lined by a layer of peri-
toneal cells, a nervous system, the principal portion of which is
in the form of a ventrally placed double nerve-cord, and there are
segmental organs, nephridia, a pair in each segment, which
provide a communication between the coelomic cavity and the
outside of the body.

Most annulates are free-living, but the division of the Hirudinea
is made up of forms which are partly parasitic. The medicinal
leech, Hirudo medicinalis, formerly used for removing blood from
the body of patients, shows externally a subsidiary segmentation
in which five external rings correspond to one true or internal
segment. The body is dorso-ventrally flattened. There is an
anterior and terminal sucker, lodging the aperture of the mouth,
and containing three cutting or lacerating jaws, a feature of the
Gnathobdellida to which the medicinal leech belongs. A pos-
terior sucker, also terminal but on the ventral side, is for attach-
ment only. The anal aperture is placed above it. The digestive
tube is expanded into a crop or reservoir for the reception of blood
upon which the animal feeds.

The Hirudinea are for the most part parasites of lower aquatic
vertebrates. In some (Rhynchobdellida) the mouth is borne on
a protrusible proboscis, ordinarily found retracted into its sheath.
Limnatis nilotica, an African form of 8 to 10 cm. in length, some-
times lives as a parasite in the human nose and pharynx, the young
gaining access to the host through drinking water. Haemadipsa,
a tropical genus, includes a number of species significant as human
parasites, one of which, H. ceylonica, inhabiting Ceylon, lives in
moist earth from which it ascends trees, afterwards dropping on
the bodies of human beings or animals passing by.

ARTHROPODA.

The phylum Arthropoda, containing many parasites and
carriers of parasites, has in many respects the fundamental organiza-
tion of the annulate phylum from which its members are in all
probability derived. The segmentation is, however, heteron-

14

omous, the metameres being differently modified in different parts
of the body, and there is more or less extensive fusion of metameres,
giving rise to patterns sometimes characteristic of large divisions.
The appendages when present are also modified into organs of a
great variety of functions, and are distinguished by their jointed
character, associated with the development of a hard external or
exo-skeleton.

The common crayfish, Cambarus (Eur. Astacus), may be taken
as an example of the lower Arthropoda, the Crustacea, the mem-
bers of which are for the most part aquatic, breathing by means
of gills. In the crayfish and numerous closely allied crustaceans
both the segmentation and limb modification are highly organized,
but there are numerous forms less completely organized, more
generally or partly segmented, some of which make up a large
portion of the microscopic life or plankton of the water, while
others are common parasites of the external surface and gills of
fishes (parasitic Copepoda).

The Myriapoda or centipedes are terrestrial arthropods in
which gills are replaced by air-tubes analogous to those of insects,
but in which the segmentation and appendages are remarkably
uniform as in annulates. A few have been known to live in the
nasal cavity and intestine of man. Many are capable of inflicting
painful and sometimes dangerous bites through the modification
of an anterior pair of appendages into curved perforated poison
claws, which carry the secretion of poison glands into the wound
inflicted.

Arachnida, mites, ticks, spiders, and scorpions are air-
breathing, for the most part terrestrial arthropods, provided, except
in most of the mites, with air-tubes or with respiratory plates or
book-lungs. The body is typically divided into a small anterior
cephalothorax and a larger posterior abdomen. The cephalo-
thorax contains six fused segments bearing characteristic appen-
dages. The first appendages, chelicerae, are curved poison claws,
the second, pedipalpi, are jointed appendages in relation to the
mouth, the remaining four pairs are walking legs. The higher
arachnids or spiders proper normally use their chelicerae for killing
their insect prey but they are frequently able to attack larger
animals and to inflict poisonous bites.

The Acarina are smaller arachnids, commonly known as mites
and ticks, and significant both as parasites and as the carriers of
protozoan parasites. The external division of the body into
cephalothorax and abdomen is not in evidence, but the character-
istic appendages are present though in a modified form. Thus the
chelicerae and pedipalpi are modified for piercing and sucking,

15

the walking legs provided with bristles, claws or sucking discs for
attachment.

Among the forms retaining more or less a free-living habit the
more characteristic are the species of Tyroglyphiis, especially T.
farinae, known as cheese mites and living upon cheese, flour or
fruit. Nearly all forms of acarines possess six-legged or hexapod
larvae, the larva sometimes being the significant parasitic stage.
Thus the harvest mite or "jigger", to be distinguished from the
penetrating flea to which the name is more properly applicable, is
the hexapod larva of Trombidium, and was formerly recognized
as a distinct form to which in America the name Leptus autumnalis
was applied. The larva becomes prevalent in the early autumn
on low vegetation, from which it gains access to the human skin.
It burrows into the skin, prodducing small red spots with a lighter
centre, and bringing about considerable itching and irritation of
the part affected.

The true itch-mites, Sarcoptidae, permanent parasites of the
skin of birds and mammals, include the human itch-mite, Sarcoptes
scabiei. The body is whitish, rounded or oval, considerably smaller
in the male, the chelicerae pincer-like, and the pedipalpi jointed.
The males are less commonly seen since they die off after copula-
tion. The females burrow in the surface layer of the skin, forming
tunnels, in the course of which the eggs are deposited. Demodex
folliculorum, the mite of the hair-follicles, is an elongated worm-like
form, showing, however, the typical structure of the acarines. It
lives in the hair follicles and sebaceous glands of man, similar
species being found in the lower mammals.

The Ixodidae or ticks are blood-sucking acarines, of larger size
and with a somewhat leathery skin. They are common surface
parasites of the mammals (e.g., bush and cattle ticks). In the
European dog-tick, Ixodes ricinus, the eggs produce a hexapod larva
which attaches itself temporarily to a mammal host. Dropping
off after a time it is metamorphosed into a nymph or pupal stage
which repeats the performance. The nymphs finally give rise to
adults, males and females, which gain access to the mammal host,
where the former wander over the surface, while the latter attach
themselves by the anterior end in the surface layer of the skin, the
free part of the body becoming greatly distended. When fully
mature the female becomes detached.

The parasitic habit of the ticks is responsible for their impor-
tance as carriers. Thus in the case of the African tick fever, the
causative organism is a protozoan spirochaete (S. duttoni) but the
transmitting organism is the tick, Ornithodorus moubata. In America
a conspicuous example is afforded by the Rocky Mountain

16

spotted fever of sheep and cattle, to which human beings are, how-
ever, susceptible. The virus is a micro-organism Dermacentroxenus
rickettsi and is transmitted by the tick Dermacentor venustus.

Insecta or Hexapoda constitute one of the largest groups not
only of arthropods but of the whole animal kingdom. The plan
of segmentation, which may be exemplified by any common insect,
such as the cricket, Gryllus, exhibits a division of the body into
three regions, the head, thorax and abdomen. The head bears
a pair of antennae and the large compound eyes, together with
three pairs of mouth parts, modified in the case of Gryllus for
cutting plant material. The thorax bears three pairs of walking
legs, associated with three divisions of the thorax itself, respec-
tively the prothorax, mesothorax and metathorax, and there
are typically two pairs of wings, the occurrence and form of which
are subject to profound modifications. The abdomen is segmented
consisting of some ten segments, lacking appendages except certain
special ones serving the function of reproduction. Respiration is
by means of air tubes or tracheae, but there are various modifica-
tions in forms which are aquatic.

An extensive metamorphosis is one of the outstanding features
of the group, especially significant in its modifications in parasitic
forms. Typically, as in the cabbage butterfly, Pieris, the female of
the adult insect, here the air-living winged form, produces eggs
which on hatching give rise to a larva "caterpillar" phase, the form
of the body of which resembles rather closely that of the annulate
worm. It is an actively feeding phase, a very destructive one in
the case of many allied forms living on cultivated plants, which in
time is replaced by a quiescent phase, known as a pupa, and often
protected by an enclosing case or cocoon spun by the larva. From
the pupa there emerges finally the adult or winged sexual form.

The life history with its seasonal, sex, and food modifications is
exceedingly various in the insect group, and its details are of
significance in the case of a very large number of species which are
either parasitic or injurious otherwise. Of the parasites, some,
such as various species of mosquitoes (Culex), blackflies {Simulium),
and bed-bugs are temporary but irritating parasites of man.
Others, such as the lice (Pediculus, Phlhirius) and fleas (Pulex),
are more or less permanent parasites of the skin, while many, such
as the malarial mosquitoes {Anopheles) and the tsetse flies (Glos-
sina) are significant both as surface parasites and as the carriers of
disease, the primary agents of which are the lower organisms.
Some species, such as the bot-flies, and screw-worm flies, have
parasitic larval phases. In the bot-fly of the horse, Gastrophilus
equi, the adult female deposits the eggs upon the hairs of the host

17

from which they pass into the stomach. The larvae attach them-
selves to the gastric mucosa, finally passing through the alimentary
canal, and assuming the pupa stage outside the body of the host.

The classification of the insects, especially as regards parasitic
and otherwise aberrant forms, is a matter of some difficulty, though
some of the common free-living orders are readily recognizable.
The features used in classification are principally those of the
mouth parts and of the wings, of which there are typically present
two pairs attached to the thoracic segments. Orthoptera, in-
cluding the grasshoppers and cockroaches, have cutting mouth
parts. The anterior wings are tough and leather-like, while the
posterior wings are membranous, and when at rest folded. Coleop-
tera or beetles have the mouth parts of a cutting type, but the
anterior wings are formed into hard protective elytra, which at
rest meet in the middle line, though they can be spread apart so
that the thin posterior wings may be expanded for flight. Lepi-
doptera, moths and butterflies, have suctorial mouth parts, and
the wings, often beautifully coloured, are encased in a covering of
minute scales. Diptera or flies have suctorial mouth parts, vari-
ously modified, the anterior wings well developed and membranous
while the posterior ones are vestigal. Hemiptera or bugs have
the mouth parts formed into a jointed sucking tube, the anterior
wings typically hard at the base and membranous at the tip,
though this condition applies more definitely to the so-called
Heteroptera, there being some Hemiptera homoptera in which
the wings are membranous throughout. Many of the parasitic
forms belong to the Diptera and Hemiptera, though highly modi-
fied insects, such as the lice, are placed in a separate group of the
Ancplura, while others, such as the fleas, are representative of the
division of the Siphonaptera, the characters of these forms being
described below.

The common housefly, Musca domestica, an example of the
Diptera, owes its prevalence to the common occurrence of exposed
waste material of an organic nature in which the eggs are deposited
and the larval and pupal stages are passed through. The life
history is analogous to that of Pieris, except that the larvae or
"maggots" are acephalous, and the pupa is protected by the
larval cuticle instead of by a coccon. Though not a biting fly,
the mouth parts being suctorial in nature, the housefly is undesir-
able on account of its indiscriminate passage from food materials
to waste and vice versa, waste materials being liable to bacterial
infection, which may be thus communicated to food intended for
human consumption.

The family Muscidae, however, contains many species in which

18

the mouth parts are adapted for piercing and sucking and in which
the parasitic role may become of importance. One of the less
significant forms is the stable fly, Stomoxys calcitrans, the resem-
blance of which to the housefly is responsible for the commonly
held but erroneous notion that the latter is a biting type. Stomoxys
has also been suspected of being the transmitting organism of
certain important diseases, anthrax and infantile paralysis. One
of the forms now definitely known to be of the greatest significance
in the transmission of disease is the African tsetse fly, Glossina, of
the various species of which distributed throughout Africa, two,
G. palpalis and G. morsitans, are the carriers of the trypanosome
parasite of sleeping sickness.

Other flies are significant because parasitic in the larval or
"maggot" stage. These flies, typified by the European flesh-fly,
Wohlfahrtia magnified, a related Canadian species, W. vigil, and
the American screw-worm fly, Chrysomyia macellaria, and indirectly
by various infestations, including skin nodules or "warbles" in
lower animals, may be said to conform to the usual habitus of egg
or larval deposition and development in decomposing flesh, with
the modification that living sites are occupied. These are repre-
sented by portions of the skin surface, open wounds, discharging
nasal or auditory surfaces, or even the interior of the digestive tube.
The condition is known as myiasis, and further as cutaneous,
nasal, or intestinal according to the particular type of localization.

The mosquitoes, Culicidae, include a large number of dipterous
insects in which the mouth parts are somewhat differently modified
to form a piercing and sucking tube. They live upon plant juices
or animal blood; some, such as the species of common mosquitoes,
Culex, being for the most part significant as temporary parasites,
and others, such as the malarial mosquitoes (Anopheles) and the
yellow fever mosquitoes (Stegomyia), important as the carriers of
lower parasitic organisms. The life history includes aquatic
phases, the eggs, larvae or "wigglers" and the pupae being found in
water, a point of importance in controlling the spread of diseases in
which they play an accessory part.

The Hemiptera include, in addition to a variety of aquatic and
leaf-inhabiting forms, the common bed-bug, Cimex lectularius. In
this form the body is dorsoventrally flattened, the anterior wings
reduced to small dimensions, while posterior wings are absent.
The species is chiefly remarkable for its nocturnal habits, living
in out-of-the-way places, such as the crevices in bed-frames, and
attacking human beings at night. The eggs are deposited in
similarly concealed situations. The hemipterous type is consid-
ered to be that to which the term "bug" is to be specially applied.

19

The lice, Anoplura, are small, soft-bodied, wingless insects
having the mouth parts of a suctorial type but unjointed and with
the appendages ending in hooks. The human lice belong to a
family of the Pediculidae. Pediculus humanus, the head-louse, lives
on the hairy parts of the head, and the eggs or "nits" are deposited
upon the hairs. The body or clothing louse, hitherto classified as
P. vestimenti, has been shown not to be separable as a species from
the foregoing one. It lives, however, on the neck and trunk, the
eggs being deposited upon the clothing. Phthirius pubis, the pubic
or crab-louse, lives on the hairy portions of the body, especially in
the pubic region. As in the case of other surface parasites attacking
the skin the organisms are less significant from their immediate
presence than from the secondary effects for which they are account-
able; for example, the results of bleeding, or scratching of the skin
on the part of the individual affected, eczematous conditions and
surface infections by extraneous organisms being not uncommon.
They are also the most important, if not the sole agents, in the
transmission of the virus of typhus fever.

The fleas, Siphonaptera, of which there are many species
attacking various animals, are well, though peculiarly, segmented
laterally compressed insects in which the mouth parts are suctorial,
and in which the limbs, especially the hinder pair, are strongly
developed. The thinness of the body enables them to pass through
very narrow spaces, while their jumping capabilities are well
known. The adults are blood-suckers, and some are important as
carriers, notably the rat-flea, Xenopsylla cheopus, now recognized
as the transmitting organism of bubonic plague. The penetrating
flea "jigger" or "chigoe", Dermatophilus penetrans, occurs in the
West Indies, Central and South America, and elsewhere. The
female burrows with the head into the human skin, the body
increasing greatly in size and assuming a spherical form. The
human flea, Pulex irritans, lives in human habitations, usually
under unsanitary conditions. It is said to be chiefly European,
the dog flea, Ctenocephalus cants, which also attacks man, being of
more general distribution. The eggs of fleas are deposited in the
dirt of cracks in the floor and similar situations.

20

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Biological
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Toronto. University. Dept.
of Biology-
Synopsis of animal parasit
ology