NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
12 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ II, I
Eggs. The structure of the eggs in Cestoda is very variable. Usually the
hexacanth embryo is surrounded by one or more envelopes. Its immediate cover
is called the embryophore and the outer covering is the egg-shell. The space between
the two is filled by the third envelope of the egg. In the genera Moniezia and
Anoplocephala the embryophore develops two processes at one end which cross each
other. This is called the pyriform apparatus. In the genus Tænia the egg shell
is lost and its place is taken by the embryophore which becomes very thick and
radially striated. In some genera of Pseudophyllidea there is an operculum at one
end of the egg.
The eggs may be colourless as in Hymenolepis and Dipylidium, light yellow as
in Diphyllobothrium, or brownish as in Tænia.
Development and life-history. All the species of Cyclophyllidea and Tetraphyll¬
idea parasitic in domesticated animals, so far as is known, require two hosts for
the completion of their life-history. The intermediate hosts become affected by
swallowing the eggs passed out by the primary hosts in their fæces. The develop-
ment does not proceed beyond the larval stage in the intermediate host. The final
host acquires infection by feeding upon the intermediate hosts or their affected
parts. In Pseudophyllidea, so far as is known, three hosts are required for the com-
pletion of the life-history. The first intermediate host is a cyclops or some other
crustacean, the second is a fresh-water fish and the third is the final host in which
the adult form develops.
Be the intermediate host a vertebrate or an invertebrate, when the egg is
swallowed by an appropriate host the egg-shell is dissolved in the digestive juices
and the embryo escapes out of it. It penetrates the intestinal wall and establishes
itself in its normal habitat, which varies in different cases. If the intermediate host
is a vertebrate one, it usually enters the vascular or lymphatic system and establish-
es itself in various organs, such as muscles as in the case of Tæniarhynchus sagina¬
tus and Tænia solium, liver as in the case of Tænia taeniformis, lungs, liver,
spleen, etc. as in the case of Echinococcus granulosus, in the body cavity as in
Tænia hydatigena, or brain as in Multiceps multiceps. When, however, the inter-
mediate host is an invertebrate, the larval stage is found in the body-cavity, e.g.,
Diphyllobothrium latum and Amoebotænia sphenoides.
Growth. After the scolex is attached to the wall of the alimentary canal by
means of the suckers or hooks, wherever these are present, it buds off segments
posteriorly. As the segments pass further posteriorly, they develop genitalia and
later become mature and ultimately the uterus is filled with eggs and the segments
become gravid.
HELMINTH PARASITES AFFECTING DOMESTIC ANIMALS
13
Larvæ.
1. Solid larvæ. There are two kinds of these larvæ, globular and elongated.
The former is called a Plerocercus and the latter a Plerocercoid.
2. Bladder larvæ. In these the cells in the centre of the segmentating embryo
liquify. Three main types of the bladder larvæ can be distinguished :—
A. Cysticercus. In this form the onchosphere enlarges and becomes converted
into a body having peripheral cells and a central cavity containing fluid. An inva-
gination of the wall takes place and at the bottom of this develops a scolex which
can be evaginated. Cysticercus cellulosæ and C. bovis grow to the size of a pea and
C. tenuicollis to that of a pigeon's egg.
The cysticerci present some variations and assume different names. A few of
them are given below :—
(i) Strobilocercus Sambon, 1924. This occurs in Tænia tæniformis in which
a chain of segments are budded off. The genitalia are, however, not
developed in them.
(ii) Dithyridium Rudolphi, 1819. This is probably the larval form of the
species of Mesocestoides. It is elongated and possesses a solid body
without a bladder. Anteriorly the scolex is invaginated into the body
and possesses four suckers only, the rostellum being absent.
(iii) Cysticeroid Braun, 1883. This resembles a cysticercus, but the bladder
is small and is usually absorbed or cast off.
B. Cœnurus Rudolphi, 1803. This consists of a bladder and many heads.
Each of the latter is produced by an invagination in the same manner as in cysticer¬
cus. It is usually of the size of a hen's egg.
C. Echinococcus Rudolphi, 1801. The larval form of Echinococcus granulosus,
a cestode found in the dog, cat and even man, is called a Hydatid cyst. It is the
largest of all the tapeworm cysts and may attain the size of a football. It is
filled with albuminous liquid which may contain brood capsules and daughter cysts.
The brood capsules are developed from invaginations of the cyst walls, which
develop scolices on them. They separate from the cyst wall and float in the liquid.
After some time the parenchyma of the brood capsule disintegrates and the scolices
become detached from one another. If the fluid from the hydatid cyst is allowed
to stand and the sediment at the bottom, which is called " sand ", examined, it
will show scolices and brood capsules. The daughter cysts which may develop
internally or externally resemble the parent in every respect. These hydatid
cysts are found in man, horse, donkey, camel, rabbit, dog, cat, pig, goat and sheep,
in almost any situation of the body.
B
14 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [II, I
NEMATODA.
Nematodes, popularly known as round worms, are parasites belonging to the
phylum NEMATHELMINTHES. They inhabit various organs of the body.
Form of body. Nematodes are elongated, cylindrical animals usually attenua-
ted at one or both ends. In the genus Trichuris the anterior end is much elongated
and drawn out into a thread-like portion; while in Oxyuris the posterior end is
drawn out into a whip-like prolongation. In some cases the head is tilted either
dorsally or ventrally. The posterior end of the male is, in some cases, spirally
coiled.
Cuticle. Covering the entire body externally there is a tough, elastic, resist-
ant cuticle, the thickness of which varies from two to several layers. It is usually
marked with transverse striations and in some genera there are longitudinal lines in
addition. In some genera, e.g., Gnathostoma, the cuticle is spiny and in the genus
Gongylonema the body is covered anteriorly with tubercles called plaques or bosses.
In the genus Nematodirus the female has a pointed process at its posterior extre-
mity. In the genus Dracunculus the anterior dome-shaped extremity is provided
with a cuticular thickening or shield and the genes Parabronema has dorsal and
ventral shields anteriorly. Some forms have cuticular cordons anteriorly. In some
genera, e.g., Œsophagostomum and Chabertia, there is a transverse groove in the
neck region called the cervical groove. In the genus Synthetocaulus the posterior
end is reinforced by chitinous arcs. The sides of the body may in some cases be
produced into longitudinal ridges or flanges called the lateral alæ which aid in loco-
motion. Usually these are present in the anterior region of the body. When
confined to the neck region only they are called the cervical alæ. Among the males
of some nematodes, e.g., in Heterakidæ, the alæ are present at the posterior extre-
mity, and are called the caudal alæ, the function of which is to aid the process of
copulation. In the family Physalopteridæ the caudal alæ meet ventrally in front
of the anus. In the same family the cuticle over the lips is reflected forwards to
form a cephalic collarette into which the head is retractile. In the genus Chlamy¬
donema there is a prepuce-like sheath of the cuticle covering the posterior extremity
of both the sexes. In Dirofilaria the cuticle is reinforced internally as well as
externally by annular thickenings and in Onchocerca there is only an external spiral
thickening of the cuticle. In the genera Setaria and Dipetalonema there is a chiti-
noid ring surrounding the mouth and in the case of the latter there are, in addi-
tion, lateral epaulet-like expansions. In the super-family Strongyloidea the males
have a bell-shaped swelling at the posterior extremity, which is called the genital
bursa or bursa copulatrix, the function of which is to assist the males during the act
of copulation.
HELMINTH PARASITES AFFECTING DOMESTIC ANIMALS
15
Syncytial layer and Musculature. Underneath the cuticle is a syncytial protop-
lasmic layer from which the former is secreted. Below it there is a single layer of
muscle cells whose fibres are arranged longitudinally and produce contraction in
a transverse direction. Locomotion is performed by a series of up and down
undulations. The musculature of the body-wall is, as a rule, divided into four
parts by the four thickenings of the subcuticular layer, two of which lie laterally
and are more prominent than the other two which lie on the dorsal and ventral
sides respectively. The musculature may be either of " polymyarian type " or
" monomyarian type ".
Mouth. The mouth is situated at the anterior extremity, being either terminal
or subterminal and is directed forwards. It may be tilted dorsally or ventrally,
depending upon the position of the head. It may be guarded by lips or a few or
more papillæ.
Buccal capsule. The mouth, in some cases, leads into a specialised cavity sur-
rounded by chitinoid walls, which is called the buccal capsule. It may be short
and ring-shaped, globular, subglobular, infundibular or elongated and cylindrical.
It may be divided into anterior and posterior portions by a transverse groove. It
may have one or more teeth on its walls or these may project into it from the
œsophageal funnel as in Tridontophorus. In the family Ancylostomidæ the ante-
rior margin of the buccal capsule is provided with the ventral teeth or cutting
plates. In some cases, as in Bathmostomum, the walls of the buccal capsule are
raised into ridges and alternate with the fissures, and in Ardueninæ the walls have
annular or spiral thickenings on them. In Strongylidæ the anterior end of the
buccal capsule has a circular fringe of leaf-like or bristle-like cuticular elements,
collectively called a leaf-crown or corona radiata. Usually one of these is present
at the opening of the buccal capsule and another a little posterior to it and spring-
ing from the walls of the buccal capsule. The former is called the external leaf-
crown and the latter the internal leaf-crown. The internal leaf-crown may be much
reduced as in some species of Trichonema or may be absent as in Equinurbia.
Pharynx. When the posterior portion of the buccal capsule is embedded, as in
some Heterakidæ, in the muscles, it is called the pharynx.
Œsophagus. The anterior portion of the alimentary canal is much specialised
and is easily distinguishable from the rest of it. This is called the œsophagus.
It lies immediately posterior to the oral cavity, the buccal capsule or the pharynx.
Its shape and size varies in differenc groups. It is sometimes short as in Ascaridæ,
in the parasitic form of Strongyloides stercoralis it occupies nearly one-third of the
body length, and in some forms, e.g., Trichuris, it is several times the length of the
rest of the alimentary canal. In Trichuroidea it is a delicate tube running through
B 2
16 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ II, I
the centre of a chain of cells placed end to end, but usually it is club-shaped and
its anterior portion is provided with powerful muscles surrounding a triangular
lumen with cuticular lining, the stomodæum. The lumen of the œsophagus is
capable of being dilated by the action of muscles, and it then acts as a suction
pump for drawing in liquid food from the tissues of the host. Generally embedded
in the walls of the œsophagus are three œsophageal glands : one dorsal and the
other two subventral. The dorsal one opens into the mouth and the other two
into the lumen of the œsophagus. In some forms, e.g., Ancylostomidæ, there are
cephalic glands lying in the body cavity and opening into the mouth ; the function
of these is supposed to be to secrete toxins. In many forms, e.g. in Heterakidæ
and Cruzidæ, there is a muscular bulb at the posterior end of the œsophagus con-
taining a masticatory apparatus consisting of three valves. In some forms, e.g.,
Spiruridæ, the posterior portion of the œsophagus is non-muscular and glandular
and is called the ventriculus.
Intestine. The œsophagus is followed by an intestine which is usually a
straight tube, either circular or flattened. Its walls are not muscular, but consist
only of epithelial cells. Occasionally, e.g., in Grammocephalus, Cruzia, and Porocæ¬
cum, it has a diverticulum or an appendage. Posteriorly it opens into the procto-
dœum, which in the case of males is a cloaca or a receptacle into which the genital
duct opens, and in females is a short rectum communicating to the exterior by an
anus situated on the ventral surface near the posterior end. Usually there are
rectal glands in connection with the proctodæm. In Filariidæ the posterior portion
of the intestine is occasionally atrophied and in Dracunculidæ atrophy of the pos-
terior portion of the intestine is the rule.
Excretory system. This consists of two longitudinal excretory canals lying
along the thickened lateral areas and extending almost the entire length of the
body. In the œsophageal region they join together into a short duct which opens
on the ventral side by a minute excretory pore.
Nervous system. This consists of a stout ring of nerve fibres surrounding the
œsophagus, various groups of ganglion cells and six longitudinal nerves given off
from these both anteriorly and posteriorly. Among the nerves the dorsal and
ventral median ones are the stoutest and are connected with one another by looped
commissures. Occasionally on the median ventral nerve there is a ganglion near the
anus.
Reproductive system. With the exception of the parasitic form of Strongyloides
stercoralis, which is a parthenogenetic female, the sexes are separate. The males
are usually smaller in the bisexual forms.
The genital glands of both the sexes are much elongated, convoluted tubes,
closed at the proximal end and opening distally into the ducts carrying their pro-
HELMINTH PARASITES AFFECTING DOMESTIC ANIMALS
17
ducts to the exterior. They are loosely attached in the body cavity and occupy
most of the body space.
Male genitalia.
Testis. In all the forms dealt with here there is only one testis which is a
straight or a coiled tube. At its free end it is solid and consists of cells which are
destined to give rise to the spermatozoa. Towards its other end, the spermatocytes
at successive stages of development are found attached to a central axis or rachis.
The testis is connected with a male duct which can be differentiated into a vas
deferens, seminal vesicle and a muscular ductus ejaculatorius opening into the cloaca.
Opening into the ductus ejaculatorius, in many forms, is a pair of cement glands,
the secretion of which serves to attach the male firmly to the female during copula¬
tion. The spermatozoa of nematodes are, as a rule, unlike those of most of the
other groups of animals in being rounded or amœboid and completely devoid of any
locomotary appendages. They attain their complete development in the uteri of
the females after they are transmitted there during copulation.
Spicules. Situated on the dorsal side of the intestine and opening into the
cloaca is a sheath or a pair of sheaths, containing chitinous rod-like body or bodies,
the spicule or spicules. Their function is considered to be four-fold: to hold fast
the female during copulation, to expand the orifice of the vagina, to act as gutters
for the transmission of the sperms and to excite the females during the act of
copulation. Special extensor and retractor muscles inserted on the sheaths bring
about their extension and retraction. Usually there are two spicules, but in the
family Trichinellidæ the spicule and the sheath is absent and in Trichurinæ and
Capillarinæ there is a single spicule with a sheath. The sheath may or may not be
armed with spines.
The size and the form of the spicules vary considerably in nematodes. In
Ascaridæ and Strongylidæ they are equal or subequal. In Filarinæ, Setarinæ,
and some other genera, e. g., Habronema and Gongylonema, they are very unequal.
They may be simple, elongated, or as in Trichostrongylidæ they may be short, spoon¬
or spatula-shaped, often having a twisted appearance owing to the development
of the ridges or other projections on their surface. They may be hooked or barbed,
or as in Osterlagia ostertagi terminate into a pincer-like process. In Synthetocaulus
rufescens they are long, tubular, with broad membranous expansions and pectinate
on the ventral surface distally. They are usually separate, but occasionally, e.g.,
in the genera Nematodirus and Mecistocirrus, they are united by a membrane
almost throughout their length excepting their anterior extremities.
Gubernaculum. In several genera, e. g., Ancylostoma, there is a chitinoid thick-
ening of the dorsal wall of the cloaca, which is called the accessory piece or the
gubernaculum. Its function is to guide the spicules during copulation.
18 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ II, I
Telamon. In some genera, e. g., Synthetocaulus, there is a chitinoid structure
developed in connection with the ventral wall of the cloaca, which is called the
telamon.
Bursa. Males of the super-family Strongyloidea have a cuticular expansion
more or less completely surrounding the posterior extremity, called the bursa,
serving as a prehensile organ during copulation. The typical bursa of the strongy¬
lidæ is divided into a dorsal and two lateral lobes. It is supported by a series
of rays which represent the stalks of the modified caudal papillæ. The bursal
rays have a definite number and arrangement although modifications may be met
with occasionally. Usually supporting the dorsal lobe is a median, unpaired
dorsal ray which may be variously branched or subdivided. In each lateral lobe
there are three rays: the ventral ray, the lateral ray and the externo-dorsal ray,
supporting its anterior, median and posterior margins respectively. The ventral
ray springs from the body and is divided into the two rays: the ventro-ventral
and the latero-ventral rays. The lateral ray also springs from the body and is
divided into the three rays: the antero-, medio-, and postero- or externo-lateral rays.
The externo-dorsal ray is undivided and may spring from the base of the stem of
the dorsal ray or independently from the body.
The bursa may be symmetrical as in the genus Strongylus, or it may be asym-
metrical as in Hæmonchus and Bunostomum. In Trichistrongylidæ the lateral
lobes are well developed, but the dorsal lobe is either vestigeal or absent. In
Metastrongylidæ the dorsal lobe is very small. In Syngamidæ the bursa is very
short. In the genus Nematodirus the dorsal lobe is divided into two small lobules.
Another super-family which is characterised by the possession of a bursa is
Dioctophymoidea. In this the bursa is bell-shaped, muscular, and is not supported
by rays.
In the genus Ostertagia within the bursa on the dorsal side is an accessory
bursal membrane with its free posterior edge supported by two slender diverging
rays.
The cloacal aperture is sometimes raised on a prominent structure situated in
the centre of the bursa, which is called the genital cone. In Trichonema it has a
well developed dermal collar and in some of the species of this genus it may bear
appendages.
Genital sucker. Among the males of the family Heterakidæ and the sub-family
Ascaridinæ there is a sucker situated in front of the cloaca, which serves the func-
tion of holding the female during copulation.
Female genitalia. In the super-families Trichuroidea and Dioctophymoidea
and the family Attractidæ there is only a single set of female genitalia. In all
HELMINTH PARASITES AFFECTING DOMESTIC ANIMALS
19
other forms there are two sets of the female reproductive organs. Each set con-
sists of a tubular ovary where the ova are manufactured, and oviduct and a uterus.
A central rachis is generally present in the ovary as in the testis.
Uterus. This is a sac-like receptacle in which the ova liberated from the
ovary are stored. It may be a single sac or it may have two, four or many
branches as in Physolopteridæ, in which case it is described as di-, tetra-, and
poly-delphys respectively. The uteri may run parallel to each other or they may
go in opposite directions in the body when they are described as divergent or
opposed. In some cases the uterus may have a specialised muscular portion called
the ovijector, the function of which is to eject the ova from the uterus. In
Dracunculidæ the uterus is enormous in size and completely fills up the body.
Vagina. This is usually a muscular duct leading from the vulva to the point
where the two uteri unite. Occasionally, e.g., in Trichinellidæ, it is difficult to
differentiate it clearly from the uterus. Its length is very variable. It may be a
short duct or a very long tube as in Dioctophyme. It is atrophid in Dracunculus.
Vulva. This in an orifice by which the female genital ducts communicate with
the exterior. Its position varies considerably. In Filaroidea it usually opens in
the œsophageal region and in Strongylidæ posterior to the middle of the body.
Its shape is also variable, but it is commonly elongated, usually transversely,
sometimes longitudinally or diagonally. It is occasionally situated on a protru¬
berence or covered with a backwardly directed flap as in Hæmonchus, It is
atrophied in Dracunculus.
Eggs. The eggs are usually oval or elliptical and rarely spherical in shape.
They are covered with a shell, the thickness of which is variable. In Trichuridæ
they are barrel-shaped, having a thick shell with an aperture at each pole closed
by a prominent plug and contain on deposition an unsegmented ovum. In the
genus Strongyloides the eggs have a thin shell containing a fully developed larva
at the time of deposition. In Strongylidæ and Filariidæ they have usually a very
thin shell. In Dioctophymoidea they have a thick shell with small depressions,
except at the poles, which are homogeneous. In Oxyuris they are large, asym
metrical, with a closed lenticular operculum and contain a tadpole-shaped larva
on deposition. In the genus Ascaris the eggs have a smooth shell surrounded by
an albuminous coat with a coarsely granular surface and contain an unsegmented
ovum when deposited. In Gnathostomidæ they have a thin shell ornamented
externally with fine granulations and in the genus Gnathostoma there is a marked
thinning at one pole, causing a weak spot through which the embryo escapes.
Development. After fertilization which takes place in the uterus the ovum
forms round itself a vitelline membrane and later an egg-shell is secreted round it.