Chapter XVI: Appendix: “Rhizopods in Poliomyelitis acuta.” (10)

The males have bodies that widen out considerably behind the ventral
sucker, the lateral parts of which in-roll ventrally, forming the
almost completely closed canalis gynæcophorus, within which the
female is enclosed. There is no cirrus pouch. The male has five or
six testes, the females are filiform; the uterus is long. There
is no Laurer’s canal. The ova almost equally attenuated at either
extremity; they have a small terminal spine, and are not provided
with a lid. They contain a miracidium, ciliated on all sides, which
is characterized by the possession of two large glandular cells,
which discharge anteriorly beside the gastric sac. They live in the
vascular system of mammals. (An allied genus [Bilharziella] lives in
the blood-vessels of birds.)

*Schistosoma hæmatobium*, Bilharz, 1852.

Syn.: _Distoma hæmatobium_, Bilh.; _Distoma capense_, Harley, 1864.

_The Male_ is whitish, 12 to 14 mm. in length, but is already mature when 4 mm. long. The anterior end is 0·6 mm. or a little over in length. The suckers are near each other, the oral sucker is infundibular, and the dorsal lip is longer than the ventral one. The ventral sucker is a little larger, 0·28 mm., and is pedunculated. A little behind the ventral sucker the body broadens to a width of 1 mm., decreasing, however, in thickness; the lateral edges in-roll ventrally, so that the posterior part of the body appears almost cylindrical, 0·4 to 0·5 mm. in diameter; the posterior extremity is somewhat more attenuated. The dorsal surface of the posterior part of the body is covered with spinous papillæ. There are delicate spines on the suckers, and larger ones invest the entire internal surface of the gynæcophoric canal, as well as a longitudinal zone at the edge of that side of the external surface that is covered by the other side rolling over it. The œsophagus is beset with numerous glandular cells (fig. 173), and presents two dilatations; the intestinal bifurcation is close in front of the ventral sucker, the two branches uniting sooner or later behind the testes into a median trunk, which may again divide at short intervals. The excretory pore is at the posterior end, but placed somewhat dorsally; the genital pore is at the beginning of the gynæcophoric canal, thus behind the ventral sucker; into it opens the vas deferens which, posteriorly, broadens into the seminal vesicle and then continues as the vasa efferentia of the four or five testes (fig. 173).

_The Female_--filiform, about 20 mm. in length, pointed at each end, and measuring 0·25 mm. in diameter in the middle. Their colour varies according to the condition of the contents of the intestine. (Posteriorly they are dark brown or blackish.) The cuticle is smooth except in the sucker, where there are very delicate spines, and at the posterior end, where there are other larger spines. The oral sucker is a little larger than the pedunculated ventral sucker (0·07 and 0·059 mm. respectively). The anterior part of the body, 0·2 to 0·3 mm. in length; the œsophagus is as in the male. The intestinal bifurcation is in front of the ventral sucker, the two branches uniting behind the ovary and the trunk running in a zigzag manner to the posterior border. There are indications of diverticula at the flexures. The ovary is median. In young females it is of an elongated oval shape; in older females the posterior end becomes club-shaped, whereas the anterior end becomes attenuated; the oviduct originates at the posterior end, but immediately turns forwards and joins the parallel vitelline duct in front of the ovary (fig. 174), where the shell gland cells open; the common canal becomes dilated to form the oötype, and then proceeds as the uterus, with only slight convolutions, along the central field to the genital pore, which lies in the middle line immediately behind the ventral sucker. The single vitellarium starts behind the ovary and extends to the posterior end. The acini are situated at the sides of the excretory duct, which runs a median course. The eggs are compact spindles, much dilated in the middle; they have no lid, and are provided with a terminal spine (rudimentary filament) at the posterior end, measuring 120 µ to 150 µ in length and 40 µ to 60 µ in breadth, but vary in size and shape (fig. 175).

_Distribution._--In order to understand the distribution of the worms and eggs in the body, it may be well to recall the blood supply of the abdominal and pelvic organs. It is generally assumed that the early life (? cercarial stage) of the worms occurs in the liver, and that the young worms travel from here, where they are invariably found, to their various sites along the portal vein and its tributaries and so _against_ the blood stream. The tributaries of the portal vein are:--

(1) _Superior mesenteric_, the tributaries of which are: (_a_) the veins of the small intestine; (_b_) ileo-colic; (_c_) right colic; (_d_) middle colic; (_e_) right gastro-epiploic; and (_f_) inferior pancreatic. By these paths infection of the small intestine, ascending and transverse colon and pancreas would occur.

(2) _Splenic._ (Ova have been recorded by Symmers in the spleen.)

(3) _Inferior mesenteric_, the tributaries of which are (_a_) superior hæmorrhoidal veins from the upper part of the hæmorrhoidal plexus; (_b_) sigmoid veins from sigmoid flexure and lower portion of _descending_ colon; (_c_) left colic vein draining descending colon.

The superior hæmorrhoidal veins form a rich plexus in the rectum, and below this level in the upper and middle parts of the anal canal. The plexus forms two networks, an _internal_ plexus in the submucosa and an external on the outer surface. The _internal_ plexus opens at the anal orifice into: (_a_) branches of the inferior hæmorrhoidal vein (from the pudic); (_b_) the external plexus. The _external_ plexus gives off: (_a_) inferior hæmorrhoidal opening into internal pudic (of _internal iliac_ vein); (_b_) mid-hæmorrhoidal into _internal iliac_ or its branches; and (_c_) superior hæmorrhoidal opening into inferior mesenteric. The external plexus further communicates with the vesico-prostatic plexus. The vesico-prostatic (vaginal) plexus opens into the _vesical veins_, which drain into the interior iliac vein. This plexus also receives afferents from the pudendal plexus, the chief tributary of which is the dorsal vein of the penis. The pudendal plexus also receives branches from the inferior pudic and the anterior surface of the bladder.

There is thus a communication between the portal vein and the vena cava by means of these plexuses, _viz._, through the inferior and middle hæmorrhoidals, and by the inferior hæmorrhoidals to the bladder and thence by the vesical veins or the pudic to the caval system (interior iliac).

It is thus by the inferior mesenteric and its tributaries that the worms reach the descending colon, rectum, anal canal, and eventually the bladder, and in some cases the caval system.

Before considering what is actually found _post mortem_ in these veins and the organs drained by them, we may further recall the fact that the calibre of “medium” veins is 4 to 8 mm., “small” veins less than 40 µ in diameter and capillaries 8 µ to 20 µ. Further, the maximum diameter of the male worm is 1 mm., that of the female 280 µ and eggs _in utero_ 80 µ to 90 µ long by 30 µ to 40 µ.

_Liver and Portal Vein._--Here worms are most easily found _post mortem_. Often only males are found and these of the same size, and if females occur only a few worms are found in copulâ. The worms are frequently not full size and the males may contain no free spermatozoa in their testes, and as regards the females some may be fertilized, others not, as shown by the presence or absence of spermatozoa in the seminal receptacle or uterus. In either case they may contain eggs--_lateral-spined_--usually one, less often two, but there may be as many as five or six. These eggs may also show some abnormality, which takes the form of: (1) abnormal contents, _viz._, disintegrating yolk cells with or without an ovarian cell; (2) abnormal shape but with normal contents and probably represented by the collapsed and empty egg-shells which are found in the tissues.

As to the interpretation of these facts, Looss believes that these lateral-spined eggs are products of young females whose egg-laying is not at first properly regulated. The shape that the eggs take, _viz._, with a lateral spine, is determined by an excess of material--ovarian and yolk cells--being present in the oötype. The shape of eggs depends upon the position they have in the oötype during their formation. In young females an excess of cells--yolk cells especially--accumulates, distending not only the dorsal wall but a portion also of the short duct joining the oötype to the uterus. The result of this is that the axis of the oötype and egg is almost transverse to the body, and the posterior funnel-shaped portion of the oötype, instead of being terminal, has now a lateral or rather a ventral position, so that the spine which occupies this portion, instead of being terminal, is now lateral. It is noteworthy that these lateral-spined eggs are thicker, owing to the excess of material present, and not uncommonly have a curved anterior border, due to a projection of the anterior end into the anterior opening of the oötype.

As these eggs are being laid by females in the portal vein they are carried back to the liver by the blood stream. The liver is one of the commonest sites for these eggs; also terminal-spined eggs may be found here for the same reason.

_Hæmorrhoidal Veins._--Mature worms, generally in copulâ, are usually found here, though young not fully grown females may also occur. The tissues of the rectal wall (or colon) show, as a rule, large quantities of lateral-spined eggs, though less often only terminal-spined eggs may be found.

_Vesico-prostatic Plexus._--Worms in copulâ are found in the veins of the submucosa in the bladder, and the eggs in the mucosa, and those voided are usually terminal-spined, though lateral-spined eggs are not so rare as generally thought. The problem next arises as to how the eggs get to the lumen of the gut or bladder.

The female worm is 280 µ in diameter. Veins in the submucosa of the rectum less than 178 µ in diameter are not affected with endophlebitis. It is probable that the female even by stretching could not penetrate much beyond this. Eggs are probably then laid in the submucosa as near the muscularis mucosa as possible. Now if the eggs are laid in a vein of larger calibre than the worm fills, the eggs would be carried back to the inferior mesenteric vein, so that presumably the worm must succeed in blocking the vein already narrowed by endophlebitis, so that by the stasis which ensues the eggs may escape from the veins. How this occurs is not exactly known; it is not necessarily due to the spine, as the same escape into the tissues occurs in spineless eggs, such as those of _Schistosoma japonicum_. The eggs, then, pass as foreign bodies through the tissues. Another hypothesis is that the worms leave the veins in order to lay their eggs, but the evidence is against this.

_Caval System._--Occasionally worms that have passed through the vesical plexus may be found in the iliac vein, inferior vena cava, and even the lungs. If the worms are young they contain a lateral-spined egg; if adult, numerous (50 to 100) terminal-spined eggs.

_Lungs._--When the liver is strongly infected with (terminal-spined) eggs it is possible that by passive movements some may pass into the intralobular veins, and thence by the inferior vena cava to the lungs.

_Gall-bladder._--Similarly terminal-spined eggs pass into the bile-capillaries and gall-bladder (where they may be abundant), and so into the fæces.

_Detection of Eggs._--Occasionally eggs may be found in various other parts of the body. They are best detected by macerating pieces of the tissue in question in about 1/4 per cent. hydrochloric acid at 50 to 60°C. (Looss).

Pathological changes:--

_Rectum._--These have been studied thoroughly by Letulle in the case of an apparently pure infection of the rectum.[274] They take the form of a chronic diffuse inflammation, which may result in--(1) ulceration, or (2) hyperplasia of the mucosa, producing adenomata.

[274] It is noteworthy that in this almost classical case no worms were found in any of the sections. It is further noteworthy that the eggs in the rectum showed great irregularity of form. Eggs with a spine at each end were not uncommon; exceptionally eggs with two polar spines and one lateral.

_Ulcerative Form._--The _mucosa_ is transformed into a mass of vascular connective tissue. The connective tissue spaces next become invaded by numerous mononuclear cells. The tissue itself undergoes diffuse sclerosis, becoming hard and fibroid. Eventually ulcerative necrosis sets in. During these changes the Lieberkühn glands are destroyed. The process does not extend to the submucosa, in this respect differing from that in chronic dysentery.

_Hyperplastic Form._--The Lieberkühn glands of the mucosa at first hypertrophy; then there is an actual hyperplasia resulting in adenomata. The interstitial tissue of the glands is also greatly hypertrophied, giving rise to very vascular granulations. These growths are often hollow and contain worms. Many eggs are found in the mucosa on their way to the lumen of the gut.

The _muscularis mucosa_ is thickened up to twice or even ten times the normal. Its vessels are dilated (36 µ to 80 µ), but they do not allow of the passage of worms.

The _submucosa_ is profoundly changed; rigid and hard instead of supple. It is here that the greatest number of eggs occur. A remarkable condition of endophlebitis exists in the veins of the submucosa, not only in the smaller ones but also in the larger ones (370 µ by 270 µ). This endophlebitis results in a more or less complete occlusion of the vessels of the lumen.

The _muscular coats_ are free from change, also their veins.

The _Serous Coats_.--The veins about 1,900 µ, also show endophlebitis. Besides the rectum, in extreme cases even the transverse colon, the cæcum and small intestine may be affected.

_Bladder._--In the early stages the mucosa is deep red and swollen like velvet, or there may be localized patches of hyperæmia or extravasation. The subsequent changes take two chief forms:--

(1) _Sandy Patches._--The mucosa looks as if it were impregnated with a fine brownish or yellowish powder (myriads of ova). This is accompanied by a gradual hypertrophy and new formation of connective tissue, so that dry, hard or plate-like patches with this sandy appearance arise; the thickening eventually affects all the coats of the bladder. In the older patches many of the eggs are calcified. These patches sooner or later break down, ulcerate and necrose. Phosphatic deposits are abundant and stone is common. These patches are not found in the rectum.

(2) _Papillomata._--Where the inflammatory change produced by the eggs gives rise to hypertrophy and hyperplasia of the mucosa, papillomata result, the axis of which is formed by connective tissue of the submucosa. These are most variable in shape and form and bleed readily, and sometimes contain cavities of extravasated blood.

As in the rectum, it is in the submucosa that eggs are most abundant, and worms in copulâ occur in the veins of this layer, but endophlebitis is not as general as described in the rectum. Malignant disease of the bladder is not an uncommon sequela of bilharziasis. Besides the bladder, the ureters and kidneys may in advanced cases be involved. The prostate and vesiculæ seminales are commonly diseased. Eggs have been recorded in the semen. The urethra is frequently attacked; the vagina in the female.

Eggs also occur in the lymphatic glands of the gut.

_Geographical Distribution._--East Africa: Nile Valley, Red Sea Coast, Zanzibar, Portuguese East Africa, Delagoa Bay, Natal, Port Elizabeth.

South Africa: Cape Colony, Orange Free State, Transvaal, Mauritius, Bourbon, Madagascar.

West Africa: Angola, Cameroons, Gold Coast, Gambia, Senegal, Sierra Leone, Lagos, Nigeria.

North Africa: Tripoli, Tunis, Algeria, parts of the Sahara.

Central Africa: Sudan, various portions. Uganda, Nyasaland.

It occurs with varying frequency in these regions. It is probably more widely spread than this list implies, as undoubtedly many cases are seen which are not recorded.

Isolated cases have been recorded from Arabia, India,[275] Greece, Cyprus.

[275] In a case from Madras, recorded by Stephens and Christophers, the eggs were long and spindle-shaped, quite unlike the eggs of _Schistosoma hæmatobium_.

The means by which infection is brought about are still uncertain; we only know that the miracidia (fig. 175) enclosed in the discharged eggs do not hatch if the eggs remain in the urine, but after cooling perish. As soon, however, as the urine is diluted with water the shell swells, generally bursting lengthways, and releases the miracidium from its investing membrane, so that it can swim about with the aid of its cilia. In its structure it differs but little from the miracidium of _Fasciola hepatica_, as, for instance, in the lack of eyes; the two large gland cells situated on either side of the intestinal sac are also present in the miracidia of _Fasciola hepatica_.

_Sarcode Globules._--This is a term applied to certain globules which at times appear in the miracidium and are later ejected. Some authors consider them as indicative that the miracidium has developed into a sporocyst, but Looss considers them to be degeneration products.

The Bilharzia mission, under R. T. Leiper, sent to Egypt by the War Office early in 1915, reports that cercariæ of bilharzia type were recognized in four of the commonest fresh-water molluscs around Cairo.

With material obtained from naturally infected _Planorbis boissyi_ acute bilharziosis was experimentally produced in rats, mice, and monkeys. Infection takes place experimentally through the skin and also through the mucous membrane of the mouth and œsophagus. The miracidium, after entering the mollusc, develops into a sporocyst. This gives rise not to rediæ, but to secondary sporocysts, which, in turn, produce cercariæ. These, like the adult worm, differ from other distomes in lacking a muscular pharynx.

*Schistosoma mansoni*, Sambon, 1907.

According to Manson, Sambon and others, the eggs with lateral spines belong to a species different from _Schistosoma hæmatobium_. Infections with this species only are said to occur in the Congo, Southern States of North America, West Indies (Guadeloupe) and Brazil (Bahia). The following characters, according to Flu, differentiate this species: (1) In the male the transition from the anterior portion of the worm to the lateral fields (the infolded portions which form the gynæcophoric canal) is not a gradual one as in _Schistosoma hæmatobium_, but in this case the lateral fields rise suddenly, almost at right angles to the anterior portion. (2) The ovaries have a well-marked convoluted course as in no other schistosome. (3) The oötype is symmetrical in reference to the long axis of the body, its duct being lateral on the ventral side (Looss’ explanation of this we have already given). (4) The worms live exclusively in portal vein and tract. (As lateral-spined eggs occur also in the bladder, this is not exactly true.)

------------------------------------------------------------------
|*Schistosoma hæmatobium*, Bilharz, 1852. |
| |
|Male, four or five large testes. Gut forks unite late, so that the|
|single gut stem is short. Female, ovary in posterior half of body.|
|Uterus very long, voluminous, with many terminal-spined eggs, some|
|lying in pairs. Vitellaria in posterior fourth of body. Cercariæ |
|in _Bullinus contortus_ and _Bullinus dybowski_ (syn.: _Physa |
|alexandrina_) in Egypt. |
| |
| |
|*Schistosoma mansoni*, Sambon, 1907. |
| |
|Male, eight small testes. Gut forks unite early, so that the |
|single gut stem is very long. Females, ovary in anterior half of |
|body. Uterus very short; usually only one lateral-spined egg at a |
|time _in utero_. Vitellaria occupy posterior two-thirds of body. |
|Cercariæ in _Planorbis boissyi_ in Egypt. |
| |
|The above morphological descriptions are founded on worms of each |
|species, derived from experimentally infected mice (Leiper, R. T.,|
|_Brit. Med. Journ._, March 18, 1916, p. 411). |
------------------------------------------------------------------

*Schistosoma japonicum*, Katsurada, 1904.

Syn.: _S. cattoi_, Blanchard, 1905.

_Male._--Eight to 19 mm., but extreme limits are 5 to 22·5 mm. Consists of a short fore-body, separated by the ventral sucker from the hind-body. The ventral sucker is stalked and somewhat larger than the oral sucker. Both suckers are larger than the corresponding ones in _S. hæmatobium_. Body usually smooth, but in the fresh state numerous fairly evident spines along the margin of the canal. Œsophagus: two bulbs. The junction of the gut forks more posterior than in _S. hæmatobium_, the median united gut stem occupying a quarter to one-fifth to one-sixth of the body length. An excretory canal runs along each side of the body, opening into the dorsal excretory pore. Testes irregularly elliptical, six to eight in number, in the anterior part of hind-body. The vasa efferentia unite into a common vas deferens which opens directly behind the ventral sucker. The seminal vesicle lies just behind this.

_Female._--Up to 26 mm., generally thinner than the male. Surface smooth. Suckers armed with fine spines. Ventral sucker larger than oral. Body thicker behind the region of the ovary. The gut forks unite immediately behind the ovary. The united gut much thicker than in _S. hæmatobium_. Ovary elliptical, almost in the mid-body, its hinder portion dilated. The oviduct arises from its posterior end and then runs sinuously forward, where it is joined by the vitellarian duct; the vitellarium well developed, extending from behind the ovary almost but not quite to the posterior end as in _S. hæmatobium_. Shell gland ducts enter at the junction point of oviduct and vitelline duct. The canal here forms an oötype and then proceeds as the uterus to open directly behind the ventral sucker. The uterus occupies almost half the hind-body. In _S. hæmatobium_ this is not so. The uterine canal is cleft-like, _i.e._, its dorso-ventral diameter is much greater than its lateral diameter. The number of eggs varies from about 50 to 300 from observations made in various hosts.

_Eggs._--_In utero_ assume various shapes, as they are soft; the lumen of the uterus is narrow. Outside they are oval, faint yellow, double contoured. In fæces the eggs measure 83·5 µ, by 62·5 µ (man); 85 µ by 61·5 µ (cattle); 98·2 µ by 73·8 µ (dog). The eggs have either small lateral spines or thickenings, and Looss at the opposite side has described cap-like thickenings. The eggs in the tissues undergo various deformities, and may contain a miracidium, as also the eggs in fæces do; or the contents may consist of granular matter or amorphous masses or they may be calcified. Lymphocytes and giant cells may also invade the eggs.

_Mode of Infection._--The miracidia hatch in water in as little as fifteen minutes, but the majority in one to three hours. They will live in water for about twenty-four hours. In water they undergo a transformation into “larvæ,” which then penetrate the skin, as has been shown by Japanese writers to hold good for man, cattle, dog and cat. The penetration of the skin is attended with an eruption on the legs, “Kabure.” The exact route by which the worms reach the portal vein is uncertain. Infection in Japan takes place from spring to autumn, especially May to July, when the soil is contaminated with manure of cattle infected with _S. japonicum_. They also appear to develop in molluscs. Leiper and Atkinson found cercariæ (in sporocysts) in the liver of a mollusc, _Katayama nosophora_. They infected mice by immersing them in water containing liver emulsion and so free cercariæ, thus confirming the similar results of Miyairi and Suzuki.

_Habitat._--The worm occurs in Japan, China, and the Philippines. The normal host is man and mammals. Cattle, dog and cat are often found naturally infected. Mice can also be experimentally infected. Their seat of election is the portal vein and its branches, especially the mesenteric veins. They either swim free in the blood or remain fixed by their suckers to the intima of the vessels. They have also been found in the vena cava and right heart of a cat, but not so far in the vesical plexus.

Eggs are found in the submucosa and mucosa of the gut, especially the colon, and at times in the serosa and subserosa of the small intestine, where they give rise to new growths. Occasionally eggs are found in the brain. The life of the worms is at least two years.

_Pathogenic Effects._--Anæmia through loss of blood due to worms; enlarged spleen, toxic in origin (?); phlebitis, thrombosis, due to portal stasis; the eggs, however, cause the greatest mischief. They are carried by the circulation to various organs where they produce inflammation, granulation tissue, and later connective tissue.

_Liver._--The eggs reaching this organ give rise to granulomata and hence enlarged liver, and later, when connective tissue is formed, to contraction. The surface is rough and irregularly granular, “parasitic embolic cirrhosis” of Yamagiwa.

_Gut._--The eggs in the mucosa and submucosa cause catarrh and destruction of tissue or new growth. In the small intestine the eggs are mainly in the serosa and subserosa, where they give rise to polypoid or branched growths.

_Spleen._--Enlarged, at first due to toxin (?) and later due to portal stasis. Eggs in the spleen are uncommon.

_Ascites_ also arises from the portal stasis, and is generally present in advanced cases.

Eggs may be found in many other situations: glands (numerous), mesentery, stomach, pancreas, kidney, etc. The bladder remains free.

Class III. *CESTODA*, Rud., 1808.

Tapeworms have been known from ancient times--at all events, the
large species inhabiting the intestines of man--and there has never
been a doubt as to their animal nature. The large cysticerci of the
domestic animals (occasionally of man also) have been known for an
equally long period, but they were generally regarded as growths,
or “hydatids,” until almost simultaneously Redi in Italy, and
Hartmann and Wepfer in Germany, concluded from their movements and
organization that they were of animal nature. From that time the
cysticerci have been included amongst the other intestinal worms, and
Zeder (1800) established a special class (_Cystici_, Rud., 1808) for
the bladder worms. Things remained in this condition until the middle
of the last century, when Küchenmeister, by means of successful
feeding experiments, demonstrated that the cysticerci were definite
stages of development of certain tapeworms. Before Küchenmeister, E.
Blanchard, van Beneden, and v. Siebold had held the same opinion in
regard to other asexual Cestodes.

Since the most remote period another question has again and
again occupied the attention of naturalists, the question of the
morphological nature--that of the INDIVIDUALITY OF THE TAPEWORM. The
ancients, who were well acquainted with the proglottids (_Vermes
cucurbitani_) that are frequently evacuated, were of the opinion
that the tapeworm originated through the union of these separate
proglottids, and this view was maintained until the end of the
seventeenth century. In 1683 Tyson discovered the head with the
double circlet of hooks in a large tapeworm of the dog; Redi (1684)
was also acquainted with the head and the suckers of several Tæniæ.
Andry (1700) found the head of _Tænia saginata_, and Bonnet (1777)
and Gleichen-Rusworm (1779) found the head of _Dibothriocephalus
latus_. Consequently most authors, on the ground of this discovery,
considered the tapeworm as a single animal, that maintains its hold
in the intestine by means of the head, and likewise feeds itself
through it. The fact was recognized that there were longitudinal
canals running through the entire length of the worm, and it was
thought that these originated in the suckers, and that the entire
apparatus was an intestine. As, moreover, the segments form at the
neck, and are cast off from the opposite extremity, the tapeworm
was also compared with the polyps, which were formerly regarded as
independent beings.

Steenstrup, in his celebrated work on the alternation of generations
(1841), was the first to give another explanation. This has been
elaborated still further by van Beneden, v. Siebold and Leuckart,
and until a few years ago all authorities adopted his views.
According to this view, the tapeworm is composed of numerous
individuals, something like a polyp colony, and, in addition to the
proglottids--the sexual individuals which are usually present in
large numbers--there is ONE individual of different structure, the
_scolex_, which not only fastens the entire colony to the intestine,
but actually produces this colony from itself, and therefore is
present earlier than the proglottids. The scolex is a “nurse,”
which, though itself produced by sexual means, increases asexually
like a _Scyphistoma_ polyp; the tapeworm chain has therefore been
termed a _strobila_. Consequently the development of the tapeworms
was explained by an alternation of generations. In support of this
opinion it was demonstrated not only that the adult sexual creatures,
the proglottids, can separate from the colony and live independently
for a time, but that in certain Tæniæ, and especially in many
Cestodes of the shark, the proglottids detach themselves long before
they have attained their ultimate size, and thus separated continue
to develop, grow and finally multiply; the scolex also exhibits a
certain independence in so far as, though not, as a rule, capable of
a free life, yet it in some cases lives as a free being, partly on
the surface of the body of marine fishes and partly in the sea. With
the more intimate knowledge of the development of the cysticerci,
the independent nature of the scolex was recognized. It is formed by
a budding of the bladder that has developed from the oncosphere, in
some cases (Cœnurus) in large numbers, in other cases (Echinococcus)
only after the parent cyst has developed several daughter cysts.
Released from its mother cyst and placed in suitable conditions,
it goes on living, and gives rise at its posterior end by budding
to the strobila, the proglottids of which eventually become sexual
individuals.

In order to make this clearer we will briefly summarize what takes
place in the jelly-fishes.

By _metamorphosis_ is meant a developmental change in the _same_
individual, while alternation of generations, or _metagenesis_,
implies a stage in which _reproduction_ of individuals takes place by
a process of budding or fission. This _asexual_ reproductive stage
_alternates_ with the _sexual_ mode of reproduction. Thus in the
development of the Scyphozoa (jelly-fishes) we have:--

(1) The fertilized egg cell divides regularly and forms a _morula_.

(2) By accumulation of fluid in the interior this becomes a closed
sac with a wall formed of a single layer of cells, forming the
_blastosphere_ or _blastula_.

(3) One end of the sac is invaginated, forming a _gastrula_.

(4) The gastrula pore or mouth closes, forming again a sac, the walls
of which have two layers, forming a _planula_.

(5) This becomes fixed to a rock, an invagination forms at one end,
a depression--the stomodæum--communicating with the enteric cavity.
Tentacles grow out and we have a _Scyphozoön polype_, _Scyphistoma_
or _Scyphula_. It is to this stage that Steenstrup gave the name
“nurse” (“wet-nurse”), because it nourished or produced asexually the
succeeding forms.

(6) _Asexual reproduction_ by transverse fission occurs in this,
forming a pile of saucer- or pine-cone-like animals which before this
time had been considered to be a distinct animal, which was called
_strobila_ from its resemblance to a pine-cone. This is the alternate
generation.

(7) The individuals of the strobila become free and are called
_Ephyrulæ_.

(8) These develop finally into adult sexual jelly-fish, _Scyphozoa_,
so that comparing a tapeworm with this we have (_a_) egg, (_b_)
scolex (= Scyphula or “nurse”), (_c_) asexual reproduction of the
tapeworm chain (= strobila), (_d_) development of the individuals of
the chain (proglottids) into sexual adults.

Van Beneden’s terminology for these stages is the following: Ciliated
embryo = protoscolex; scyphistoma = deutoscolex (or scolex); free
Ephyrula = proglottis. According to this view, as is the case in many
endoparasitic Trematodes, asexual reproduction by budding occurs
at two stages of the whole cycle of development, _viz._ (1) in the
formation of the scolex by budding from the bladder (“nurse”), (2) in
the formation of the strobila by budding from the scolex (“nurse”).

But in cysticercal larval forms it appears that the scolex does not
arise in this way but is simply a part of the proscolex (hexacanth
embryo), becoming invaginated into it for protection, so that there
is no asexual gemmation here. It has been questioned also whether
the strobila also arises by gemmation. If it does, the tapeworm is a
_colony_ of zoöids produced by budding from the asexual scolex; if
it is not produced in this way, then the tapeworm is to be regarded
as an _individual_ in which growth is accompanied by segmentation.
Against the “colony” view are the facts that the muscular, nervous,
and excretory systems are continuous throughout the worm, and that
some tapeworms, such as _Ligula_, are unsegmented.

Finally, if the tapeworm is an individual the question arises
which is the head end. As new segments are formed at the neck,
and as this point in annelids is the antepenultimate segment, the
scolex must be the last or posterior segment. The caudal vesicle
or bladder of larval forms is consequently anterior. According to
this view, in tapeworms as among many endoparasitic flukes, an
_asexual_ multiplication occurs at two points of the whole cycle
of development, which is as follows: (1) egg, (2) oncosphere or
hexacanth embryo, (3) bladder (cysticercus or hydatid), (4) (after
digestion of the bladder) by budding, the scolex, (5) by budding from
the scolex the sexual proglottids, (6) the egg; (4) and (5) being the
two asexual stages.

ANATOMY OF THE CESTODA.

If we except the tapeworms with only one proglottis, the CESTOIDEA MONOZOA, Lang = _Cestodaria_, Monticelli, we can always distinguish in the Cestodes, in the narrower sense, one scolex or head and a large or small number of segments (proglottids). The SCOLEX serves the entire tapeworm for fastening it to the internal surface of the intestinal wall, and therefore carries at its end various organs which assist in this function, and which are as follows: (i) SUCTORIAL ORGANS, _i.e._, the four suckers (acetabula), which are placed crosswise at the circumference of the thickened end of the scolex; further, the double or quadruple groove-like suckers (bothridia), which are diversely shaped in the various genera and families.[276] (2) FIXATION ORGANS (hooklets)[277] that likewise occur in varying numbers and different positions; they may be in the suckers, or outside them on the apex of the scolex; for instance, in many of the _Tæniidæ_ they appear in a circle around a single protractile organ, the rostellum, or the latter may be rudimentary, and is then replaced by a terminal sucker. (3) PROBOSCIS. One family of the Cestodes, the _Rhynchobothriidæ_, carries four proboscides, moved by their own muscular apparatus, on the scolex, and they are beset with the most diverse hooks. (4) TENTACLE-LIKE formations are only known in one genus (Polypocephalus).

[276] They may remain simple, and are then not separated from the remaining muscles of the scolex; or they project as roundish or elongated structures over the scolex, hollow on their free surface, and often divided into numerous areas by muscular transverse ribs. They may also carry accessory suckers on their surface.

[277] The various parts of a hooklet are thus named from the point backwards: (1) blade or prong, (2) guard or ventral or posterior root, (3) handle or dorsal or anterior root.

The thickened part of the scolex that carries the suckers is usually called the head; the following flat (unsegmented) part connecting it with the proglottids is called the neck, and is sometimes quite small. In a few cases the entire scolex (or head) disappears, and its function is then undertaken by the contiguous portion of the chain of proglottids, which is transformed into a variously shaped PSEUDO-SCOLEX.

The proglottids are joined to the scolex in a longitudinal row, and are arranged according to age in such a manner that the oldest proglottis is farthest from the scolex, and the youngest nearest to it.

The number of segments varies, according to the species, from only a few to several thousands; they are either quadrangular or rectangular; in the latter case their longitudinal axis falls either longitudinal or transverse to that of the entire chain, according as the segments are longer than broad or broader than long. When the number of segments is very large, the youngest ones are, as a rule, transversely oblong, the middle ones are squarish, and the mature ones longitudinally oblong. The posterior border of the segments, as a rule, carries a longitudinal groove for the reception of the shorter anterior border of the following proglottis. The two lateral borders of the segment are rectilinear, but converge more or less towards the front, or they are bent outwards. In most of the Cestodes the segments, just as the neck, are very flat; in rare cases their transverse diameter is equal to their dorso-ventral diameter. As a rule the segments, singly or several united together, detach themselves from the posterior end, in many cases only after complete maturity is attained, and in others much earlier; they then continue to live near their parent colony, to still call it by that name, in the same intestine and continue their development. Even when evacuated from the intestine the proglottids under favourable circumstances can continue to live and creep about, until sooner or later they perish.

The first proglottis formed, and which in a complete tapeworm [_i.e._, sexually complete] is the most posterior, is as a rule smaller and of different shape, it also frequently remains sterile, as likewise happens in the next (younger) segments in a few species; otherwise, however, sooner or later the generative organs develop in all the segments, mostly singly, sometimes in pairs; in the latter case they may be quite distinct from each other or possess some parts in common. The term “mature” is used for a proglottid that has the sexual organs fully developed, while “gravid” is used for one containing eggs. Most of the species combine male and female genitalia in the same segment, only a few are sexually distinct (Diœcocestus). In the hermaphrodite species one male and one female sexual orifice are always present, and, in addition, there may be a second female orifice, the uterine opening; as a rule, however, this is lacking, and in one sub-family, the _Acoleinæ_, to which also the genus Diœcocestus belongs, the other sexual orifice, the opening of the vagina, is also absent. The position of these orifices varies; the cirrus and vagina usually open into a common atrium on one lateral border or on a surface of the segments; the orifice of the uterus may be on the same surface or on the opposite one.

The surface on which the uterus opens is termed the VENTRAL SURFACE; if this orifice is absent, one must depend on the ovary, which almost always approaches one of the two surfaces; this surface is then called the ventral.

The length of the Cestodes--independently of their age--depends on the number and size of the segments, as well as on their contraction; the smallest species (_Davainea proglottina_) is 0·5 to 1·0 mm. in length; the largest may attain a length of 10 m., and even more.

The entire superficial surface of the tapeworms is covered with a fairly resistant and elastic layer, which exhibits several indistinctly limited layers and which is usually called a cuticle, which also covers the suckers, and is reflected inwardly at the sexual orifices. In some species fine hairs appear, either on the entire body or only in the region of the neck, on the external surface. In the cuticle there can be recognized, besides the pores, which no doubt are concerned with nutrition, spaces in which lie the ends of sensory cells. Close under the cuticle lies the external layer of the parenchyma (basal membrane), and below this the circular and longitudinal muscles forming the dermo-muscular coat. The matrix cells of the cuticle occur as in the Trematodes, only on the inner side of the peripheral muscles in the external zone of the parenchyma; they are fusiform cells, forming one or two layers, but are not arranged in the manner of epithelial cells (fig. 184, _Sc.c._). They have fine branching processes which run between the dermal muscles, pass through the basal membrane and penetrate the internal surface of the cuticle with small pistil-like enlargements, expanding on the internal surface of the cuticle into a thin plasma layer.

In addition to the above mentioned, there are other cuticular formations occurring on the cuticle of some Cestodes, such as immobile hairs and variously formed hooks, such as are seen principally on the scolex. Their development is only roughly known in a few species; they are usually already present in the larval stage, and of the same arrangement and shape as in the fully developed tapeworms; a matter of importance, because by these structures larvæ can be recognized as being those of a certain species of tapeworm.

The CUTICULAR GLANDS in Cestodes are scarce.

The PARENCHYMA forms the chief tissue of the entire body, and in all essentials its structure is similar to that of the Trematodes.

The same doubt exists here also as to the nature of the parenchyma. Recent authors consider that it consists of highly branched cells, the processes of which ramify in all directions. These cells lie in a non-cellular matrix containing fluid vacuoles. This matrix spreads in between and so breaks the continuity of the epidermal cells.

In the parenchyma of almost all the Cestodes there are found in adult specimens, as well as in larvæ, light-refracting concentrically striated structures, of a spherical or broad elliptical shape, which, on account of their containing carbonate of lime, are termed CALCAREOUS CORPUSCLES (fig. 184, _C._). Their size, between 3 µ and 30 µ, varies according to the species; their frequency and distribution in the parenchyma also varies, but they are chiefly found in the cortical layer. They are the product of certain parenchymatous cells, in the interior of which they lie like a fat globule in a fat cell, but according to others they are _intercellular_ in origin.

The MUSCULAR SYSTEM of the proglottids is composed of--(1) the subcuticular muscles (figs. 184 and 185), as a rule consisting of a single layer of annular muscles; (2) longitudinal muscles; (3) dorso-ventral fibres extending singly from one surface to the other, and at both ends expanding in a brush-like manner, and inserted into the basal membrane, consisting of an outer, more numerous, and an inner, less numerous but more powerful layer (the number of bundles in this layer being in certain cases of specific importance); (4) transverse fibres, the elements of which penetrate to the borders of the segments, thus passing through the longitudinal muscles and reaching the cuticle. In the region of the septa the transverse and dorso-ventral muscles form a kind of plate.

The mass of parenchyma bounded by the transverse muscles is termed the MEDULLARY layer, while the mass lying outside them is termed the CORTICAL LAYER.

It was known long ago that the myoblasts adhere to the dorso-ventral fibres as thickenings, but it is only recently that large star-shaped cells (fig. 184), separated from but connected with them by processes, have been recognized as the myoblasts of other fibres (Blochmann, Zernecke).

Within the scolex the direction and course of the muscular layers change.

The SUCKERS are parts of the musculature, locally transformed, with a powerful development of the dorso-ventral muscles, now become radial fibres.

The ROSTELLUM of the armed Tæniæ, like the proboscis of the _Rhynchobothriidæ_, also belongs to the same category of organs.

In the simplest form, the rostellum, or top of the head (as in _Dipylidium caninum_), appears as a hollow oval sac, the anterior part of which, projecting beyond the upper surface of the head, carries several rows of hooks (fig. 186). The entire internal space of the sac is occupied by an elastic, slightly fibrous mass, while the anterior half of the surface of the rostellum is covered by longitudinal fibres and the posterior half by circular fibres. On contraction of the latter the entire mass is protruded through the apical aperture, the surface of the rostellum becomes more arched, and the position of the hooks is, in consequence, altered. The rostellum of the large-hooked _Tæniidæ_, which inhabit the intestine of man and beasts of prey, is of a far more complicated structure, for, in addition to the somewhat lens-shaped rostellum carrying the hooks on its outer surface, there are secondary muscles grouped in a cup-like manner (fig. 187). Every change in the curvature of the surface of the rostellum induces an alteration in the position of the hooks. In the hookless _Tæniidæ_ the muscular system of the rostellum is altered in a very different manner; in a few forms a typical sucker appears in its place.

The NERVOUS SYSTEM commences in the scolex and runs through the neck and the entire series of proglottids. Within the proglottids it consists of a number of longitudinal nerve fibres of which those at each lateral border are usually the largest. In the Tæniæ the lateral nerves are accompanied both dorsally and ventrally by a thinner nerve (accessory nerve) (fig. 185); on each surface, moreover, between the lateral nerve and the median plane, there are two somewhat stronger bundles (sub-median), so that there is a total of ten longitudinal nerve bundles. They lie externally to the transverse muscle plates, and the lateral and accessory bundles lie externally to the principal excretory vessels, and are everywhere connected by numerous anastomoses and secondary anastomoses; one typical ring commissure is usually found at the posterior border of the segments. In the _Bothriocephalidæ_ the distribution of the nerve bundles is different (for instance, two lie in the medullary layer), or they are split up into a larger number of branches. In the scolex the nerve bundles are connected in a very remarkable manner by commissures with that which is generally termed the central part of the entire nervous system. There occurs normally a commissure between the two lateral nerves; at the same level, the dorsal and ventral median nerves are also connected at each surface as well with each other as with the lateral nerves, so that a hexagonal or octagonal figure is formed. The so-called apical nerves pass from this commissural system anteriorly, embrace the secondary muscular system of the rostellum semicircularly, and form an annular commissure (rostellar ring) at the inner part of the rostellum.

The peripheral nerves arise from the nerve bundles as well as from the commissures situated in the scolex; some go direct to the muscles, while others form a close plexus of nerves external to the inner longitudinal muscles, which plexus likewise sends out fibres to the muscles, but principally to numerous fusiform sense organs (fig. 184, _Pl._); they lie internal to the subcuticular cells and, piercing the cuticle with their peripheral processes, end as projecting “receptor” hairs. Higher organs of sense are not known.

The EXCRETORY APPARATUS of the Cestodes is similar to that of other flat worms. The terminal (flame) cells, which hardly differ in appearance from those of the Trematodes, are distributed throughout the parenchyma, but are more common in the cortical than in the medullary layer (fig. 184, _T.c._). Before opening into a collecting tube, the capillaries run straight, tortuously, or in convolutions, anastomosing frequently with one another or forming a _rete mirabile_. The collecting tubes, which have their own epithelial and cuticular wall, and which also appear to be provided with muscular fibres, occur typically as four canals passing through the entire length of the worm (fig. 189); they lie side by side, two (a wider thin-walled ventral, and a narrower thick-walled dorsal one) in either lateral field; in the head the two vessels on each side unite by means of a loop, at the posterior extremity they open into a short pyriform or fusiform terminal bladder which discharges in the middle of the posterior edge of the original terminal proglottis.

This primitive type (fig. 189) of arrangement of collective tubes is subject to variation in most Cestodes, in the scolex as well as in the segments. Indeed, even the lumen of the four longitudinal tubes does not remain equal, as the dorsal or external tubes are more fully developed and become thicker, whereas the ventral or internal ones remain thin, and in some species quite disappear in the older segments (figs. 185, 187). Moreover, very frequently connections are established between the right and left longitudinal branches, as in the head, where a “frontal anastomosis” develops, which in the _Tæniidæ_ usually takes the form of a ring encircling the rostellum (fig. 190), and in the segments of a transverse anastomosis at each posterior border, especially between the larger branches, and more rarely between the smaller collecting tubes also (fig. 191).

The so-called “island” formation is another modification, _i.e._, at any spot a vessel may divide and after a longer or shorter course the two branches reunite, and this may appear in the collecting tubes themselves as well as in their anastomoses. The above-mentioned ring in the frontal commissure of the _Tæniidæ_ is such an island; similar rings also frequently encircle the suckers (fig. 190). In extreme cases (_Triænophorus_, _Ligula_, _Dibothriocephalus_, etc.) this island formation extends to all the collecting tubes and their anastomoses. Instead of two or four longitudinal canals only, connected by transverse anastomoses at the posterior border of the segments, there is an irregular network of vessels, situated in the cortical layer, from which the longitudinal branches, having again subdivided, can only be distinguished at intervals, and even then not in their usual number.

The opening of the longitudinal branches at the posterior end requires more accurate investigation; it is true that a single terminal bladder is mentioned as being present in many species, but this is also disputed; when the original end proglottis has been cast off, the longitudinal branches discharge separately. Some species possess the so-called foramina secundaria, which serve as outlets for the collecting tubes; they are generally at the neck, but may be situated on the segments.

The contents of the excretory vessels is a clear fluid, the regurgitation of which is prevented by the valves present at the points of origin of the transverse anastomoses. The fluid contains in solution a substance similar to guanine and xanthine.

_Genital Organs._--With the exception of one genus (_Diœcocestus_, Fuhrm.), in which the species are sexually differentiated, all the Cestodes are hermaphroditic; the genitalia develop gradually in the segments (never in the scolex), the male organs, as is usual in hermaphroditic animals, forming earlier than the female. The youngest proglottids generally do not exhibit even traces of genitalia: these, as a rule, develop first in the older segments, and the development proceeds onwards from segment to segment. In a few exceptional cases (_Ligula_) the sexual organs are already developed in the larval stage, but are only functional after the entry of the parasite into the final host.

With the exception of the end portions of the vagina, cirrus and uterus, all the parts of the genital apparatus lie in the medullary layer, except only the vitellaria, which in many species are in the cortical layer. The male apparatus consists of the testes, of which, as a rule, there are a large number,[278] and which lie dorsal to the median plane (fig. 185, _T._); a vas efferens arises from each testis, unites with contiguous vasa, and finally discharges into the muscular vas deferens that is situated in about the middle of the segment. According to the position of the genital pore, the vas deferens opens on the lateral margin or in the middle line in the front of the segment; it is much convoluted or twisted, and frequently possesses a dilatation termed the vesicula seminalis. It finally enters the cirrus pouch, which is usually elongated; within the cirrus pouch lies the protrusible cirrus, which is not uncommonly provided with hooklets.

[278] There are, however, tapeworms with only one, others with only two or three testes in each segment.

The male sexual orifice almost always opens with that of the vagina into a genital atrium, the raised border of which rises above the edge of the segment and forms the genital papilla (fig. 191).

The vagina, like the vas deferens, usually runs inwardly and posteriorly, where it forms a spindle-shaped dilatation (receptaculum seminis); its continuation, the spermatic duct, unites with the oviduct, the common duct of the ovaries (fig. 191). The ovaries, usually two in number, are compound tubular glands in the posterior half of the proglottis, which extend into the medullary layer, but ventral to the median plane.