THE ALIMENTARY CANAL AND ITS APPENDAGES.
_SPECIAL REFERENCES._
CHOLODKOWSKY. Zur Frage über den Bau und über die Innervation der
Speicheldrüsen der Blattiden. Horæ Soc. Entom. Rossicæ, Tom. XVI.
(1881). [Salivary Glands of Cockroaches.]
SCHINDLER. Beiträge zur Kenntniss der Malpighi’schen Gefässe der
Insekten. Zeits. f. wiss. Zool., Bd. XXX. (1878). [Malpighian
Tubules of Insects.]
CHUN. Ueber den Bau, die Entwickelung, und physiologische Bedeutung
der Rectaldrüsen bei den Insekten. Abh. der Senkenbergischen
Naturforschers Gesellschaft, Bd. X. (1876). [Rectal Glands of
Insects.]
LEYDIG. Lehrbuch der Histologie, &c., and VIALLANES. (_Loc. cit._
_supra_, chap. iv.) [Histology of Alimentary Canal.]
BASCH. Untersuchungen über das Chylopoëtische und Uropoëtische
System der Blatta orientalis. Kais. Akad. der Wissenschaften.
(Math-Nat. Classe.), Bd. XXXIII. (1858). [Digestive and Excretory
Organs of Blatta.]
SIRODOT. Recherches sur les Sécrétions chez les Insectes. Ann. Sci.
Nat., 4^e Série, Zool., Tom. X. (1859). [Digestive and Excretory
Organs of Oryctes, &c.]
JOUSSET DE BELLESME. Recherches expérimentales sur la digestion des
Insectes et en particulier de la Blatte (1875).
PLATEAU. Recherches sur les Phénomènes de la Digestion chez les
Insectes. Mem. de l’Acad. Roy. de Belgique, Tom. XLI. (1874).
[Now the principal authority on the Digestion of Insects. The
other physiological memoirs cited (Nos. 5, 6, 7) are chiefly of
historical interest.]
PLATEAU. Note additionelle. Bull. Acad. Roy. de Belgique, 2^e Sér.,
Tom. XLIV. (1877). [Contains some corrections of importance.]
_The Alimentary Canal._
The alimentary canal of the Cockroach measures about 2-3/4 inches in
length, and is therefore about 2-3/4 times the length of the body. In
herbivorous Insects the relative length of the alimentary canal may be
much greater than this; it is five times the length of the body in
Hydrophilus. Parts of the canal are specialised for different digestive
offices, and their order and relative size are given in the following
table:--
Œsophagus and crop ·95 in.
Gizzard ·1
Chylific stomach ·5
Small intestine ·1
Colon ·875
Rectum ·25
-----
2·775
=====
[Illustration: Fig. 56.--Alimentary Canal of Cockroach. × 2.]
The principal appendages of the alimentary canal are the salivary
glands, the cæcal diverticula of the stomach, and the Malpighian
tubules.
Considered with respect to its mode of formation, the alimentary canal
of all but the very simplest animals falls into three sections--viz.,
(1) the mesenteron, or primitive digestive cavity, lined by hypoblast;
(2) the stomodæum, or mouth-section, lined by epiblast, continuous with
that of the external surface; and (3) the proctodæum, or anal section,
lined by epiblast folded inwards from the anus, just as the epiblast
of the stomodæum is folded in from the mouth. The mesenteron of the
Cockroach is very short, as in other Arthropoda, and includes only the
chylific stomach with its diverticula. The mouth, œsophagus, and crop
form the stomodæum, while the proctodæum begins with the Malpighian
tubules, and extends thence to the anus. Both stomodæum and proctodæum
have a chitinous lining, which is wanting in the mesenteron. At the
time of moult, or a little after, this lining is broken up and passed
out of the body.
[Illustration: Fig. 57.--Section of Wall of Crop. _Cc_, chitinous
layer; _C_, chitinogenous cells; _Mi_, inner muscular layer; _Mo_,
outer do. × 275.]
The mouth of the Cockroach is enclosed between the labrum in front,
and the labium behind, while it is bounded laterally by the mandibles
and first pair of maxillæ. The chitinous lining is thrown into many
folds, some of which can be obliterated by distension, while others are
permanent and filled with solid tissues. The lingua is such a permanent
fold, lying like a tongue upon the posterior wall of the cavity and
reaching as far as the external opening. The thin chitinous surface of
the lingua is hairy, like other parts of the mouth, and stiffened by
special chitinous rods or bands. The salivary ducts open by a common
orifice on its hinder surface. Above, the mouth leads into a narrow
gullet or œsophagus, with longitudinally folded walls, which traverses
the nervous ring, and then passes through the occipital foramen to the
neck and thorax. Here it gradually dilates into the long and capacious
crop, whose large rounded end occupies the fore-part of the abdomen.
When empty, or half-empty, the wall of the crop contracts, and is
thrown into longitudinal folds, which disappear on distension. Numerous
tracheal tubes ramify upon its outer surface, and appear as fine white
threads upon a greenish-grey ground.
[Illustration: Fig. 58.--Wall of Crop, in successive layers. References
as in fig. 57. × 250.]
Three layers can be distinguished in the wall of the crop--viz., (1)
the muscular, (2) the epithelial, and (3) the chitinous layer.[122]
The muscular layer consists of annular and longitudinal fibres,
crossing at right angles. (See fig. 58.) In most animals the muscles
of organic life, subservient to nutrition and reproduction, are very
largely composed of plain or unstriped fibres. In Arthropoda (with
the exception of the anomalous Peripatus) this is not generally the
case, and the muscular fibres of the alimentary canal belong to the
striped variety. The epithelium rests upon a thin structureless
basement-membrane, which is firmly united in the œsophagus and crop
to the muscular layer and the epithelium. The epithelium consists of
scattered nucleated cells, rounded or oval. These epithelial cells,
homologues of the chitinogenous cells of the integument, secrete
the transparent and structureless chitinous lining. Hairs (setæ) of
elongate, conical form, and often articulated at the base, like the
large setæ of the outer skin, are abundant. In the œsophagus they are
very long, and grouped in bundles along sinuous transverse lines. In
the crop the hairs become shorter, and the sinuous lines run into a
polygonal network. The points of the hairs are directed backwards, and
they no doubt serve to guide the flow of saliva towards the crop.
[122] Here, as generally in the digestive tube of the adult
Cockroach, the peritoneal layer is inconspicuous or wanting. It
occasionally becomes visible--_e.g._, in the outer wall of the
Malpighian tubules, and in the tubular prolongation of the gizzard.
[Illustration: Fig. 59.--Transverse section of Gizzard of Cockroach.
The chitinous folds are represented here as symmetrical. See next
figure. × 30.]
The gizzard has externally the form of a blunt cone, attached by its
base to the hinder end of the crop, and produced at the other end
into a narrow tube (1/4 to 1/3 in. long), which projects into the
chylific stomach. Its muscular wall is thick, and consists of many
layers of annular fibres, while the internal cavity is nearly closed by
radiating folds of the chitinous lining. Six of the principal folds,
the so-called “teeth,” are much stronger than the rest, and project
so far inwards that they nearly meet. They vary in form, but are
generally triangular in cross section and irregularly quadrilateral
in side view. Between each pair are three much less prominent folds,
and between these again are slight risings of the chitinous lining. A
ridge runs along each side of the base of each principal tooth, and
the minor folds, as well as part of the principal teeth, are covered
with fine hairs. The central one of each set of secondary folds is
produced behind into a spoon-shaped process, which extends considerably
beyond the rest, and gradually subsides till it hardly projects from
the internal surface of the gizzard. Behind each large tooth (_i.e._,
towards the chylific stomach) is a rounded cushion set closely with
hairs, and between and beyond these are hairy ridges. (See fig. 61.)
The whole forms an elaborate machine for squeezing and straining
the food, and recalls the gastric mill and pyloric strainer of the
Crayfish. The powerful annular muscles approximate the teeth and folds,
closing the passage, while small longitudinal muscles, which can be
traced from the chitinous teeth to the cushions, appear to retract
these last, and open a passage for the food.[123]
[123] Plateau has expressed a strong opinion that neither in the
stomach of Crustacea nor in the gizzard of Insects have the so-called
teeth any masticatory character. He compares them to the psalterium
of a Ruminant, and considers them strainers and not dividers of the
food. His views, as stated by himself, will be found on p. 131.
[Illustration: Fig. 60.--The Six Primary Folds (teeth) of the Gizzard,
seen in profile.]
[Illustration: Fig. 61.--Part of Gizzard laid open, showing two teeth
(_T_) and the intermediate folds, as well as the hairy pads below.
_A-A_ and _B-B_ are lines of section (see figs. 62 and 63). × 50.]
[Illustration: Fig. 62.--Section through one tooth and two intermediate
spaces (see figure 61, _A-A_). _Cc_, chitinous cuticle; _C_,
chitinogenous layer; _am_, annular muscles; _p_, peritoneal layer.
× 75.]
[Illustration: Fig. 63.--Section through one principal hairy ridge and
two intermediate spaces (see fig. 61, _B-B_); _rm_, radiating muscles;
_tr_, trachea. The other references as before. × 75.]
The gizzard ends below, as we have already mentioned, in a narrow
cylindrical tube which is protruded into the chylific stomach for about
one-third of an inch. Folds project from the wall of this tube, and
reduce its central cavity to an irregular star-like figure. Below it
ends in free processes slightly different from each other in size and
shape. The chitinous lining and the chitinogenous layer beneath pass
to the end of the tube and are then reflected upon its outer wall,
ascending till they meet the lining epithelium of the cæcal tubes.
Between the wall of the gizzard-tube and its external reflected layer,
tracheal tubes, fat-cells, and longitudinal muscles are enclosed.
[Illustration: Fig. 64.--Longitudinal section through Gizzard and
fore-part of Chylific Stomach. _G_, gizzard; _Tu_, cæcal tube; _St_,
stomach; _Ep_, its lining epithelium. _A_ and _B_ are enlarged in the
side figures. × 35.
_A._--The Reflected Chitinogenous Layer of the Tubular Gizzard. _Tr_,
tracheal tube. × 400.
_B._--One of the Tubular Extensions of the same, enclosing muscles and
tracheæ. × 400.]
[Illustration: Fig. 65.--Transverse section of tubular prolongation of
Gizzard, within the Chylific Stomach, part of which is shown at its
proper distance. _R C_, reflected chitinogenous layer; _Tr_, tracheal
tube; _M_, cross section of muscle; _Ep_, epithelium of chylific
stomach. × 100.]
The chylific stomach is a simple cylindrical tube, provided at its
anterior end with eight (sometimes fewer) cæcal tubes, and opening
behind into the intestine. Its muscular coat consists of a loose
layer of longitudinal fibres, enclosing annular fibres. Internal to
these is a basement membrane, which supports an epithelium consisting
of elongate cells which are often clustered into regular eminences,
and separated by deep cavities. The epithelium forms no chitinous
lining in the chylific stomach or cæcal tubes; and this peculiarity,
no doubt, promotes absorption of soluble food in this part of the
alimentary canal. Short processes are given off from the free ends of
the epithelial cells, as in the intestines of many Mammalia and other
animals.
[Illustration: Fig. 66.--Epithelium of Chylific Stomach. In the upper
figure the digestive surface is indented, while in the lower figure
it is flat. Both arrangements are common, and may be seen in a single
section. The epithelial buds are shown below, and again below these the
annular and longitudinal muscles. × 220.]
Between the cells a reticulum is often to be seen, especially where the
cells have burst; it extends between and among all the elements of the
mucous lining, and probably serves, like the very similar structure
met with in Mammalian intestines,[124] to absorb and conduct some of
the products of digestion. Different epithelial cells may be found in
all the stages noticed by Watney--viz., (1) with divided nuclei; (2)
small, newly produced cells at the base of the epithelium; (3) short
and broad cells, overtopped by the older cells around; (4) dome-shaped
masses of young cells, forming “epithelial buds”;[125] (5) full-grown
cells, ranging with those on either side, so as to form an unbroken and
uniform series. The regeneration of the tissue is thus provided for.
The cells come to maturity and burst, when new cells, the product of
the epithelial buds, take their place.
[124] See Watney, Phil. Trans., 1877, Pt. II. The “epithelial buds”
described and figured in this memoir are also closely paralleled in
the chylific stomach of the Cockroach.
[125] These epithelial buds have been described as glands, and we
only saw their significance after comparing them with Dr. Watney’s
account.
The epithelium of the chylific stomach is continued into the eight
cæcal tubes, where it undergoes a slight modification of form.
[Illustration: Fig. 67.--Section of Chylific Stomach, showing the six
bundles of Malpighian tubules. × 70.]
At the hinder end of the chylific stomach is a very short tube about
half the diameter of the stomach, the small intestine. At its junction
with the chylific stomach are attached, in six bundles, 60 or 70 long
and fine tubules, the Malpighian tubules.[126] The small intestine has
the same general structure as the œsophagus and crop; its chitinous
lining is hairy, and thrown into longitudinal folds which become much
more prominent in the lower part of the tube. The junction of the small
intestine with the colon is abrupt, and a strong annular fold assumes
the character of a circular valve (fig. 68).
[126] Development shows that these tubules belong to the proctodæum,
and not to the mesenteron.
[Illustration: Fig. 68.--Junction of Small Intestine with Colon. × 15.]
From the circular valve the colon extends for nearly an inch. Its
diameter is somewhat greater than that of the chylific stomach, and
uniform throughout, except for a lateral diverticulum or cæcum, which
is occasionally but not constantly present towards its rectal end.
The fore part of the colon is thrown into a loose spiral coil. A
constriction divides the colon from the next division of the alimentary
canal, the rectum.
The rectum is about 1/4 inch long, and is dilated in the middle when
distended. Six conspicuous longitudinal folds project into the lumen of
the tube. These folds are characterised by an unusual development of
the epithelium, which is altogether wanting in the intermediate spaces,
where the chitinous lining blends with the basement-membrane, both
being thrown into sharp longitudinal corrugations. Between the six
epithelial bands and the muscular layer are as many triangular spaces,
in which ramify tracheal tubes and fine nerves for the supply of the
epithelium. The chitinous layer is finely setose. The muscular layer
consists of annular fibres strengthened externally by longitudinal
fibres along the interspaces between the six primary folds.[127]
[127] The epithelial bands of the rectum of Insects were first
discovered by Swammerdam in the Bee (Bibl. Nat., p. 455, pl. xviii.,
fig. 1). Dufour called them muscular bands (Rech. sur les
Orthoptères, &c., p. 369, fig. 44).
[Illustration: Fig. 69.--Transverse section of Small Intestine and
Colon, close to their junction. × 50.]
The corrugated and non-epitheliated interspaces may be supposed to
favour distension of the rectal chamber, while the great size of the
cells of the bands of epithelium is perhaps due to their limited
extent. Leydig[128] attributed to these rectal bands a respiratory
function, and compared them to the epithelial folds of the rectum
of Libellulid larvæ, which, as is well known, respire by admitting
fresh supplies of water into this cavity. It is an obvious objection
that Cockroaches and other Insects in which the rectal bands are well
developed do not take water into the intestine at all. Gegenbaur has
therefore modified Leydig’s hypothesis. He suggests (Grundzüge d.
Vergl. Anat.) that the functional rectal folds of Dragon-flies and
the non-functional folds of terrestrial Insects are both survivals of
tracheal gills, which were the only primitive organs of respiration
of Insects. The late appearance of the rectal folds and the much
earlier appearance of spiracles is a serious difficulty in the way
of this view, as Chun has pointed out. It seems more probable that
the respiratory appendages of the rectum of the Dragon-fly larvæ
are special adaptations to aquatic conditions of a structure which
originated in terrestrial Insects, and had primarily nothing to do with
respiration.
[128] “Lehrbuch der Histologie,” p. 337.
[Illustration: Fig. 70.--Transverse section of Rectum. × 50.]
The number of the rectal bands (six) is worthy of remark. We find six
sets of folds in the gizzard and small intestine of the Cockroach,
six bundles of Malpighian tubules, with six intermediate epitheliated
bands. There are also six longitudinal bands in the intestine of the
Lobster and Crayfish. The tendency to produce a six-banded stomodæum
and proctodæum may possibly be related to the six theoretical elements
(two tergal, two pleural, two sternal,) traceable in the Arthropod
exoskeleton, of which the proctodæum and stomodæum are reflected folds.
The anus of the Cockroach opens beneath the tenth tergum, and between
two “podical” plates. Anal glands, such as occur in some Beetles, have
not been discovered in Cockroaches.
_Appendages. The Salivary Glands._
The three principal appendages of the alimentary canal of the Cockroach
are outgrowths of the three primary divisions of the digestive tube;
the salivary glands are diverticula of the stomodæum, the cæcal tubes
of the mesenteron, and the Malpighian tubules of the proctodæum.
[Illustration: Fig. 71.--Salivary Glands and Receptacle, right side.
The arrow marks the opening of the common duct on the back of the
lingua. A, side view of lingua; B, front view of lingua.]
A large salivary gland and reservoir lie on each side of the œsophagus
and crop. The gland is a thin foliaceous mass about 1/3 in. long, and
composed of numerous acini, which are grouped into two principal lobes.
The efferent ducts form a trunk, which receives a branch from a small
accessory lobe, and then unites with its fellow. The common glandular
duct thus formed opens into the much larger common receptacular duct,
formed by the union of paired outlets from the salivary reservoirs. The
common salivary duct opens beneath the lingua. Each salivary reservoir
is an oval sac with transparent walls, and about half as long again
as the gland. The ducts and reservoirs have a chitinous lining, and
the ducts exhibit a transverse marking like that of a tracheal tube.
When examined with high powers the wall of the salivary gland shows
a network of protoplasm with large scattered nuclei, resting upon a
structureless chitinous membrane.
The salivary glands are unusually large in most Orthoptera.[129] In
other orders they are of variable occurrence and of very unequal
development.
[129] Except in Dragon-flies and Ephemeræ.
_The Cæcal Tubes._
There are eight (sometimes fewer) cæcal tubes arranged in a ring
round the fore end of the chylific stomach; they vary in length,
the longer ones, which are about equal to the length of the stomach
itself, usually alternating with shorter ones, though irregularities of
arrangement are common. The tubes are diverticula of the stomach and
lined by a similar epithelium. In the living animal they are sometimes
filled with a whitish granular fluid.
Similar cæcal tubes, sometimes very numerous and densely clustered, are
attached to the stomach in many Crustacea and Arachnida. The researches
of Hoppe Seyler, Krukenberg, Plateau, and others have established the
digestive properties of the fluid secreted in them, which agrees with
the pancreatic juice of Vertebrates.
_The Malpighian Tubules._
The Malpighian tubules mark the beginning of the small intestine, to
which they properly belong. They are very numerous (60–70) in the
Cockroach, as in Locusts, Earwigs, and Dragon-flies; and unbranched,
as in most Insects. They are about ·8 inch in length, and ·002 inch
in transverse diameter, so that they are barely visible to the naked
eye as single threads. In larvæ about one-fifth of an inch long,
Schindler[130] found only eight long tubules, the usual number in
Thysanura, Anoplura, and Termes; but the grouping into six masses, so
plainly seen in the adult, throws some doubt upon this observation. In
the adult Cockroach the long threads wind about the abdominal cavity
and its contained viscera.
[130] Zeitsch. f. wiss. Zool., Bd. XXX.
[Illustration: Fig. 72.--Malpighian Tubules of Cockroach. _A_,
transverse section of young tubule; _p_, its connective-tissue or
“peritoneal” layer; _B_, older tubule, crowded with urates; _tr_,
tracheal tube; _C_, tubule cut open longitudinally, showing three
states of the lining epithelium. × 200.]
In the wall of a Malpighian tubule there may be distinguished (1) a
connective tissue layer, with fine fibres and nuclei; within this, (2)
a basement-membrane, between which and the connective tissue layer runs
a delicate, unbranched tracheal tube; (3) an epithelium of relatively
large, nucleated cells, in a single layer, nearly filling the tube, and
leaving only a narrow, irregular central canal. Transverse sections
show from four to ten of these cells at once. The tubules appear
transparent or yellow-white, according as they are empty or full;
sometimes they are beaded or varicose; in other cases, one half is
coloured and the other clear. The opaque contents consist partly of
crystals, which usually occur singly in the epithelial cells, or heaped
up in the central canal. Occasionally, they form spherical concretions
with a radiate arrangement. They contain uric acid, and probably
consist of urate of soda.[131] In the living Insect the tubules remove
urates from the blood which bathes the viscera; the salts are condensed
and crystallised in the epithelial cells, by whose dehiscence they pass
into the central canals of the tubules, and thence into the intestine.
[131] The contents of the Malpighian tubules may be examined by
crushing the part in a drop of dilute acetic acid, or in dilute
sulphuric acid (10 per cent.). In the first case a cover-slip is
placed on the fluid, and the crystals, which consist of oblique
rhombohedrons, or derived forms, are usually at once apparent. If
sulphuric acid is used, the fluid must be allowed to evaporate. In
this case they are much more elongated, and usually clustered. The
murexide reaction does not give satisfactory indications with the
tubules of the Cockroach.
The Malpighian tubules develop as diverticula from the proctodæum,
which is an invagination of the outer integument and its morphological
equivalent. They are, therefore, similar in origin to urinary organs
opening upon the surface of the body and developed as invaginations
of the integument, like the “shell-glands” of lower Crustacea, and
the “green glands” of Decapod Crustacea. The segmental organs of
_Peripatus_, Annelids, and Vertebrates do not appear to be possible
equivalents of the excretory organs of Arthropods. They arise, not as
involutions, but as solid masses of mesoblastic tissue, or as channels
constricted off from the peritoneal cavity, and their ducts have
only a secondary connection with the outside of the body or with the
alimentary canal.
_Digestion of Insects._
The investigation of the digestive processes in Insects is work of
extreme difficulty, and it is not surprising that much yet remains to
be discovered. Plateau has, however, succeeded in solving some of the
more important questions, which, before his time, had been dealt with
in an incomplete or otherwise unsatisfactory way. The experiments of
Basch, though now superseded by Plateau’s more trustworthy results,
deserve notice as first attempts to investigate the properties of the
digestive fluids of Insects.
Basch set out with a conviction that where a chitinous lining is
present, the epithelium of the alimentary canal secretes chitin only,
and that proper digestive juices are only elaborated in the chylific
stomach, or in the salivary glands. The tests applied by him seemed
to show that the saliva, as well as the contents of the œsophagus
and crop, had an acid reaction, while the contents of the chylific
stomach were neutral at the beginning of the tube and alkaline further
down. From this he concluded that the supposed deep-seated glands of
the chylific stomach secreted an alkaline fluid, which neutralised
the acidity of the saliva. Finding that the epithelial cells of the
stomach were often loaded with oil-drops, he concluded that absorption,
at least of fats, takes place here. The chylific stomach, carefully
emptied of its contents, was found to convert starch into sugar at
ordinary temperatures. The saliva of the Cockroach gave a similar
result, and when a weak solution of hydrochloric acid was added,
Basch thought that the mixture could digest blood-fibrin at ordinary
temperatures.
Plateau’s researches upon _Periplaneta americana_,[132] modified by
subsequent experiments upon _P. orientalis_,[133] and by still more
recent observations, lead him to the following conclusions[134]:--
[132] Bull. Acad. Roy. de Belgique, 1876.
[133] Ib., 1877.
[134] We are indebted to Prof. Plateau for the statement of his views
given in the text.
1.--The saliva of the Cockroach changes starch into glucose; but the
saliva is not acid, it is either neutral (_P. orientalis_) or alkaline
(_P. americana_). Any decided acidity found in the crop is due to the
ingestion of acid food; but a very faint acidity may occur, which
results from the presence in the crop of a fluid secreted by the cæcal
diverticula of the mesenteron.
2.--The glucose thus formed is absorbed in the crop, and no more is
formed in the succeeding parts of the digestive tube.
3.--The function of the gizzard is that of a grating or strainer. It
has no power of trituration. If the animal consumes vegetable food rich
in cellulose, a substance not capable of digestion in the crop, the
fragments are found unaltered as to form and size in the mesenteron. If
it is supplied with plenty of farinaceous food, such as meal or flour,
the saliva is not adequate to the complete solution and transformation
of the starch, and the intestine is found full of uninjured starch
granules, which must have traversed the gizzard without crushing.
4.--The cæcal diverticula secrete a feebly acid fluid. To demonstrate
its acidity an extremely sensitive litmus solution, capable of
indicating one part in twenty thousand of hydrochloric acid, must be
used. The fluid secreted by the cæca emulsifies fats, and converts
albuminoids into peptones.
In all Insects digestion is effected in the following way (which is
particularly easy of demonstration in _Carabus_ and _Dytiscus_). The
crop is filled with food coarsely divided by the mandibles, and the
gizzard being shut to prevent further passage, the fluid secretion of
the cæca ascends to the crop, and there acts upon the food. Digestion
is effected in the crop, and not beyond it. This is clear beyond
doubt. In Decapod Crustacea also it is very easy to prove that the
fluid secreted by the so-called liver ascends into the stomach (which
corresponds to the crop, together with the gizzard of the Insect). To
satisfy ourselves on this point we have only to open a Crayfish during
active digestion.
When digestion in the crop is finished, the gizzard relaxes, and the
contents of the crop, now in a semi-fluid condition, pass into the
mesenteron, which is devoid of chitinous lining, and particularly
fitted for absorption.
5.--There are no absorbent vessels properly so called, and Plateau has
long thought that the products of digestion pass by osmosis directly
through the walls of the digestive tube, to mix with the blood in
the perivisceral space. If we may rely upon what is now known of the
process in Vertebrates, we should be led to modify this explanation.
It is very likely that in Insects, as in Vertebrates, absorption is
effected by the protoplasm of the epithelial cells, which select
and appropriate certain substances formed out of the dissolved
food. Not only do the epithelial cells transmit to the neighbouring
blood-currents the materials which they have previously absorbed, but
they subject certain kinds to further elaboration. The protoplasm of
the epithelial cells of Vertebrates is capable of forming fat. Thus,
a mixture of soap and glycerine, injected into the intestine of a
Vertebrate, is absorbed by the lacteals in the form of oil-drops.
Modern physiologists allow, too, that part of the peptone is similarly
changed into albumen, without transport to a distance, by the activity
of the epithelial lining.
These facts explain why Plateau was unable to isolate the secretion of
the epithelium of the chylific stomach of Insects. The cells are not
secretory, but absorbent; and the secretion vainly sought for does not
actually exist.