Chapter XXVIII: Part II: Embryology of Insects (2)

In the completely segmented primitive band may be distinguished two regions of a peculiar appearance (Figs. 515, 527), one at the anterior, and the other at the hinder end. The anterior, the primary head-section, contains the mouth-opening, and is characterized by its lateral expansions, or procephalic lobes. The other section, or posterior section, the so-called anal segment or telson, contains the anus. Between the two sections lies the segmented primary trunk-segment, which in insects consists of 17 segments. Of these the three most anterior are those destined to bear the mandibles and two pairs of maxillæ; the three following are the thoracic, which are succeeded by 10 abdominal segments, besides the 11th or telson (pygidium, or suranal plate).

It is now generally believed that there are primarily eleven
abdominal segments, while Heymons has detected twelve in the embryos
of Blattids and Forficula (see p. 162). In the later stages of
embryonic development the number of abdominal segments is
diminished, the 10th and 11th abdominal segments becoming fused. In
Hydrophilus and Lina this is the case, but according to Graber, in
the Lepidoptera there is a fusion of the 9th and 10th abdominal
segments, the llth remaining free.

According to Wheeler, in Doryphora, and also in Chalicodoma
(Carrière), between the primary head-region and the mandibular
segment is interpolated a rudimentary and transitory body-segment,
the premandibular segment. According to Carrière this segment
corresponds to a rudimentary pair of limbs, and also to a ganglion,
which participates in the formation of the œsophageal commissure
(see p. 51).

FIG. 524.—Three embryonic stages of a leaf-beetle (Lina): _A_,
unsegmented primitive streak; in _B_ and _C_ the segmentation
becomes distinct on the lower layer (_u_). _B_, with the germs of
the gnathal segments (_k′-k‴_), and in _C_ the three thoracic
segments (_t’-t‴_), with the first two abdominal segments (_a_′,
_a″_): _bl_, blastopore; _kl_, head-lobes; _th_, extension of the
primitive streak into the thoracic region.—After Graber, from
Korschelt and Heider.
]

=The procephalic lobes.=—The head-lobes, or procephalic lobes, appear at a very early period (Fig. 524, _kl_), before any traces of the segments of the trunk region. Ayers has shown that in Œcanthus the primitive band, in its earliest condition and before the appearance of the head-lobes, is a simple oval plate or almond-shaped thickening near the posterior end of the egg (Fig. 525, 1, 2). This plate is “soon divided into two tolerably well-marked regions by the enlargement of the head-end,” the first indication of the head-lobes (3). A depression next forms in what is to be the middle of the forehead. “It indicates the position of the future labrum, and forms the inner boundaries of the two cephalic ganglia, which are developed on either side of this depression at a much later stage.” Almost simultaneously with the appearance of this depression, two lateral folds are formed in the trunk portion of the band, which are the first indications of the maxillary and thoracic regions, the abdominal portion not yet showing traces of a division into segments (Fig. 525, 5). The thickened outer edges of the head-fold next gradually grow in towards the median line (Fig. 525, 5), and bend forward towards the region of the future mouth. The rounded angle made by the posterior end of the head-fold is the first indication of the antennæ. The embryo is now composed of four well-marked regions: cephalic, maxillary, thoracic, and abdominal. The primitive band then grows much longer, the primitive mouth and anus appear, and the appendages bud out, and eventually the embryo revolves and appears on the ventral side of the egg (Fig. 525, 6).

FIG. 525.—Early stages in the embryology of _Œcanthus niveus_. Fig. 1,
the youngest observed primitive band, the serosa not yet formed; 2,
longitudinal optic section (diagrammatic) of Fig. 1; 3, the
primitive band after the appearance of the head-fold, which is
indicated at this time by the more rapid growth and consequent
greater breadth of the lower end of embryo, x 25; 4, a young embryo
after the appearance of the primitive segment-folds, x 50; 5, a more
advanced embryo, with the antennal folds distinctly marked off; the
free ends of the primitive folds have united across the embryo
posterior to the antennal folds, x 50; 6, ventral view of the embryo
with the appendages budding out, x 25 (the embryo in this stage lies
dormant through the six colder months of the year): _am_, amnion;
_m_, micropylar end; _ch_, chorion; _gb_, primitive band; _bf_,
brain-fold; _yl_, yolk; _tf_, caudal fold; _kf_, head-fold
(procephalic lobe); _p.fd.t_, primitive thoracic fold; _p.fd.m_,
primitive maxillary fold; _p.fd.a_, primitive abdominal fold;
_ab.c_, abdominal constriction; _t.c_, thoracic constriction;
_at.l_, antennal lobe; _M_, mesoderm; _h.g_, head groove; _mo_,
mouth; _sk_ invagination of ectoderm to form head-apodeme; _md_,
rudiment of mandible; _m^1_, 1st, _m^2_, 2d maxilla; _T^1–T^5_,
legs: _ab.p_, 1st abdominal appendage; _ap_, other appendages; _tb_,
caudal expansion; _mf_ median furrow; _B_, primitive unpaired organ
(metastomum).—After Ayers.
]

These primitive regions of the primitive band, before the segments
are formed, are called by Graber _macrosomites_, and the secondary
segments into which they divide (which afterwards become the
body-segments), _microsomites_. The macrosomites are peculiar to
insects, and may be an inheritance from a hypothetical ancestral
form. With Korschelt and Heider, we should hardly share this view.

FIG. 526.—Older embryo of Œcanthus with the appendages budded out,
those of the abdomen distinct: _abp_, first pair; _a.s_, anal
stylet; _pr_, proctodæum; _am_, amnion.—After Ayers.
]

Our observations on locusts show clearly (1) that the procephalic lobes are the pleural portion of the first cephalic or antennal segment; (2) that the antenna is an appendage or outgrowth of the procephalic lobes; (3) that the eyes are a specialized group of epidermal cells of the upper part of the procephalic lobes, and are not homologues of the antennæ or of the appendages in general; and (4) it seems to follow from a study of the relations and mode of development of the clypeus and labrum, that they arise between the procephalic lobes, and probably represent the tergal part of the antennal segment, forming the roof of the mouth, _i.e._ closing in from above the pharynx.

In general the formation of the body-segments into the primitive band is in succession from before to the hinder end. This successive appearance has been observed by Graber in genera of different orders (Stenobothrus, Lina, and Hylotoma). For example, in the beetle Lina, after the appearance of the mandibular and two maxillary segments, appear the three thoracic segments, together with the two anterior abdominal segments, the other abdominal segments arising afterwards. In other cases, the formation of segments seems to be simultaneous along the entire length of the primitive band. An exception to the rule has been noticed by Heider in Hydrophilus, as in this beetle the development of the segments of the middle region appears somewhat delayed, while the fore and hind parts of the primitive band are more rapid in development. In Pieris, according to Graber, the thoracic segments are more rapidly developed than the others; soon after, the gnathic segments (mandibles and two pairs of maxillæ) appear, and finally the abdominal segments are formed.

=Fore-intestine (stomodæum) and hind-intestine (proctodæum), Labrum.=—The digestive canal of insects consists, as in other animals, of three portions, the fore, mid, and hind gut or intestine. The next change after the completion of the segments of the primitive band is the development of the fore and hind intestine and the appendages. The fore-intestine (stomodæum) arises as an invagination in the area of the primary head-section, and the hind-intestine (proctodæum) in the terminal section (Figs. 300 and 546).

In insects generally the formation of the fore-intestine occurs
earlier than that of the hind-intestine. An exception was discovered
by Graber and also by Voeltzkow in Muscidæ, where the proctodæum
appears earlier.

FIG. 527.—Rudiments of the appendages of the embryo of Hydrophilus:
_an_, antenna; _md_, mandibles; _mx_{1}_, 1st, _mx_{2}_, 2d maxilla;
_vk_, clypeal region; _m_, mouth; _p^1-p^3_, legs; _p^4-p^9_,
rudiments of abdominal appendages, 1–9; _st_, stigma; _a_,
anus.—After Heider, from Lang.
]

Usually at the time of origin of the stomodæum a projection arises at the anterior edge of the primary head-region, the so-called forehead (Fig. 527, _vk_), which is the common rudiment of the clypeus and labrum. In many cases (certain Coleoptera and Lepidoptera) these rudiments first assume the form of paired hooks (see Figs. 83, 102, 104, 105, of Graber’s Keimstreif der Insekten, also Figs. 529 and 546), which afterwards, by fusion in the median line, become single, though notched in the middle; but in the more generalized Blatta and Mantis, as well as in bees, the rudiment is single at the outset.

The view advanced by Patten, and also by Carrière, that the labrum
is a first pair of antennæ, is scarcely tenable, and we quite agree
with Korschelt and Heider in regarding the clypeo-labral region as
homologous with the upper lip of Crustacea, and, we may add, of
Merostomes and of Trilobites.

It should be observed that in many insects, in their earlier
embryonic state, directly behind the mouth arises, from paired
rudiments, what seem provisional lower lip structures (not to be
confounded with the 2d maxillæ of insects). This under lip structure
was first discovered by Bütschli in the bee (his inner or 2d
antennæ), and afterwards by Tichomiroff in Lepidoptera. Heider, in
his work on Hydrophilus, describes it as the “lateral mouth-lips,”
while, more recently, Nusbaum has observed it in Meloë. This under
lip structure may be regarded as analogous to the paragnaths of
Crustacea, although to attempt to homologize it with these seems
useless. (Korschelt and Heider.)

=Completion of the head.=—Sufficient attention has not been paid to this subject by embryologists. The head is at first, dorsally, mostly composed of the head-lobes, or antennal segment only, and the dorsal or tergal portion of the oral appendages develop at a later period. We have observed in the embryo of dragon-flies (Æschna) that the tergites of the mandibles and first maxillæ are simultaneously fused with the head-lobes, while the much larger tergal region of the 2d maxillæ remains for some time separate from the anterior part of the head, and is continuous with the thoracic segments, and it is only just before hatching that this segment becomes fused with the rest of the head (Fig. 36). In a sense, the 2d maxillary segment when it is free from the head reminds us of the foot-jaw, or 5th segment of chilopod myriopods (see also p. 53).

_g._ The appendages

As we have seen, nearly or quite simultaneously all the limbs as a rule bud out from each side of the median line of the primitive band. They arise as saccular evaginations or outgrowths of the ectoderm, directed a little backwards. They are at first filled with mesoderm cells, and in the Orthoptera diverticula of the cœlom-sac are taken up into the rudimental limbs, as in Peripatus and Myriopoda. (Graber, Cholodkowsky.) As the antennæ, mouth-parts, legs, and abdominal appendages are all alike at first, their strict homology with one another is thus demonstrated. In insects never more than a single pair of limbs is known to arise from one segment.

=The cephalic appendages.=—The antennæ evidently arise from the hinder edge of the procephalic lobes (Fig. 527, _an_). As in Limulus, the first pair of appendages are at first postoral (Fig. 528, _at_), afterwards moving forward owing to changes in the relative proportions of the parts of the head, and they are in all respects, in their development and position in relation to the segment from which they arise, homologous with the appendages succeeding them.

The occurrence of rudiments of a pair of preantennal appendages in
Chalicodoma which is claimed by Carrière, needs confirmation, as
other embryologists have not observed them.

The postoral appendages of the head are the mandibles and the 1st and 2d maxillæ, besides the supposed premandibular segment already referred to on pp. 50–54, which only temporarily exists.

The trophi or oral appendages are all alike at first, but soon differ in shape, acquiring their characteristic form shortly before the embryo leaves the egg. The mandibles of Œcanthus are said by Ayers at the time of revolution of the embryo to be slightly bilobed, and in his Fig. 5, Pl. 19, they are represented as deeply trilobed, but in general they are undivided. The 1st maxillæ are at this time distinctly trilobed. The 2d maxillæ are separate, and distinctly though unequally bilobed, becoming united shortly before birth. In the embryos of dragon-flies they are at an early date very large and long, and directed backwards, and are not fused together until just before hatching, when the extraordinary mask-shaped labium is fully developed.

FIG. 528.—Two embryonic stages of the primitive streak of Melolontha.
_A_, younger stage, with rudiments of eight pairs of abdominal
appendages (_a^1–a^8_). _B_, older stage, the primitive band now
very broad: _a_, 1st abdominal appendage, in _B_ sac-like; _x_,
place of adhesive disc; _g_, brain; _l_, clypeo-labrum; _s_, lateral
cord of the ventral nervous cord; other lettering as in previous
figures.—After Graber, from Korschelt and Heider.
]

The distal parts of the labium, such as the ligula, palpifer, and palpus are elaborated before the mentum and submentum. Many details as to the final changes in the mouth-parts before hatching remain to be worked out.

=The thoracic appendages.=—The three pairs of legs arise at the same period and in the same manner in all insects; it is not until the end of embryonic life that they become jointed, and that the claws and onychia are developed. Especial attention has not yet been given to the details of the development of the parts of the last joint of the tarsus.

In many forms the antennæ are the first to appear, the mandibles,
maxillæ, and legs appearing at a latter date, though simultaneously.
It is thus in Stenobothrus, Hydrophilus, and Melolontha. In Lina,
according to Graber, the mandibles precede the antennæ in
appearance. In the Libellulidæ, according to Brandt, the legs first
appear, then the jaws, and lastly the antennæ. This did not seem to
be the case in the embryos of Æschna observed by us, although our
observations were more superficial.

On the other hand, in those insects whose larvæ are footless, the
rudiments of the legs are retarded and aborted just before hatching
(fossorial Hymenoptera and Apidæ), or the rudiments of the legs are
not developed at all.

=The abdominal appendages.=—These appear soon after the thoracic limbs, corresponding in most cases to the latter in shape and position, and their position in the embryo is a matter of the greatest interest. Von Rathke was the first embryologist to detect those of the first abdominal segment, in his examination of the development of Gryllotalpa. Long afterwards Bütschli detected them in the embryo of the honey-bee, observing a pair on each segment. Patten observed them in Trichoptera; Kowalevsky first perceived them in Lepidoptera, Tichomiroff confirming his observations. Graber, Ayers, and Wheeler have observed them in Orthoptera and Coleoptera, and the latter has detected them also in Hemiptera and Neuroptera; and while they do not arise in the embryos of Diptera and of Siphonaptera, they are to be looked for in any or all the lower or more generalized orders.

As the result of these discoveries of polypodous embryos occurring in all but the most specialized order (Diptera), it appears to be a rational deduction that the winged insects have descended from insects in which there were functional legs on each abdominal segment. Such an ancestor was the forerunner of the Thysanura, in which abdominal locomotive appendages still survive, though in a modified, more or less aborted condition. This polypodous ancestral form was apparently allied to Scolopendrella, which has a pair of functional legs on each abdominal segment.

The subject, then, of polypodous embryo insects is one of special
interest, and has attracted much attention from Graber, Wheeler,
Haase, and others. That these are genuine, though transitory
appendages, is shown by the fact that certain pairs persist
throughout adult life. The embryology of the Thysanura when worked
out will throw much light on this subject, but we know that the
spring (elater) of Collembola (and possibly the collophore) and the
cerci of the winged insects are survivals of these limbs. That the
three pairs of appendages forming the ovipositor, or sting, are most
probably derived from these appendages is claimed by Wheeler (p.
167), and seems proved by the fact that Ganin and also Bugnion has
detected three pairs of imaginal disks in the embryo of parasitic
Hymenoptera. Hence the abdominal appendages may ultimately be found
to arise in nearly all cases from imaginal disks like those giving
origin to the cephalic and thoracic appendages.

As regards the Diptera, Pratt has observed that each of the three
thoracic and eight abdominal segments of the embryo brachycerous
Diptera (seen especially well in Melophagus) has two pairs of
imaginal disks, a dorsal and a ventral pair. He thinks there is no
doubt but that the ventral abdominal disks are homologous with the
rudimentary appendages which appear in the embryos of all other
insects, though not in the brachycerous dipters.

=Appendages of the first abdominal segment (pleuropodia).=—As early as 1844 Rathke observed in the embryo of the mole-cricket a pair of appendages on the 1st abdominal segment, which he described as mushroom-shaped bodies, and supposed to be embryonic gills. They are called _pleuropodia_ by Wheeler, who, with Patten, Graber, and Nusbaum, ascribes a glandular function to them, while Wheeler suggests that they were odoriferous repugnatorial organs. In Blatta (Phyllodromia) they are of large size, in Melolontha enormous (Fig. 528, _B_) and filled with blood. Wheeler distinguishes as varieties, beside the mushroom-shaped appendages of Gryllotalpa and Hydrophilus, the reniform (Œcanthus), the broadly pyriform (Blatta), and the elongate pyriform (_Mantis carolina_). In the European Mantis they are most limb-like, with a digitiform continuation divided by a constriction into two sections. (Graber.) In Meloë they assume the shape of a stalked cup. (Nusbaum.) In the bee and in Lepidoptera the pleuropodia are not present, though the temporary appendages on the succeeding segments appear; Carrière, however, found them on the two first abdominal segments of very young larvæ of the wall-bee (Chalicodoma).

Their cellular structure is peculiar, and they are either formed by evagination or invagination, those of the latter type being subspherical and solid. Those of the former type have a cavity communicating by means of a narrow duct through the peduncle with the body-cavity (Blatta). No tracheæ, nerves, or muscles enter them, though blood-corpuscles have been seen in the cavities. “In some species the pleuropodia produce a secretion from the ends of their enlarged cells. This secretion may be a glairy albuminoid substance (Cicada, Meloë), a granular mass (Stenobothrus), a bundle of threads (Zaitha), or a thick, striated, cuticula-like mass (Acilius).” They attain their greatest size during the revolution of the embryo, and they are “mere rudiments of what were probably in remote ages much larger and more complex organs.” (Wheeler.)

Lameere has observed that in Phyllodromia the first pair of
abdominal appendages, after becoming of considerable size, undergo
an enlargement at their free end, become detached, and fall into the
amnion.

Wheeler also calls attention to the homology of these pleuropodia
with the 1st abdominal appendages of Campodea, shown by Haase to be
originally glandular, but with at present a respiratory function. In
the embryos of later, higher orders of insects, these appendages are
in size and shape similar to those of the succeeding segments. (See
also p. 164.)

FIG. 529.—Primitive band of _Bombyx mori_, showing the temporary legs
on abdominal segments 2–11: _A_, early stage, in which the abdominal
legs _al^2–al^10_ appear. _B_, later stage, when they are very faint
and all except _al^3–al^6_ and _al^10_ are about to disappear. _C_,
the persistent abdominal legs _al^3–al^6_ and _al^10_; _st^2_,
_st^9_, the 2d and 3d pair of stigmata; _sgl_, silk duct.—After
Tichomiroff.
]

=Are the abdominal legs of larval Lepidoptera and phytophagous Hymenoptera true limbs?=—The presence of these abdominal legs in the embryos of Sphinx (Kowalevsky), of _Bombyx mori_ (Tichomiroff), and both _Bombyx mori_ and _Gastropacha quercifolia_ (except those of the first segment), as well as in Hylotoma, which has 11 pairs of such appendages, has suggested that the prop or prolegs of caterpillars and saw-fly larvæ are survivals of these outgrowths, and not secondary, adaptive structures. Opinions on this point vary. Balfour, and also, more recently, Cholodkowsky, hold that the prolegs are survivals of the embryonic appendages. Graber cautiously, after a lengthy and interesting discussion, says that the question cannot be, in the present state of our knowledge, solved. He, however, seems inclined to believe that the prolegs are not merely secondary structures, and that the rudiments of limbs may remain for a long time in a latent state before their final development. Korschelt and Heider are disposed to regard the abdominal appendages of Lepidoptera and Hymenoptera as true limbs, referring to Balfour’s statement that in the Crustacea there are different examples of the loss and later appearance of limbs, such as the loss of the mandibular palpi of the zoëa of decapods, and the loss in the zoëa of appendages in the Erichthus form of the Squilla larva corresponding to the third pair of maxillipedes and first two pairs of legs of Decapoda, and which are afterwards reproduced; similar cases occurring in the Acarina. In the wasps and bees also, as is well known, the imaginal disks of the thoracic appendages appear, the legs themselves being suppressed in the larva (the imaginal disk probably existing in an indifferent state), to reappear in the pupa and imago. It does not, however, necessarily follow that the numerous pairs of hooked ventral tubercles of certain dipterous larvæ (Ephydra) are true appendages.

It seems to us that it is a strong argument for the view that these prolegs are survivals of primitive limbs, that from similar embryonic paired outgrowths on different segments arise the spring of Podurans, the anal cerci, and three pairs of appendages forming the ovipositor, and the anal legs of the Corydalus larva, as well as those of caddis-worms; at least five abdominal segments throughout the class of insects as a whole bearing appendages in the adult.

On the other hand the view of Haase, that the prolegs of caterpillars are secondary, adaptive characters, is supported by the fact of the rapidity with which two pairs on the 3d and 4th segments nearly disappear in the larvæ of certain Noctuidæ (Catocala, etc.), a reduction evidently due to disuse.

=The tracheæ.=—The tracheal system arises as ectodermal invaginations on one side of the appendages, appearing soon after the latter. The earliest condition of the tracheal invagination is seen in section at Fig. 539, _E_, _tr_; as it deepens, it sends off diverticula or tracheal branches, while the narrow mouth of the invagination forms the stigma. The cup-like cavities situated serially one behind the other, and arising from the single tracheal invaginations, become at the end or bottom of the cup elongated along the length of the body and fused together at their ends; then the two longitudinal stems of the system arise, by a breaking through at the place where the original invagination had become fused, thus forming a continuous tube, the lumina opening into each other. (Bütschli.)

The cuticular tracheal intima is differentiated late in embryonic life. The entrance of the air is accomplished in part before the embryo hatches, the air being derived from the tissues and fluids of the body.

The farther development of the tracheal branches is due to the
progressive formation of diverticula. The branches thus arising are
intercellular formations. On the other hand, the finest twigs are
intercellular structures. However, as Schaeffer states, the
differences between the two modes of formation are not important.

Wheeler mentions the existence of “two pairs of very indistinct
tracheal openings in the 10th and 11th somites” of the abdomen of
Doryphora (Fig. 546, _t_{19}_, _t_{20}_), and Heider believes that
they exist in Hydrophilus.

The tracheal invaginations as a rule begin to appear after the appendages commence to bud out. An exception is met with in the bee (Apis), where the tracheal ingrowths are seen before the rudiments of the legs. Most of the tracheal invaginations appear simultaneously. Only rarely do we see an indication of their successive development from before backwards. Thus in Hydrophilus, Graber observed that the mesothoracic stigmata appeared somewhat earlier than those of the other segments.

_h._ Nervous system

The rudiments of the nervous and tracheal systems essentially contribute to the building up of the relief of the primitive band of insects. The nervous system is the earliest to appear, being indicated very early, in fact before the appendages begin to grow out. The first traces of the nervous system are two ridges extending along the primitive band, the depression between them being called the primitive furrow. At an early period the segmentation is observed in the primitive ridges, while widened spaces (the rudiments of the ventral ganglia) alternate segmentally with the narrow places which are the incipient longitudinal commissures (Fig. 527, _A_, _g_).

The primitive ridges extend anteriorly into the head-lobes; this part must be regarded as the rudiment of the œsophageal commissure. The rudiments of the brain are from their first appearance directly connected with the ventral chain of ganglia.[83]

=Completion of the definite form of the body.=—This is accomplished by the growth of the primitive band around the yolk, the band widening, so that its edges behind the head extend up, and finally meet on the back, forming the back or tergum of the embryo, thus enclosing the yolk (Fig. 530, _F_). The tergal wall of the head is due to the dorsal growth of the head-lobes, and of the clypeo-labral region. In the course of this process the anterior end of the primitive band becomes turned up dorsally, forming a dorsal curve or bend. By this bending up of the primitive band the forehead nearest the mouth forms a transverse ridge, the labrum, while the basal or earlier part of the forehead now is differentiated into the clypeus. This clypeo-labral region likewise forms the roof or palatal region of the mouth. The head-lobes cause by this dorsal growth a rotating motion which carries the rudimental antennæ back over the mouth.

FIG. 530.—Diagram of the formation of the dorsal organ in Hydrophilus.
_A_, cross-section through an egg, whose primitive streak is still
covered over by amnion (_a_) and serosa (_s_). _B_, amnion and
serosa are grown together in the middle line, then separated and
drawn back to form a fold on each side. _C_, by the contraction of
the serosa (_s_),which becomes converted into the dorsal plate, the
folds become drawn up dorsally. _D_, the contracted serosa becomes
partly overgrown by the folds. _E_, the folds grow together to form
the dorsal tube. _F_, the mid-gut has closed over dorsally and
enclosed the dorsal tube (_s_): _a_, amnion; _d_, yolk; _ec_,
ectoderm; _h_, heart; _l_, body-cavity; _m_, rudiment of the
mid-gut; _n_, nervous system; _s_, serosa (in _C_ and _D_ = dorsal
plate, in _E_ and _F_, dorsal tube); _tr_, the chief tracheal
stem.—After Graber and Kowalevsky, from Lang, and Korschelt and
Heider.
]

The gnathal or post-antennal segments at first bear but a small part in completing the tergal region of the head, but shortly before hatching the mandibles and their muscles enlarge, giving fulness to the upper and back part of the head.

_i._ Dorsal closure and involution of the embryonic membranes

FIG. 531.—Schematic figure of the formation of the dorsal tube by
invagination of the dorsal plate (transformed serosa); following
after stage Fig. 520, _C_, and Fig. 521, _D_; _am_, amnion (now
forming the provisional dorsal closure); _r_, dorsal tube, whose
cells are already breaking away.—After Korschelt and Heider.
]

In most other Arthropoda (Crustacea, Arachnida, Myriopoda, etc.) development goes on by the formation of a so-called primitive band, but without the appearance of peculiar embryonic membranes. The outer surface of the entire egg becomes, then, in part covered by the band-like embryonic germ, and partly by a portion of the blastoderm which remains unchanged. The dorsal region is formed by the widening and spreading of the primitive band over the greater part of the surface of the egg, while the area of the unchanged section of the blastoderm continually becomes more restricted. It is generally accepted that the latter is concerned in the dorsal closure, because, together with a histological transformation, it becomes involved in the formation of the ectoderm of the primitive band.

A similar form of retrograde structure possibly occurs in the
embryos of Poduridæ, in which a dorsal organ has been observed to
develop in an early embryonic stage, which bears some relation to
the cuticula enveloping the embryo, but whose significance is in
general rather obscure.

In most insects the relations are more complicated, since in such
cases, the amnion-folds rise on the edges of the primitive band and
of the unchanged section of the blastoderm, whose retrograde
development is intimately connected with the closure of the back.

A very simple case of dorsal closure, but which certainly is not a
primitive one, occurs in Muscidæ and certain other Diptera whose
amnion-folds are developed in a rudimentary way. In this case
(according to Kowalevsky and Graber), the amnion-folds become
smoothed out again. Amnion and serosa become then a simple
epithelium, which throughout corresponds to the unmodified type of
blastoderm of Crustacea, Arachnida, and Myriopoda, and here seems to
share in the formation of the back. More complicated and very
manifold relations of dorsal closure and involution of the embryonal
membranes occur in other insects, of which Korschelt and Heider
distinguish four different types:

1. Involution under the formation of a continuous dorsal
amnion-serosa-sac (Odonata).

2. Involution with exclusively dorsal absorption of the amnion
(Doryphora).

3. Involution with exclusively dorsal absorption of serosa and
separation of the amnion (Chironomus and Trichoptera).

FIG. 532.—Diagram of the formation of the dorsal walls in Doryphora
in cross-sections: _am_, amnion; in _B_, serving as a provisional
dorsal closure, in _C_, about to break up; _k_, primitive band;
_s_, serosa.—After Wheeler, from Korschelt and Heider.
]

4. Involution with separation of both embryonic membranes
(Lepidoptera and Hymenoptera, Hylotoma).

FIG. 533.—Involution of the embryonic membranes of Chironomus: _am_,
amnion; _r_, dorsal umbilicus; _s_, serosa, which has withdrawn
into the region of the dorsal umbilicus, and in _C_ has passed
into the interior of the embryo.—After Graber, from Korschelt and
Heider.
]

The first type occurs in the most primitive order of winged insects.
The second type (Coleoptera) appears to be an independently
inherited form of dorsal closure. In the first type, the formation
of the amnion-serosa-sac is initiated by a rupture of the two fused
embryonic membranes. This rupture in the ventral middle line occurs
in Odonata only in the region of the head-section. In the second
type only the amnion, in the third only the serosa are concerned in
this rupture, while in the fourth type both membranes remain intact
until the slipping out of the larva. (Korschelt and Heider.)

_j._ Formation of the germ-layers

FIG. 534.—Diagram showing the formation of the embryonic membranes in
Lepidoptera (_A_, after Kowalevsky, _B_ and _C_, after Tichomiroff):
_k_, primitive band; _am_, amnion: _se_, serosa; _do_, yolk; _vd_,
invagination of the fore-gut, _ed_, of the hind-gut; _m_, mouth;
_an_, anus; _x_, dorsal umbilical passage.—From Korschelt and
Heider.
]

The older views on the structure of the layers of the primitive band of insects were thoroughly unsatisfactory. Bütschli first found that in the bee, by a kind of folding process, an inner layer of the primitive band arose. Soon afterwards Kowalevsky, by the employment of section-cutting and thorough researches, laid the foundation of a more exact knowledge of these layers. He found that in Hydrophilus a furrow extended along the whole length of the primitive band (Fig. 515, _A_, _B_, _r_), which, while invaginating or sinking in, gave rise to the inner layer of the primitive band, i.e. the common rudiment of endoderm and mesoderm (Fig. 539, _A-C_).

Kowalevsky also found similar conditions in the honey-bee (Apis), Lepidoptera, and other forms. The furrow above mentioned must be regarded as a very long gastrula invagination, extending along the entire ventral side of the embryo, and the edges of the furrow as a long-drawnout blastopore. The tube arising in Hydrophilus through the closing of the furrow we may regard as a primitive intestinal canal.

The first rudiment of the gastrula furrow appears in insects as two folds extending along both sides of the median line in the thickened ventral plate (Fig. 536, _f_), through whose formation a more median section of the ventral plate, the so-called middle plate (_m_), becomes separated from the side plates (_s_). As the middle plate curves in and becomes overgrown by the folds forming the edges of the blastopore, the gastrula-tube (Fig. 539, _A_, _r_) is formed, and furnishes the rudiments of the lower (inner) layer. The ectoderm, then, according to Heider, arises from the lateral plates of the primitive band. The growth of the edges of the blastopore, by which the closure of the gastrula-tube is effected, takes place latest in the region of the most anterior part of the furrow (Fig. 515, _B_ and _C_), corresponding to that place in the primitive band in which the stomodæum afterwards develops.

FIG. 535.—Two embryonic stages of a saw-fly (_Hylotoma berberidis_) in
schematic median section: _a^1–a^{10}_, 1st to 10th abdominal
segments; _bg_, ventral nervous cord; _og_, brain; _ol_, germ of
labrum; _sp_, salivary gland; _ed_, hind-gut; _x_, _x′_, inner folds
of amnion: other letters as before.—After Graber, from Korschelt and
Heider.
]

FIG. 536.—Gastrula stage of the wall-bee (Chalicodoma), so-called
flask-shaped stage: _f_, folds which on each side border the middle
plate (edge of the blastopore); _m_, the partly segmented middle
plate (here = rudiment of the mesoderm); _s_, the segmented lateral
plate (becoming afterwards the ectoderm of the primitive band);
_ve_, fore, _he_, hinder entodermal rudiment.—After Carrière, from
Korschelt and Heider.
]

Fig. 537.—Two successive stages in the gastrulation of Apis.
Cross-section through the primitive band: _b_, lower (inner) layer;
_ec_, ectoderm.—After Grassi, from Korschelt and Heider.
]

During the invagination of the middle plate and its transformation into the gastrula-tube a change takes place in its histological character (Fig. 539, _A_ and _B_). While it originally consists of a high cylinder epithelium, which after farther changes becomes divided into several layers, since the wedge-shaped single cells push themselves over each other, the cells in later stages become more and more cubical or irregularly polygonal (Fig. 539, _B_), and are irregularly arranged. At the same time the gastrula-tube is compressed in a dorso-ventral direction. While it in this way spreads out laterally under the side plates (ectoderm), its originally circular primitive lumen passes into the form of a horizontal fissure, which in Hydrophilus long remains as the boundary between the two layers of the inner (or lower) membrane. (Korschelt and Heider.)

There are numerous variations of the process of gastrulation, which
are by Korschelt and Heider divided into three types, as follows:—

1. Through invagination and formation of a tube (Fig. 539, _A_,
Hydrophilus, Musca, Pyrrhocoris, etc.).

2. By a lateral overgrowth (Fig. 537, Lepidoptera and Hymenoptera).

3. By an inward growth of cells from a median furrow (Aphides and
Trichoptera).

In Doryphora and Lina (Fig. 524) the hinder end of the gastrula
furrow is forked.

FIG. 538.—Diagrammatic sketch of the formation of the germinal layers
in Doryphora: _A_, view of upper surface. _B_, cross-section through
the fore end of the primitive streak at the line _a-a_. _C_, section
through the middle of the primitive streak corresponding to the line
_b-b_. _D_, section through the hinder end of the primitive band
corresponding to the line _c-c_: _bl_, blastopore; _ec_, ectoderm;
_en′_, anterior U-shaped; _en″_, hinder U-shaped germ of the
endoderm; _ms_, mesoderm.—After Wheeler, from Korschelt and Heider.
]

The cellular layer arising from the gastrula invagination (lower layer) forms the common germ of the endoderm and mesoderm. It has only recently become known how these two germ-layers of insects have become differentiated. Kowalevsky first discovered in Musca that the greatest part of the lower (inner) layer yielded mesoderm exclusively, and that a cell-mass only corresponding to the most anterior and posterior end of the primitive band was used in the formation of the endoderm. We must therefore, in insects, speak of a fore and a hinder endodermal rudiment. In proportion, now, as the ectodermal invaginations, which are destined to form the stomodæum and the proctodæum sink beneath the surface of the embryo, the cell-masses of which the two endodermal rudiments are composed are pushed farther in, and a separation between them and the mesoderm is thus effected. The two endodermal rudiments now form accumulations of cells which lie closely adjacent to the blind ends of the stomodeal and the proctodeal invaginations. They soon widen out into two hour-glass-shaped rudiments, which are directed with their concavities towards each other, but with their convex side towards the nearest pole of the egg. They soon change their form; two lateral stripes grow out from them, and each now assumes the form of a U (Fig. 538, _en′_). The limbs of the fore and hind U-shaped rudiment are directed toward each other, and grow towards each other until they meet, and are fused together. Thus the endodermal rudiments arising out of the fusion of the two U-shaped rudiments form two stripes extending along the primitive band and situated mostly under the primitive segments. At the two ends the endodermal rudiment fuses with the stomodeal and proctodeal invaginations. These lateral endodermal streaks now spread out, and gradually begin to grow over the yolk, on whose outer surface they lie. This overgrowth makes the greater advance on the ventral side, so that the two endodermal streaks first unite in the ventral median line and afterward in the dorsal. The yolk in this way passes completely into the interior of the rudiment of the mid-intestine.

Kowalevsky has already proved that it is the median parts only of
the inner layer which at the two ends of the primitive band become
separated as endodermal rudiments through the advance of the
stomodeal and proctodeal invaginations: the lateral portions become
mesoderm.

Kowalevsky has compared the germ-layers of insects with those of
Sagitta. This comparison is supported by the later researches of
Heider and of Wheeler on Coleoptera. (See Korschelt and Heider, p.
809.)

Relations somewhat different from the common type of formation of
germ-layers occur in Hymenoptera. Kowalevsky and also Grassi agree
that here also the endoderm originally forms a part of the lower
(inner) layer. But the separation of the endoderm from the mesoderm
goes on in Apis in such a way that the two ends of the inner layer
pass up to the dorsal side of the egg, where the fore and hind
rudiments of the endoderm extending along the back of the embryo
grow together. When the two horseshoe-shaped rudiments have met each
other and become fused, the enclosing of the yolk begins, which
accordingly here proceeds from the dorsal towards the ventral side,
instead of _vice versa_. As a result the endodermal cell-layer in
Apis (and also Chalicodoma) at first does not lie under the
primitive band, but on the dorsal side of the egg under that flat
epithelium, which, arising from the amnion-fold, completes the
provisional closure of the back.

The yolk-cells and secondary yolk-segmentation are discussed by
Korschelt and Heider at this point. The yolk-cells are elements
scattered throughout the yolk and which partly remain in the yolk
during the formation of the blastoderm (Fig. 507, _C_ and _D_), but
which in part through a later immigration pass out of the blastoderm
into the yolk. Graber has proved the fact of the migration of cells
from the lower layer into the yolk, and his observations have been
confirmed by other authors. Indeed, in certain cases (Melolontha),
these later immigrant cells are clearly distinguishable by their
histological characters from those originally found in the yolk.

The yolk-cells are regularly scattered throughout the yolk. Their
use to the embryo lies in the fact that they absorb the particles of
yolk, which they digest and thus reduce to a fluid condition. It
usually happens that after the complete formation of the primitive
band there results a delimitation of the areas enclosing each
yolk-cell, and this occurrence is called _secondary yolk-division_.
In special cases (Apis, Musca) this occurrence seems not to take
place. The yolk-cells are still, after the complete formation of the
mid-intestine, to be recognized in the yolk-remnants filling the
interior of the same, and gradually become absorbed.

_k._ Farther development of the mesoderm. Formation of the body-cavity

We have seen that by means of an invagination extending throughout the entire length of the primitive band a layer of cells is produced which soon spreads out on the inner side of the band and thus forms a second lower (inner) layer (Fig. 539, _C_). From this inner layer is separated at the anterior and posterior ends of the primitive band, the endoderm, which lies in direct contact with the invaginations of the proctodæum and stomodæum. The remainder, by far the most extensive part of the inner layer, is the mesoderm.

The mesoderm now becomes divided into two lateral streaks (mesodermal streaks), by the withdrawal of its cells from the median line (Fig. 539, _D_). This withdrawal is not, however, always a complete one. In the free median space thus formed, the yolk often forms the so-called _median yolk-ridge_. Segmentally arranged cavities soon appear in the lateral region of the mesoderm (the primitive segmental cavities), and the bordering mesoderm-cells arrange themselves in the form of an epithelium, and constitute the wall of the primitive segments or cœlom-sac. (Korschelt and Heider).

The primitive segmental cavities in general arise through a split in
the mesoderm. In Phyllodromia, according to Heymons, the primitive
segments are very extensive. The mesoderm, at the time of the
formation of the rudiments of the appendages, is raised with the
ectoderm from the surface of the yolk, and in this way there arise
in each segment cavities, which, since they are surrounded by
mesodermal elements, become the closed cœlom-sacs (Fig. 540, _c_,
_c′_, _c″_).

The cœlom-sacs differ in different groups. They are largest in
Orthoptera (Phyllodromia), where they take up almost all the cell
material of the mesoderm in their formation, and exhibit certain
conditions recalling those of Peripatus. The very large primitive
segmental cavities, which in Orthoptera also extend into the
rudiments of the appendages (Fig. 540, _B_, _ex_), in their later
stages are, through the formation of a constriction, divided into a
dorsal and a ventral half (Fig. 540, _B_, _c′_, _c″_). The ventral
portions of these cavities extending into the extremities soon
disappear, while the cells of their walls lose their epithelial
nature, and group themselves irregularly into a sort of mesenchym.
In this tissue, then, arises, partly through a separation among its
cells, partly through the elevation of the same from the upper
surface of the yolk, _the definite body-cavity_. The dorsal portions
of the primitive segmental cavities remain unchanged a longer time
in order to play a rôle in the formation of the intestinal muscular
layer, of the heart, pericardial septum, and sexual organs.

FIG. 539.—Cross-section through the primitive streak of Hydrophilus in
six successive stages: _A_, gastrula-stage (compare Fig. 515, _A_,
corresponding to the point _a_). _B_, cross-section through stage,
Fig. 515, _D_, in the most anterior section of the primitive band,
where the same is not completely overgrown by the amnion-folds. _C_,
cross-section through the trunk-segment of stage, Fig. 515, _E_.
_D_, _E_, _F_, cross-sections through later stages: _am_, amnion;
_b_, lower (inner) layer; _d_, yolk; _dz_, yolk-cells; _ec_,
ectoderm; _en_, entoderm; _l_, definite body-cavity; _pr_, primitive
groove (= neural groove); _pw_, primitive roll, or strip, of the
ventral nerve-cord; _r_, blastopore; _sp_, fissure in the mesoderm
(remains of the cavity of the primitive intestine); _se_, serosa;
_s_, lateral cord of the rudiment of the nervous cord; _spm_,
splanchnic layer of the mesoderm; _tr_, rudiment of a trachea (in
_E_ appearing as an invagination of the ectoderm) in _F_ in
cross-section; _us_, primitive segment (= cœlomic sac).—After
Heider, from Lang.
]

FIG. 540.—Cross-sections through the abdominal part of three
successive stages of evolution of _Phyllodromia germanica_: _am_,
amnion; _bg_, rudiment of the ventral nervous chord; _c_, cœlomic
cavity; _c′_, dorsal, and _c″_, ventral, section of the cœlomic sac;
_cz_, cells of the walls of the primitive segment, which are joined
to the genital rudiments; _gz_, genital cells; _dw_, dorsal wall of
the cœlomic sac; _d_, yolk; _ec_, ectoderm; _ep_, epithelium-cells;
_ex_, rudiment of the abdominal appendages; _f_, germ of the
fat-body; _lw_, lateral wall of the cœlomic sac; _m_, mesoderm
cells, which take no part in the formation of the cœlomic sac; _mw_,
median wall of the cœlomic sac; _so_, somatic mesoderm layer; _vm_,
ventral longitudinal muscle.—After Heymons, from Korschelt and
Heider.
]

In the highest groups of insects (Coleoptera, Lepidoptera, and
Hymenoptera) the primitive segments are not so extensively developed
(Fig. 539, _D-F_, _us_). They here form only relatively small sacs
situated in the lateral parts of the primitive band which correspond
to the dorsal section of the cœlom-sacs of Orthoptera. The ventral
part is here from the very outset replaced by a mesenchym. As a
result in these forms also no cœlomic diverticula occur in the
rudiments of the extremities.

The definite body-cavity of insects arises entirely independent of
the cœlom cavities, and in fact, as Bütschli showed, through the
separation of the primitive band from the yolk (Fig. 539, _F_, _l_).
It appears bounded on the one hand by the surface of the yolk, on
the other side by the irregularly arranged mesenchym cells.
Originally we can in cross-sections distinguish three separate
cavities of the definite body-cavity (in Hydrophilus according to
Heider), a median and two larger paired lateral ones which later
fuse with each other and with wide lacunæ (_e.g._ in the appendages)
arising by the separation of the mesenchym cells. We refer the
compartments of the definite body-cavity, as in Peripatus, to the
primary body-cavity or segmentation-cavity. They are only lacunæ in
the area of the mesenchym, and throughout bear the character of a
pseudocœl.

In later stages of embryonic development the cœlom-sacs and the
definite body-cavity enter into communication with one another (Fig.
523, _A_, _us_, _lh_). (Korschelt and Heider.)

Then the hinder cœlom-sacs unite through the degeneration of the
transverse dissepiments which separate them. After this a fissure
opens in the median wall of the cœlomic sac, through which its
cavity unites with the definite body-cavity. In the subsequent
changes which the wall of the cœlom-sacs undergoes, these can be
recognised no longer as separate divisions of the whole body-cavity.

_l._ Formation of organs

=The nervous system.=—As we have already seen (p. 554), the rudiments of the ventral nervous cord arise, after the gastrula invagination is completed, as two ectodermal thickenings situated on each side of the median line, the so-called primitive rolls or strips (Fig. 528, _s_), which extend from the centre of the procephalic lobes of the head to the last segment, enclosing between them the single median “primitive groove” (Fig. 539, _C_, _pr_, and _pw_).

Soon after the appearance of the primitive strips, the first traces of segmentation may be detected. The ventral cord is from the first in direct connection and continuous with the brain. From the segmental expansions of the primitive strip arise the ventral nervous ganglia, and from the intersegmental constrictions are developed the paired longitudinal commissures.

Transverse sections of the ectoderm in the region of the primitive strips (Figs. 539, _C_, and 517) show several layers of cells. Of these cellular layers the deeper ones afterwards, by a kind of delamination, separate from the superficial ones and form the “lateral cords,” _i.e._ the germs of the longitudinal cords of the ventral ganglionic cord. Meanwhile the primitive groove (_pr_) deepens and forms an invagination extending between the lateral cords. The cells at the bottom of this invagination form the so-called “median cord,” and give rise to the transverse commissures connecting the ganglia.

FIG. 541.—Transverse section through the rudiment of the ventral
nervous cord of Xiphidium: _f_, fibrous mass; _m_, neuroblast cells
of the median cord; _n_{1}-n_{4}_, neuroblasts of the lateral cord;
_z_, pillar of ganglion-cells arising from the neuroblasts.—After
Wheeler.
]

Wheeler has detected in the rudiment of the ventral cord of several
Orthoptera, on the upper surface of the lateral cords, four large
cells which he calls _neuroblasts_ (Figure 541, _n_{1}-n_{4}_), from
which cells arise by budding and become arranged in vertically
arranged layers or pillars (_z_). Graber has observed them in
Stenobothrus and Viallanes in Mantis. These neuroblasts are only
present in the inter-ganglionic region, and soon move back to the
hinder side of the transverse commissures.

At first there is a pair of ganglia to each of the 16 trunk-segments of the embryo, but afterwards these become more or less fused together; thus those of the three gnathal segments unite to form the subœsophageal ganglion of the adult, and the last abdominal ganglia are fused together and move a little anteriorly (see also pp. 227, 228).

=Development of the brain.=—The supraœsophageal ganglion is due to the spreading out of the procephalic lobes. The rudiment of the brain is due to a thickening of the ectoderm on the sides of the mouth and of the forehead, this expansion of germinal brain-cells being the direct continuation of the primitive rolls or strips, and which finally becomes differentiated into the protocerebrum, deutocerebrum, and tritocerebrum, as stated on p. 228.

The ganglion opticum, now regarded as a part of the compound eye, arises as an ectodermal thickening on each side of the rudimentary brain. The optic ganglion belongs exclusively to the foremost division of the brain (see also p. 227).

=Development of the eyes.=—Compound eyes do not appear until the beginning of pupal life, the single eye (ocellus) being the primitive organ of vision. The ocellus of Acilius, according to Patten, arises as a pit or depression of the ectoderm (Fig. 542). The long hypodermal cells which form the walls of this pit or hollow are arranged in a single layer, and bear at their free ends a striated cuticular edge (_c_), while from their inner or basal end arise the fibres destined to form the common optic nerve.

At a later stage (Fig. 542, _B_), the eye-pit is closed over, the edges growing over and covering the deeper part of the eye. In this way there arises out of the pit-like rudiment a two-layered optic cup. The outer or superficial layer (_l_) becomes in its central part the crystalline lens, while the peripheral parts form the iris. From the cuticular striated border of these cells arise the chitinous or corneal lens. On its outer edge the superficial layer of the eye passes gradually into the unmodified hypodermis (_h_).

FIG. 542.—Two stages of development of the 5th of the six ocelli of
larva of Acilius: _c_, cuticular striated band; _cl_, germ destined
to form the corneal lens; _h_, hypodermis; _l_, crystalline-lens
layer; _n_, optic nerve; _r_, retinal germ; _sp_, vertical fissure
of the retina; _x_, the retina-cells bordering this fissure.
]

FIG. 543.—Two later stages of development of the same eye as in Fig.
542: _i_, iris; _m_, middle inverted layer of the eye; _r_, retina;
_sp_, vertical fissure of the retina; _st_, rods; other letters as
in Fig. 542.—This and Fig. 542 after Patten, from Korschelt and
Heider.
]

The inner, deeper layer of the eye, which forms the contracted cup-shaped portion, appears to be the rudimentary retina (_r_). From its cuticular rod-like or fibrous edge arise the visual rods. There soon arise certain peculiarities characteristic of the eye of Acilius, _i.e._ the fissure (_sp_) bordered by the horizontally situated rods of the large retina-cells (_x_).

In the farther developed eye (Fig. 543) there is a flattening of the cup-shaped inner edge, by which the bottom of the eye is levelled and the little rods belonging to it stand up vertically (Fig. 543, _B_, _st_). Then the cells belonging to the edge of the retinal cup (_m_) are turned in, forming an inverted layer constituting the germs of a third layer interpolated between the two chief layers of the eye. (Korschelt and Heider, from Patten.) Patten concludes that the structure of the retina in the larval ocelli of insects is much like that of myriopods, and that the whole eye is constructed on the same plan as that of Peripatus and most molluscs.

=Intestinal canal and glands.=—The intestinal or digestive canal is primitively divided, as already stated on p. 299, into three sections, of which the anterior and posterior are called respectively the stomodæum and proctodæum, and are invaginations of the ectoderm, forming sacs whose blind ends face the future site of the mid-intestine. The fore-intestine (stomodæum) in most cases arises earlier than the proctodæum. Its muscles are derived from the mesoderm. From the stomodæum arises at an early date an unpaired dorsal invagination out of which develops the ganglion frontale and the pharyngeal nerve.

The absorption of the ends of the blind sacs of the fore and hind
intestine, and opening up of the passage into the mid-intestine,
occur rather early in embryonic life. In the wasps and bees, as well
as the larva of the ant-lion, the mid-intestine remains closed at
the end, not communicating with the proctodæum, which has an
exclusively excretory function (Fig. 497).