H. Charlton Bastian.

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system closely resembles that found amongst the larvae or
Caterpillars of higher Insects (fig. 39). The supra-cesopha-
geal ganglia, or brain, receive nerves from the two pairs of
antennae, and from the groups of ocelli on each side of the
head. They are connected by oesophageal cords with a
bilobed infra-oesophageal ganglion, which distributes nerves
to the jaws and other parts about the mouth. This bilobed
infra-oesophageal ganglion is the first and largest of a
series of ventral ganglia, numbering about twenty, which
are connected together by a double ventral cord. Every



96



THE NERVOUS SYSTEM OF ARTHROPODS.



FIG. 34.



PIG. 35.




ganglion sends off lateral nerves to a pair of limbs. Th0
stomato-gastric nerves are connected with the posterior
part of the brain or with the ossopha-
geal cords, and they distribute them-
selves over the alimentary canal in
the usual manner.

Among Crustacea great differ-
ences are met with in the degree of
concentration of the nervous system,
the variations being, in the main, de-
pendent upon differences of external
form and in the arrangement of
locomotor appendages, in the different
representatives of the class. In some
of the lower terms of the series, such
as the Sandhopper and its allies, in
which the body is elongated and com-
posed of many almost similar seg-
ments, the nervous system is not very
/fi T\\ ^fe different from that of many Worms.
^ ' In the Sandhopper, indeed, the ven-

12. 34) of the



(Taiurus locusta). (Grant.) two sides of the body are separate
from one another as they are in Ser-

size as other ganglia situated p u j a /fipr 32) although the ganglia
on the separate ventral cords. r ,

FIG. 35. -Nervous System are here fewer in number and much

of Cymothoa. (Grant.) Cere- mQre Distinct.

In slightly higher forms of Crus-
tacea, however, the two divisions of
^he originally double ventral cord

_ _ n i M i

always become fused together, whilst,
at the same time, the equality of the
several ganglia diminishes. Thus, in such forms as the



bral ganglia almost wholly

absent from cesophageai



a single ventral cord, with

compound ganglia at inter-



CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS.



97



Lobster and the Crayfish, the ganglia of the thorax, which
supply nerves to the limbs, are distinctly larger than those
of the abdominal segments, though these are also of
good size, since the tail- segments are actively called into
play during locomotion.

In the Prawn a further development and concentration
of the nervous system is seen. The thoracic ganglia are
fused into a single elliptical mass, whilst
those of the abdominal segments still re-
main separate.

But in the ordinary edible Crab (fig. 36)
and its allies, an even more remarkable
concentration of the nervous system is met
with. All the thoracic and all the abdo-
minal ganglia are here fused into one large
perforated mass of nervous matter (c, c),
situated near the middle of the ventral
region of the body.* From this large
and compound ganglionic mass nerves are
received from, and given off to, the limbs,
to the abortive tail, and to other adjacent
parts. The brain (a) of the Crab is repre-
sented by a rather small bilobed ganglion.
It receives nerves from the pedunculated

FIG. 36. Nervous

compound eyes, from the two pairs of system of a crab

T f> ii 1*1 (Ptilinurusvulftans).

antennae, and from the palpi-bearing man- V Fused cerebra i
dibles. The posterior antennae (or anten- ganglia receiving

, . _ x optic, tactile, and

nules, as they are sometimes termed) con- olfactory (?) nerves ;



tain in their basal joint a body which is

J * cords ; c, c, great

supposed to represent an olfactory organ, ventral gangiiomc
though others have regarded it (on very ^ s .-, (Mi
insufficient grounds) as an organ of hear-
ing. This small bilobed brain is, indeed, thought by
* A large artery passes through the aperture in this ganglion.



98 NERVOUS SYSTEM OF ARTHROPODS.

many naturalists really to embody three pairs of ganglia,
in relation with three pairs of sensory organs, viz., eyes,
tactile antennae, and the supposed olfactory antennules.

The hrain is connected, by means of a long cord on
each side (b, b) of the oesophagus, with the anterior ex-
tremity of the great ventral ganglion. Nerves in relation
with the organs of mastication join the cords about mid-
way between the brain and the great abdominal ganglion,
and small ganglia are to be found in this situation. Just
behind these small ganglia a transverse commissure con-
nects the cords with one another. The unusual length of
the cesophageal cords is one of the most notable character-
istics of the nervous system of the higher Crustacea, and
this seems due in part to the fact that the sub-oesophageal
ganglia remain separate instead of uniting with one
another, as they do in fig. 18.

The ' stomato-gastric ' system of Crustacea is very
similar to that which exists in Centipedes. One part of it
is given off from the cesophageal cord on each side, while
another median branch proceeds from the posterior part of
the united cephalic ganglia, as in lulus (fig. 33, /). Where
the main nerve lies on the upper surface of the stomach,
in the higher Crustacea, it is connected with one or
two ganglia from which branches pass to the walls of this
organ. They send filaments also to the right and left,
into the liver. This principal visceral nerve is brought
into communication with the above-mentioned nerves,
going to the organs of mastication, by means of two
filaments which join the ganglionic swellings on the
C3sophageal cord at the part whence they issue.

Among Arachnida forms of the nervous system exist
which agree in many respects with those belonging to
members of the class last described especially where



CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS. 99

there are general similarities in the external configuration
of the hody. Thus in Scorpions the arrangement of
the nervous system is not very dissimilar from that met
with in the Prawn and its allies. The thoracic ganglia
have coalesced with one another and with the anterior
abdominal ganglia ; thereby forming a large stellate ner-
vous mass which supplies the limbs and the anterior part of
the abdomen. The ventral cord throughout the remainder
of the abdomen, and its caudal prolongation, is marked at
intervals by a series of small ganglionic swellings.

In Spiders proper, the nervous system attains its maxi-
mum amount of concentration. The bilobed brain (fig. 37, c)
receives nerves on each side (o), corresponding in number
with the ocelli which the animal may happen to possess.




FIG. 37. Head and Nervous System of a Spider (Mygale). (Owen after Duges.)
c, Cerebral ganglia* (side view), receiving (o) optic nerves, and (m) nerves (sensory and
motor) from the powerful mandibles, m'. The cerebral ganglia are connected by
very short cesophageal cords with a large stellate ventral ganglion (s), from which
five large nerves issue on each side (p, I, I); a, mouth; b, oesophagus ; d, stomach.

It also receives two large nerves (m), which probably con-
tain outgoing as well as ingoing fibres, from the so-called
mandibles (m).

Owing to the suctorial habits of these fierce and
predatory creatures, the oesophagus is very narrow ; and
as a consequence, the oesophageal cords are very short, so
that the brain is unlike the arrangement which obtains

H 2



100



NERVOUS SYSTEM OF ARTHROPODS.



in the common Crab and its allies (fig. 36) quite close
to the great stellate systemic ganglion (s), into which
are fused the representatives of the sub-oesophageal, the
thoracic, and the abdominal ganglia.

From this ganglion (fig. 38, i)
five principal nerves are sent off
on each side, " the first to the
pediform maxillary palpi; the
second to the more pediform
labial palpi, which are usually
longer than the rest of the legs,
and used by many Spiders rather
as instruments of exploration
than of locomotion ; the three
posterior nerves supply the re-
maining legs, which answer to
the thoracic legs of hexapod
Insects." (Owen.)

Since the sub - oesophageal
ganglia are in part analogous,
as already stated, with the ' me-
dulla oblongata' of vertebrate
animals, their fusion with the
thoracic ganglia in Arachnida,
as well as in Myriapoda, tends,

FIG. 38. Nervous System of a . />

great scorpion-iike Spider (Thdy- m a measure, to confirm the

phonuscaudatus). (Gegenbauer, aft- r y j ew }Q\J{ by S0 me anatomists,
Ulanchard.) s, Cerebral ganglia ; i, ', -\' LL j. J4.1,

great ventral ganglion, communicat- that it IS better to regard the

ing with fiv. large nerves on each < medulla > as a prolongation of
side; o, eyes; p, palpi; p'-p", feet; $ *

c. taii-iike prolongation. the spinal cord, than as an in-

tegral part of the brain. The

artificial line, that is, which for convenience is drawn
between the brain and the cord in Vertebrates, should be
placed at the upper rather than the lower or posterior







CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS. 101

boundary of the ' medulla/ so that the latter part may
be regarded as the more highly developed portion of the
spinal cord by which fusion with the brain is effected.

The visceral nerves are well developed in the higher
Arachnida. They consist of one or two filaments, on
which a ganglion may exist, in connection with the posterior
part of the brain, and thence proceeding to the stomach
and other internal organs. There are, moreover, two or
three branches given off from the great ventral ganglion
which, after passing through smaller ganglia, distribute
numerous filaments to the intestines, the respiratory
and genital organs, as well as other viscera. The former
set may be in the main afferent, and the latter perhaps
principally efferent visceral nerves.

Organs of vision are much more elaborate in Crustacea,
Spiders and Insects, than among Worms or Centipedes.
And, whilst organs of touch and taste are further perfected,
two sensory endowments, found among higher Mol-
lusks, seem also to manifest themselves. These higher Ar-
thropods are capable of being impressed by, and of discri-
minating, the different odours of some substances anterior
to their contact with the mouth. This power must mate-
rially aid them in their ' search ' for or recognition of food.
Some Arthropods seem to be also capable of appreciating
those vibrations of the medium they inhabit which induce
impressions recognizable by us as sounds or noises. Still,
in some of the most highly organized forms of Insects
a sense of hearing appears to have no existence. Much
uncertainty, in fact, exists in regard to this sense-endow-
ment.* Extreme sensibility of the tactile order may
cause the organism to display an apparent sensitiveness
to sounds. A delicate general ability to appreciate aerial
vibrations, therefore, must not be confounded with the
* See pp. 65 and 205.



102



NERVOUS SYSTEM OF ARTHROPODS.



more special auditory perception. On the other hand,
it is quite possible that sounds not appreciable by our



V





Pio. 39. Nervous System of full-grown Caterpillar of Privet Hawk-Moth
igustri), about two days previous to its change to the chrysalis state.

FIG. 40. Nervous System of the Privet Hawk-Moth thirty days after changing to
the chrysalis state. The abdominal cords are now seen to be much shortened, and
bearing five instead of seven ganglia.

FIG. 41. Nervous System of the perfect Insect. A, Greatly enlarged cerebral,
and B, optic ganglia. The numerals refer to the numbers of the ganglia, o, o, o, o,
respiratory nerves, 'nervi transversi.' (Solly after Newport.)

organization may be perceptible by the sensory organs
and centres of some of the lower organisms.

Additional sensory endowments like Smell and Hearing



CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS.



103



would, of course, be of importance to any organisms, but
more especially to those possessing active powers of loco-
motion. They would serve, on the one hand, to assist in
bringing their possessors into relation with food, or with
sexual mates, and, on the other, to warn them of the
approach of enemies.

The nervous system of Insects varies not only among
different classes and orders, but even in the same indivi-
dual in different stages of its development. The cater-
pillar of a Moth (fig. 39) or Butterfly presents a nervous
system not very different from that met with in the
Centipede; while in the imago
or perfected Insect, the same
system has undergone some re-
markable changes there is, for
instance, an increased size of
the cerebral ganglia, and also a
notable development of some of
the ganglia pertaining to the
ventral cord, while concentra-
tion or even suppression of
others is met with.

T T -T-, . FIG. 42. Brain and Adjacent Paitg

In such insects as Butter- of Nervous Sy&tem of a rat her

flieS, BeeS, DraffOn-flieS, and &"&&, apterous Beetle, Timarcka

' tenebricosa. (Newport.) A, Brain re-

Othei'S Where the Visual Organs ceiving the antennal nerves, and also




are enormously developed, aad

in which the pOWer Of vigOrOUS mencement of the oesophageal cords;

T . . T n . , . . D, the sub-oesophageal ganglia ; 6, the

and Sustained flight IS COrreS- vaguS) or visce ral nerve before reach-

pondingly increased, the nerVOUS ing its ganglion; c, lateral visceral

nerves.

system as a whole attains its

maximum of development among the Arthropoda. The
brain of these creatures differs from that existing in all
other members of the class by reason of the great develop-



104 NERVOUS SYSTEM OF ARTHROPODS.

ment of those portions of it in relation with the visual
organs, as may be seen by fig. 45, representing the nervous
system of the Common Fly, and by fig. 42, representing
the brain of a Beetle. A ganglionic swelling is fre-
quently found where the optic nerve joins the brain, and
in some Insects there are also small ganglionic swellings
at the corresponding parts of the antennal nerves.

It is in Ants, Bees, and Flies, however, that the brain
of Insects seems to attain its greatest development. Speak-
ing of the brain of the Blow-fly, B. T. Lowne says* :
" Next to bees and ants that of the blow-fly is the largest
known in any insect proportionally to its size, being about
thirty times larger than the cephalic ganglia of the larger
beetles." The same writer adds : " But a more positive
indication of a higher type of organization than even the
relative bulk of the sensory ganglia is found in the fact
that two very remarkable convoluted nerve centres, con-
nected by a commissure, each about l-30th of an inch in
diameter, surmount the cephalic ganglion, and are con-
nected to it by a pair of distinct peduncles ; t these are
extremely like the pedunculated convoluted nerve centres
which occupy the same position in bees and ants, first
described by M. Felix Dujardin (" Ann.des Sc. Nat." (Ser.
in.), t. xiv. p. 195), and considered by him as analogous to
the cerebral lobes of the higher animals. That naturalist
failed to distinguish these organs in the fly, probably
owing to their being imbedded in the substance of the
cephalic ganglion." In the Bee, according to Dujardin,
these peculiar bodies are attached to the sensory ganglia
by a single peduncle, and their united bulk is said by
him to equal -J-th of the whole brain. Farther details
concerning these interesting structures are much needed.
The double cerebral ganglion is connected in nearly
* " Anat. of the Blow-fly," p. 14. f LOG. cit., PI. vii. fig. 4.



CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS.



105



all Insects with a separate sub-oesophageal ganglion, from
which nerves are given off to the mandibles, the maxillae,
and the labium. But, as in Spiders, the oesophageal ring is
often very narrow, owing to the greatly diminished size of
the O3sophagus in the imago forms of higher Insects. In
Spiders and Myriapods, as before stated, the sub-oeso-



FIG. 43.



FIG. 44.



FIG 45





FIG. 43. Nervous System of a White Ant (Termes}. (Gegenbauer after Lespes.)

FIG. 44. Nervous System of a Water Beetle (Diitiscus). (Gegenbauer.)

FIG. 45. Nervous System of a Fly (Musca). (Gegenbaner after Blanchard.) o,

Eyes ; gs, supra-oesophageal ganglia (brain) ; gi, sub-oesophageal ganglion ; gr, g*, g 3

fused ganglia of the thorax.

phageal ganglion has no separate existence apart from
the thoracic ganglia.

In many Insects the three thoracic ganglia preserve
a separate existence (fig. 43), though in others of tho
higher types above referred to these ganglia are more
frequently fused into a single lobed mass (fig. 45). The



106 NERVOUS SYSTEM OF ARTHROPODS.

eight abdominal ganglia, which are always much smaller
than the thoracic, also continue to have a separate exist-
ence among some 6f the less developed types of Insects
(fig. 43) though it. is more frequent for some, or even all,
of them to disappear (figs. 44, 45).

The ' stomato-gastric ' system of nerves attains a con-
siderable degree of complexity in these animals. In front
there is a median ganglion (fig. 42) lying below and often
anterior to the brain. This oral ganglion is a swelling
situated on the great median (afferent) visceral nerve, at
the spot where it bifurcates in order to proceed to each
half of the brain. It receives branches from the mouth
and adjacent parts. The main nerve, or else the ganglion,
is also connected with other branches (c), proceeding from
one or two pairs of lateral ganglia situated close to the
cesophageal cords, and often in structural relation with
them. This visceral system of nerves receives branches
from the stomach, the intestines, and other internal
organs.

In Insects, moreover, we meet with another semi-
independent set of visceral nerves, connected with a chain
of minute ganglia lying upon the great ventral ganglionated
cord, and united thereto by means of minute nerve fila-
ments. The nerves (fig. 41, o, o, o) in connection with this
chain of minute ganglia are received from and distributed
to the all-pervading respiratory organs (air tubes) of the
Insect. They are known to anatomists, on account of the
disposition of their main branches, as ' nervi transversi,'
and are much more highly developed in these animals
than are anything corresponding to them amongst other
Arthropods.



CHAPTER VTL

DATA CONCERNING THE BRAIN DERIVED FROM THE STUDY
OF THE NERVOUS SYSTEM OF INVERTEBRATES.

THIS survey of some of the principal varieties of the
Nervous System among the Invertebrata, brief though it
has been, should have sufficed to call attention to many
important facts and to show the warrant for certain related
inferences, many of which are embodied in the following
propositions :

1. Sedentary animals, though they may possess a
Nervous System, are often headless, and they then have
no distinct morphological section of this system answering
to what is known as a Brain.

2. Where a Brain exists, it is invariably a double
organ. Its two halves may be separated from one
another ; though at other times they are fused into what
appears to be a single mass.

3. The component or elementary parts of the Brain
in these lower animals are Ganglia in connection with
nerves proceeding from special impressible parts or Sense
Organs ; and it is through the intervention of these united
Sensory Ganglia that the animal's actions are brought into
harmony with its environment or medium.

4. That the Sensory Ganglia, which in the aggregate
constitute the Brain of invertebrate animals, are connected



108 THE BRAIN OF INVERTEBRATES.

with one another on the same side and also with their
fellows on opposite sides of the body. They are related
to one another either by what appears to be continuous
growth or by means of * commissures.'

5. The size of the Brain as a whole, or of its several
parts, is therefore always fairly proportionate to the develop-
ment of the animal's special Sense Organs. The more
any one of these impressible surfaces or organs becomes
elaborated and attuned to tak^ part in discriminating
between varied external impressions, the greater will be
the proportionate size of the ganglionic mass concerned.

6. Of the several sense-organs and Sensory Ganglia
whose activity lies at the root of the Instinctive and
Intelligent life (such as it is) of Invertebrate Animals,
some are much more important than others. Two of
them especially are notable for their greater proportional
development : viz., those concerned with Touch and
Vision. The organs of the former sense are, however,
soon outstripped in importance by the latter. The visual
sense, and its related nerve-ganglia, attain an altogether
exceptional development in the higher Insects and in the
highest Mollusks.

7. The sense of Taste and that of Smell seem, as a
rule, to be developed to a much lower extent. In the
great majority of Invertebrate Animals it is even difficult to
point to distinct organs or impressible surfaces as certainly
devoted to the reception of either of such impressions.
Nevertheless, as we shall subsequently find, there is reason
to believe that in some Insects the sense of Smell is mar-
vellously keen, and so much called into play as to make it
for such creatures quite the dominant sense endowment.
It is pretty acute also in some Crustacea.

8. The sense of Hearing seems to be developed to a very
slight extent. Organs supposed to represent it have been



CHAP. VII.] THE BRAIN OF INVERTEBRATES. 109

discovered, principally in Mollusks and in a few Insects.
It is, however, of no small interest to find that where these
organs exist, the nerves issuing from them are most fre-
quently not in direct relation with the Brain, but imme-
diately connected with one of the principal motor nerve-
centres of the body. It is conjectured that these so-called
' auditory saccules ' may, in reality, have more to do with
what Cyon terms the sense of Space than with that of
Hearing (p. 218). The nature of the organs met with
supports this view, and their close relations with the motor
ganglia also become a trifle more explicable in accord-
ance with such a notion.

9. Thus the associated ganglia representing the double
Brain are, in animals possessing a head, the centres in
which all impressions from sense-organs, save those last
referred to, are directly received, and whence they are
reflected on to different groups of muscles the reflection
occurring not at once but after the stimulus has passed
through certain ' motor ' ganglia. It may be easily under-
stood, therefore, that in all Invertebrate Animals perfection
of sense-organs, size of brain, and power of executing
manifold muscular movements, are variables intimately
related to one another.

10. But a fairly parallel correlation also becomes estab-
lished between these various developments and that of the
Internal Organs. An increasing visceral complexity is
gradually attained ; and this carries with it the necessity
for a further development of nervous communications.
The several internal organs with their varying states are
gradually brought into more perfect relation with the
principal nerve centres as well as with one another.

11. These relations are brought about by important
visceral nerves in Vermes and Arthropods those of the
' Stomato-Gastric System ' conveying their impressions



110 THE BRAIN OF INVERTEBRATES.

either direct to the posterior part of the Brain or to its
peduncles. They thus contribute internal impressions
which impinge upon the Brain side by side with those
coming through external sense organs.

12. This Visceral System of Nerves in invertebrate ani-
mals has, when compared with the rest of the Nervous
System, a greater proportional development than among
vertebrate animals. Its importance among the former is
not dwarfed, in fact, by that enormous development of the
Brain and Spinal Cord which gradually declares itself in
the latter.

13. Thus impressions emanating from the Viscera and
stimulating the organism to movements of various kinds,
whether in pursuit of food or of a mate, would seem to
have a proportionally greater importance as constituting
part of the ordinary mental life of Invertebrate Animals.
The combination of such impressions with the sense-
guided movements by which they are followed, in complex
groups, will be found to afford a basis for the development
of many of the Instinctive Acts which animals so fre-
quently display.



CHAPTEB VIII.

THE BRAIN OF FISHES AND OF AMPHIBIA.

IN all Vertebrates the relation of the principal nervous



Online LibraryH. Charlton BastianThe brain as an organ of mind → online text (page 8 of 58)