order on the ventral side of the animal, and
are connected by longitudinal commissures
(connectives), and also by transverse com-
missures connecting the left and right
ganglia. The first pair of the series is.
formed by the infra-oesophageal ganglion,
which sends out commissures right and left,
surrounding the pharynx, to the supra-
O3sophageal ganglion. The supra- and infra-
oesophageal ganglia together with the
oesophageal commmissures form the cesopha-
geal ring, a nerve-ring surrounding the
Tubular System. The tubular type of
nervous system is found only in the chordates
(fig. 76). The vertebrate brain and spinal
cord may be regarded as parts of a tube with
greatly thickened walls, developed in differ-
ent ways. In the centre lies the extremely
narrow central canal, which widens anteriorly
into the several ventricles of the brain. In
a transverse section the nervous elements
FIG. 75. Ladder nervous -i T , -, -> ^ .
system of Porceiuo scatter are seen grouped around the central canal m
a manner almost the reverse of that of the
ionic type. On the periphery lies a
f nerve-fibres (the < white matter' of
human anatomy) ; next is a central portion
formed of ganglion-cells and nerve-fibres (the 'gray matter'),
which is marked oil from the central canal by a special epithelium
Relations between the Nervous System and the Skin. For
almost all animals it has been ascertained that the nervous system
arises from the ectoderm. Therefore, in many animals, the nerve-
cords and the ganglionic masses lie pemanently in the skin: in
others only during the development, later becoming separated by
splitting off or by infolding, and thus coming to lie in the deeper
layers of tlje body (fig. 9).
GENERAL ORGANOLOGT. 125
III. Sensory Organs.
Sensations of the Lower Animals. What we know of the
character of the external world is founded upon experiences gained
through our sensory organs. We thus know the external world
only in so far as it is accessible to the senses, controlled by the
judgment. If things exist outside of ourselves which have no
influence upon our senses, we can form no conception of them.
It follows from this proposition that we can gain knowledge of the
FIG. 76. Cross-section of the human spinal cord. (From Wiedersheim.) Black repre-
sents the gray, white the white substance of the cord ; Cc, central canal, sur-
rounded by the anterior and posterior commissures (C and C'); <Sa, Sp, anterior
and posterior fissures ; V\V, JfW, anterior and posterior nerve-roots ; VH, HH,
anterior and posterior horns of gray matter; V, S, If, anterior, lateral, and pos-
terior columns of white matter.
natural capacity of the sensory organs of animals only by analogy
with our own experiences. Hence the distinction of five senses,
touch, taste, smell, hearing, and sight, based upon human
physiology has been extended to the whole animal kingdom. A
priori, however, it cannot be denied that sensations may occur in
animals which we do not experience; following out this course of
thought has led to the idea of a ' sixth sense/ which, however,
must remain to us a meaningless abstraction, since it is impossible
for us to conceive of the character of a sense which we lack.
Anatomy gives Insufficient Knowledge of Sensory Organs.
A further, and still more important reason for our very fragmen-
tary knowledge of animal sensations is the fact that, in regard to
the physiological meaning of the sensory apparatus, it is seldom
that we can depend upon experiments, and consequently we must
base our conclusions upon structure. But the anatomy of many
sensory organs, like those of smell and taste, is by no means so
characteristic that it alone is sufficient to determine the physio-
Tactile Organs. The skin of animals functions as a tactile
organ, usually over the whole area, although not everywhere with
GENEEAL PRINCIPLES OF ZOOLOGY.
equal intensity. Prominent parts, like the tentacles of polyps
and of many worms, the antennae of arthropods and the snails,
need only mention. Special epithelial cells with stiff hairs pro-
jecting above the surface, the tactile bristles or tactile hairs, are
FIG. 77. FIG. 78.
FIG. 77. Skin of an insect with an ordinary hair (h) and a tactile hair ((); n, nerve;
*, sensory cell; e, epithelium; c, cuticle. (After vom Rath.)
FIG. 78. Vater-Pacinian corpuscle of the mesentery of a cat. a, axis cylinder; /, fat;
0, blood-vessel; i, inner bulb; /c, capsule with nuclei; n, medullated nerve-fibre.
tactile (fig. 77). Only in the vertebrates do the nerves of touch
terminate in specially modified end organs (Vater-Pacinian cor-
puscles, corpuscles of Meissner, etc., fig. 78); these usually lie
under the epithelium.
Organs of Smell and of Taste are accurately known only in
vertebrates. The olfactory organ of fishes consists of two small
pits in the skin, above or in front of the mouth.
In the air-breathing vertebrates this pair of pits which here
also arise from the skin are taken into the dorsal wall of the two
respiratory canals leading from the outside to the pharynx. Now
since the olfactory cells distributed in these pits (fig. 37, /?) are
frequently characterized by bundles of olfactory hairs, while the
surrounding epithelium is often ciliated, one is inclined to regard
as organs of smell sensory organs of invertebrates (e.g., medusas,
cephalopods), which have the form of ciliated pits and lie near the
respiratory apparatus (e.g., the osphradium of molluscs). Yet
there are exceptions. Experiments seem to show that in the
arthropods the antennae probably serve for smelling. Here the
sensory perception can be connected only with certain modified
GENERAL ORGANOLOGT. 127
hairs, the olfactory tubules of the Crustacea and the olfactory
cones of insects. In a similar way certain nerve end organs in
the region of the mouth are considered as organs of taste, since the
taste organs of vertebrates, the so-called taste buds, are abundant
in the mouth cavity, especially on the tongue.
Organs of Hearing and of Sight are called the higher sense-
organs, because they are of much greater importance for the
totality of our perceptions than the other organs, since they fur-
nish sensations which are quantitatively and qualitatively much
more definite. Ears and eyes have therefore a complicated and
characteristic structure, which renders them easily recognizable
by the almost invariable presence of certain structures accessory to
History of the Auditory Organs. The auditory organs of
vertebrates and of most of the other animal groups can be traced
back to a simple fundamental form, the auditory vesicle (fig. 79).
FIG. 79. Auditory vesicle of a mollusc (Pterotr ached). JV, auditory nerve : Jfz, audi-
tory cells with the central cell, Cz ; Wz, ciliated cells ; Oi, otolitn. (After Claus.)
This has an epithelial wall, a fluid contents, the endolymph, and
an auditory ossicle or otolitli, formed from a single or from several
fused auditory concretions. In some instances the otoliths, to the
number of thousands, may remain separate. In a definite region
of the epithelial wall the cells are developed into the crista
acustica, the auditory ridge; they are in connexion with the
auditory nerve and bear the auditory hairs projecting into the
endolymph. The otoliths themselves are concretions of carbonate
or of phosphate of lime (exceptionally in My sis of fluoride of
calcium). They usually float free in the centre of the vesicle, and
are often held in place by bundles of cilia which project from the
non-sensitive epithelial cells.
128 GENERAL PRINCIPLES OF ZOOLOGY.
Auditory Pit. Every auditory vesicle develops from a pitlike
invagination of the skin, and consequently is for a time an auditory
pit. Therefore it is not surprising that in many animals the organ
has stopped at the lower stage of development; for 'example, the
crayfish has an open auditory pit. On the other hand, the
auditory vesicle may develop a complicated system of cavities as
in mammals (fig. 80), where it is divided by a constriction into
FIG. 80. Diagram of the human labyrinth. 17, titriculus with the semicircular
canals; S, sacculus connected with the cochlea (G) by the canalis reuniens; It,
recessus labyrinth!; V, blind sac of the cochlea; K, apex of the cochlea.
the sacculus and the utriculus. The sacculus is provided with a
spirally-wound blind sac, the cochlea, the utriculus with the three
semicircular canals. In addition there is formed in the mammals,
as also in most vertebrates, a sound-conducting apparatus, so that
the auditory organ acquires an extremely complicated structure.
Other Forms of Auditory Organs. Since there are animals
without auditory vesicles which hear well, like the spiders and
insects, we must assume that there are auditory organs which are
formed after another type. Still we have no certain knowledge of
these except in the case of the tympanal auditory organs of the
grasshoppers (which compare).
Function of the Semicircular Canals. Experiments upon
representatives of the most diverse classes of vertebrates have led
to the conclusion that the three semicircular canals, standing at
right angles to each other, are organs of equilibrium, for, after
these canals are destroyed, the animals begin to stagger and lose
their balance. It is probable that in fishes this is the sole function
of the labyrinth; for it has not been determined that fishes hear.
Starting from this assumption, recent investigators have attempted
to prove that the auditory vesicles of invertebrated animals are
exclusively, or at least largely, organs of equilibration. This would
explain the otoliths, for these bodies, of relatively large specific
gravity, would affect the crista in different ways according to the
position of equilibrium of the body. Statoliths would thus be a
The Eye is in all animals recognized by the character of the
sensory epithelium, the retina. This always has a large amount
of pigment which lies either in the sensory cells or in special cells
arranged between or behind them. The simplest-formed eye,
therefore, appears as a sharply circumscribed pigment-spot in the
epithelium of the skin, provided with nerves, commonly also with
a lens (fig. 81).
Rods and Cones. The sensory cell itself bears usually at its
peripheral end a process, the rhabdom. This is a cuticular struc-
ture, probably serving to collect the
rays of light and thus to stimulate the
cell, and has, particularly in the verte-
brates, a complicated structure, each
rhabdom consisting of an inner and an
outer portion. Here can be frequently
distinguished two kinds of rhabdoms,
rods and cones (fig. 82).
The Optic Ganglion. Before the
optic nerve divides into the separate
visual cells it forms a swelling, the
FIG. 81. Flo 82>
FIG. 81. Ocellus (oc) of a medusa (Lizzia Koellikeri) with lens (0.
FlG n !!fc I lman - retina V (After Gegenbaur.) P, pigment-layer; E, layer of sensory
; G, optic ganglion; 1, limitans mterna; 2, nerve-fibre layers; 3, ganglion-
cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer- 7.
V Muller^^bres 761 ' 5 8 ' limitans externa ? 9 i r ds and cones; 10, tapetum nigrum;
optic ganglion, which either lies as a detached body outside of the
eye, or is united with the retina into a connected whole. The
GENERAL PRINCIPLES OF ZOOLOGY.
considerable thickness of the vertebrate retina is due to the fact
that it includes the optic ganglion. The parts (fig. 82) called
reticular layers, inner granular layer, ganglion cells, and nerve-
fibre layer, constitute the optic ganglion; the layer of visual cells
consists only of the outer granular layer and the connected rods
and cones. The outer granules are the nuclei of the visual cells
to which rods and cones belong.
Accessory Structures. The eye may be further complicated
by special refractive bodies (cornea, lens, vitreous body) which
G NO vo
FIG. 83. Horizontal section through the human eye. (After Arlt, from Hatschek.)
.E, epithelium of the cornea (conjunctiva) ; C, cornea ; vA, anterior chamber of
the eye; I, iris; hA, posterior chamber of the eye; Z, zonula Zinnii; Os, ora ser-
rata ; Sc, sclerotic coat ; Ch, choroidea ; U, retina ; p, papilla of optic nerve ; m,
macula lu tea, area of most distinct vision; VO, sheath of the optic nerve; NO,
optic nerve; 0, arteria centralis; Cc, corpus ciliare; J/, lens; Or, vitreous body.
concentrate the light in order to cast an image upon the retina;
and an iris to regulate the amount of light. Then, too, means for
nutrition (the choroid coat) and for protection (sclerotic coat)
must be provided. If all these parts be present, a structure results
such as is found in the squid and in the vertebrates (fig. 83).
GENERAL OROANOLOGY. 131
The Eye of the Vertebrates. The eye of the vertebrates
usually is an approximately spherical body whose surface is formed
by a firm membrane. Over the greater part of the circumference
this is an opaque, fibrous or cartilaginous covering, called the
sclera, or sclerotica; it is transparent only in the most anterior
part, and here it forms by its greater convexity a projecting por-
tion like a watch-glass, the cornea. Internally to the sclera lies
the choroidea, an envelope of connective tissue, rich in pigment
and blood-vessels, which, at the junction of sclera and cornea, is-
changed into the iris. The iris, the seat of the color of the eye,
is pierced in the centre by the pupil, an opening the varying size
of which regulates the amount of light. Next internal to the
choroid follows a layer of black cells, the tapetum nigrum (pig-
mented epithelium), and finally the retina itself, the expansion of
the optic nerve which enters the eye at the hinder part. The
tapetum nigrum and the retina arise together, and hence both end
at the edge of the pupil, although the retina loses its nervous
character at the ora serrata, some distance from the outer edge of
The cavity of the eye is completely filled by the vitreous body,
aqueous humor, and the lens. For vision the lens is the most
important, since, next to the cornea, it influences most the course
of the rays of light. It lies behind the iris, fixed to the anterior
wall of the choroidea, which here is changed into the ciliary
process. In front of it is a serous fluid, the aqueous humor, partly
in the so-called posterior chamber of the eye, between the lens
and iris, partly in the anterior chamber, between the iris and
cornea. The single, larger cavity behind the lens is filled up by
a jelly-like mass of tissue, the vitreous body. The image formed
on the retina is inverted.
The Various Types of Eyes. Between the simple pigment-
spot and the highly organized vertebrate eye are many transitional
stages: pigment-spots with lens and vitreous body, with enveloping
and nourishing coverings, etc. The faceted eye of insects and
Crustacea shows a special type of development, described later
under the Arthropoda.
SUMMARY OF THE MOST IMPORTANT POINTS OF ORGANOLOGY.
1. Organs are composed of tissues, and by their environment
are led to the formation of a body of definite shape and to the
performance of a single function; consequently every organ is
132 GENERAL PRINCIPLES OF ZOOLOGY.
characterized morphologically (according to its structure and its
relations) and physiologically (according to its function).
2.- Organs of different animals may be physiologically equiva-
lent , analogous organs (i.e., with similar functions).
3. Organs of different animals may be morphologically equiva-
lent, homologous (developing in similar relations).
4. In the comparison of the organs of two animals three
possibilities become evident.
a. They may be at the same time homologous and analogous.
b. They may be 'homologous, but not analogous (swim-bladder
of fishes, lungs of mammals).
c. They may be analogous, but not homologous (gills of fishes,
lungs of mammals).
5. Organs are divided into animal and vegetative.
6. Animal functions are those which are not completely foreign
to plants, but are only slightly developed in them; in the animal
kingdom, on the contrary, they undergo an increase and become
7. Vegetative functions are developed with equal completeness,
though in a different manner, in plants and animals.
8. To the animal organs belong the organs of motion and
sensation, such as the muscles, the sense-organs, the nervous
9. To the vegetative organs belong the organs of nutrition and
10. Under nutrition, in the widest sense, are included not only
the taking in and digestion of food and drink, but also the taking
in of oxygen (respiration), the distribution of food to the parts of
the body, and the removal of matter which has become useless.
11. With nutrition, therefore, are concerned not only the
digestive tract and its accessory glands, but also the organs of
respiration, the blood-vascular system, and the excretory organs
12. The male and female sexual organs serve for reproduction.
13. The male and female organs may occur in different indi-
viduals (diwcious), or both may be found in one and the same
14. The highest degree of hermaphroditism is attained when
one and the same gland (the hermaphroditic gland) gives rise to
both eggs and spermatozoa.
15. Very often the sexual organs and the ducts from the
kidneys are closely united; we then speak of a urogenital system.
PROMORPHOL OOT. 133
IV. PROMORPHOLOGY, OR STUDY OF THE FUNDAMENTAL FORMS.
Organic and Inorganic Bodies. The structure of the individual
animal rests upon the regular combination of differently-function-
ing organs. The organs thus assume a relation to one another
which is definite for each animal group, or varies only in subordi-
nate ways. If the various groups be compared with reference to
the principle of the arrangement of parts, there appear a few
fundamental forms which play a role in morphology similar to that
of the fundamental forms of crystals in mineralogy. But we must
not carry this comparison too far, and attempt to compare the
study of the fundamental forms, the promorphology, of animals
FIG. 84. Spongilla fluriatilis, fresh-water sponge. (After Huxle
with dermal pores; be, region of the ampullae; c/,
(After Huxley.) a, superficial layer
with crystallography as of equal value. A crystal is a mass made
up of similar parts; its form is the necessary and immediate result
of the chemico-physical constitution of its molecules. A direct
connection of this kind between molecular structure and funda-
mental form does not, and cannot, exist in the organism, since
each organ is composed of many chemical combinations. Conse-
quently there is lacking also the mathematical regularity which
occurs in crystals. Even in the case of animals which have the
greatest regularity in the arrangement of their parts there is not
an entire conformity to the demands of the fundamental form, so>
that we are compelled to ignore certain greater or less variations.
If, for example, we call man bilaterally symmetrical, we overlook
not only the slight asymmetry of a nose awry, etc., but also what
is more important that the liver has been pushed to the right,
GENERAL PRINCIPLES OF ZOOLOGY.
the heart to the left; and that the digestive tract throughout its
entire course runs asymmetrically.
FIG. 85. Halinmma erinaceus, a radiolarian. , external, i, internal, latticed spheri-
cal skeleton ; cfr, central capsule; wk , extra capsular soft parts ; ?, nucleus.
FIG. 86. Nausithoe^ an acraspedote medusa (after Lang), seen from the end of the
greatly shortened main axis, pr, perradii; tr, interradii; ar, adradii (perradii
and interradii mark the four planes of symmetry of the animal); *r, subradii: rf,
mantle-lobes; f, tentacles; s/f, sensory organs: g, sexual organs; gff, gastric fila-
ments; rn, subumbrellar circular muscle; in the centre the cross-shaped mouth-
Symmetry. Now, according to the three dimensions of space,
can pass three axes, perpendicular to each other, through the
body of an animal, and up to a certain degree may characterize it
according to the nature of these axes; further, we may characterize
it according to the planes by which it can be symmetrically halved,
the planes of symmetry. Thus we find the following fundamental
1. Anaxial, asymmetrical, irregular, or amorphous funda-
mental form (fig. 84).
2. Homaxial, symmetrical in all directions, spherical funda-
mental form (fig. 85).
3. Monaxial, radially symmetrical (fig. 86).
4. Simple heteraxial, biradially symmetrical (figs. 87, 88).
5. Double heteraxial, bilaterally symmetrical (fig. 89).
1. Anaxial or asymmetrical animals, so called, are those in which the
arrangement of parts is not regularly de-
fined in any direction or space, and they
therefore may grow irregularly in any direc-
tion. There is no fixed central point, and
there is no possibility of running definite
axes through the body or of dividing it into
symmetrical parts. (Many sponges and
2. Homaxial or spherical animals have
the sphere as their fundamental form ; the
parts of the body are arranged concentri-
cally around a fixed central point, so that
any number of axes and planes of symmetry
<?an be passed through it ; that is to say,
all lines and planes which run through the
central point of the sphere. (A few spheri- FIG. 87.-Diagramof anactinian
cal Protozoa, chieny radiolarians.) ( |t^^5S^So?ltoti^
3. Monaxial or radial symmetry is much-lengthened main axis,
brought about, if growth go on in a definite direction, and correspond-
ingly also if the formation of organs take place in directions other than
perpendicular to this. The line which marks this direction of growth is
the main axis, in distinction from the accessory axes or radii, which are
all similar to each other. The main axis can be determined, because it is
longer or shorter than the accessory axes ; but it may also be of the same
length and still be entirely distinct, since it contains certain organs (e.g.,
the mouth-opening) which are lacking in the other planes. In radially
symmetrical animals the same organs are always present in greater num-
ber and are distributed regularly around the main axis in the direction of
the radii. Through such an animal a great number of sections can be
made, which pass through the long axis and halve the body symmetri-
cally. If we cut the animal in the direction of all the possible planes of
symmetry, we obtain pieces which, in essential points, are similarly con-
GENERAL PRINCIPLES OF ZOOLOGY.
structed. Great groups of animals, as most eohinoderms and ccelenter-
ates, are more or less completely radially symmetrical.
4 and 5. The next two fundamental forms have in common the fact
that three unequal axes perpendicular to each other are distinguishable ;
these may be designated as the main axis, the transverse axis, and the
sagittal axis : this is the case if, leaving the main axis out of considera-
tion, an arrangement of organs occur different in the sagittal direction
from that in the transverse direction if organs lie in the former which
FIG. 88. Cross-section of an actinian (Adamsia diaphana). AB, directive septa,
which are at the same time ends of the sagittal axis, which marks one plane or
symmetry of the body, while the second stands perpendicular to it ; I-IV, cir-
cles of paired septa of first to fourth order.
are lacking in the latter or the reverse. There are then, if w r e take into
consideration the dissimilarity of the axes, two possible planes of sym-
metry : the animal can be symmetrically divided, (1) if the division passes
through the main and transverse axes, (2) if it passes through the main
and the sagittal axes. Such biradially symmetrical animals are the
ctenophores, actinians (figs. 87, 88), and corals.
Bilateral Symmetry. If now we further suppose that the ends of the
sagittal axes become unlike, that at one end are organs quite different
from those of the other, we then reach the most usual form, bilateral
symmetry. The dissimilar ends of the sagittal axes are called ' dorsal r
and * ventral,' and further the terms ' right ' and * left ' are given to the
ends of the transverse axis ; a bilaterally symmetrical animal can be