figures of the larval stages of Sao hirsuta which BARRANDE has given
in his magnificent work, vol. I, pl. 7. In the _first_ stage figured
(figs. 1 d-e) the whole animal consists only of the head shield which
is completely smooth, the glabella scarcely segmented, no facial ridge,
no facial suture. In the _second_ stage, according to me (figs. 2-4
a), the glabella has become distinctly segmented, and the pygidium and
partly the thorax have been added. In the _third_ stage, 4 c, d-9, one
small narrow string exits from each side of the front of the glabella,
making the first faint beginning of the facial ridge. They form a right
angle with the glabella. In continued growth they become by degrees
a little more curved (figs. 5 c, 6 b) and the lengthened ridge bends
parallel to the outer lateral margin of the head (fig. 9 b). In fig.
9 d it has become so far complete that it reaches nearly back to the
posterior cephalic margin, but is still of the same narrowness all
along. In the _fourth_ stage, in a specimen (fig. 10 a) of 3 mm. in
length, the facial suture makes its first appearance, setting in from
the posterior margin of the head and meeting in its forward progress
the posterior extremity of the facial ridge which now begins to swell
out. It seems that both have a mutual influence on each other, the
suture being deviated from a straight course[21] to take a bend outside
the ridge, and the extremity of the ridge again at this contact to
increase in size so as to form the elongated tubercle, often called
palpebral lobe (fig. 14 b). This now augments in the same rate as the
whole body. The characters of the four stages of the development of the
larva consequently are: 1:o The archaic stage, only head shield with
ridgelike glabella. 2:o The coming and growth of the pygidium and the
complete segmentation of the glabella. 3:o The coming and growth of the
facial ridge. 4:o The coming and development of the facial suture.
[Footnote 21: In Trinucleus and Ampyx where there is no facial ridge,
the suture has a straight direction along the lateral margins.]
BARRANDE regards the whole ridge as a prolongation of the eye and
the tubercle at its posterior extremity as the eye itself.[22] But,
again, in page 399 he says »Sa surface (of the eye) est toujours mal
conservée, pour nous permettre de voir si elle était réticulée.» And
he also confounds the ridge, »filet» as he names it, with the eye
itself, and the first faint beginnings of this ridge in his fourth
stage he considers as the eye.[23] By a partly schematical figure
of the free cheek (fig. 25) he places the eye on this cheek, and in
the same manner in fig. 29, »restaurée d'après divers fragments» he
figures a reticulated surface of the eye on the free cheek, outside the
tubercle. I have sought for a reticulated surface on sufficiently good
specimens, but never found any, and I must consider Sao as one of the
blind genera. BARRANDE himself also in the table on the eyes of the
trilobites places Sao in the group with »Surface visuelle inconnue», p.
131.
[Footnote 22: Page 383 »l'œil argue est prolongé par un filet en
relief, vers le front de la glabelle».]
[Footnote 23: p. 389.]
[Illustration: (0 is new, added here to supplement fig. I.)]
BRÖGGER has also succeeded[24] in finding a series of small larva,
which he considers as belonging to a species of Liostracus. As the
figures drawn by BRÖGGER twenty five years ago may now be very little
known, I here reproduce them with the kind permission of the Editors
of »Geologiska Föreningens Förhandlingar», where they were published
in 1875. This development proceeds nearly upon the same plan as in
Sao. The first stage, however, (I) seems to be much earlier than any
of Sao, the rhachis or glabella consisting of an unsegmented ridge of
more primitive appearance. Before this first stage of BRÖGGER'S larva a
still older phase of development can be imagined, (0) a simple rounded,
smooth head shield without any indication of a glabella at all. This
stage might correspond to the head of certain species of Agnostus, as
A. glandiformis, A. nudus, which have no glabella. This stage 0 is
also valid for the larva of Sao. Several stages are evidently wanting
between I and II, in the later of which the thickened glabella is
divided in four segments. In III we have six segments, all these three
stages consisting only of the ovate head with narrow fixed cheeks. In
IV the pygidium has been added to the primitive head, but the segments
of the glabella have been reduced to four and in V slightly altered in
shape In VI, again, we see the head with five glabellar segments and
scarcely the first sign of the facial ridge. Between VI and VII there
must be links missing, as the change can not be so abrupt, and likewise
between VIII and IX as in VIII there are still no free cheeks nor any
facial ridge. This interesting discovery of BRÖGGER confirms, together
with those of several other authors[25], the supposition that the
development of the later Cambrian and older Silurian forms is a quite
different one from that of the Olenellidæ and the Paradoxidæ. They have
a rhachis, but no pleura proper, as the single facial ridge has a
quite different signification and appears at a comparatively much later
stage than the facial ridge of the Olenellidæ, which is present from
the earliest stages known.[26]
[Footnote 24: Fossiler fra Öxna og Klettna, Geol. För. Förhandl. 1875,
p. 572, pl. 25, fig. I-X.]
[Footnote 25: Foremost among these stands MATTHEW in »Illustrations
of the Fauna of St. John» IV, where he, p. 143, pl. II figs. 1 f.
etc., describes a few stages very like those given by BRÖGGER, so the
glabella as an unsegmented, narrow ridge etc.]
[Footnote 26: A deviating form of the ridges is shown by »Liostracus»
tener HARTT, Acad. Geol. 2d Ed. p. 652 (see also MATTHEW Illustrations
of the Fauna of St. John [1887] p. 132 p. 1 f. 3 a-3 c). Beside the
usual facial ridge there is a second pair of ridges between the first
and the glabella, arching in an opposite direction. See also WALCOTT,
Bulletin U. S. Geol. Survey, N:o 10, 1884 pl. V, figs 6, 6 a, 6 b, new
figures and copy of HARTT'S description.]
This form of facial ridge, although prevalent in the middle and later
Cambrian times, dates back so far that genera coeval with Holmia, viz.
Ellipsocephalus and Arionellus show it along the facial suture. This
early occurrence of the ridge coeval with the less developed Olenellidæ
leads to the assumption of a different origin of these trilobites as a
branch, which already far back in the oldest Cambrian or precambrian
times had deviated from a common ancestor and I have tried to give a
view in tables of these two coordinated lines of evolution further
on, at pages 24 and 25. The most remarkable genera which, as far
as I have found, belong to this fourth group are Ellipsocephalus,
Arionellus, Liostracus, Olenus, Leptoplastus, Parabolina, Corynexochus,
Parabolinella, Sao, Ptychoparia, Doropyge, Oryctocephalus etc.
I have not been able with an absolute degree of certainty to recognize
whether some of these genera now enumerated, have been oculate or
blind like Sao, like the Paradoxidæ and similar. The precarious state
of preservation prevents all definite conclusions in that respect. It
seems, however, that the evidence gathered through the examination
of numerous specimens rather points in a negative direction. As the
free cheeks in these old Cambrian trilobites have been in a very loose
connection with the fixed cheeks and generally deciduous, contrary
to the condition in the Silurian ones, it is in many instances very
difficult to tell whether species with facial ridge, especially those
from the earlier Olenus schists have been blind or provided with eyes.
The order of succession of the genera in the Swedish uppermost
Cambrian, in the Olenid slates is, according to S. A. TULLBERG'S
researches[27] at Andrarum in Scania as follows:
1. Parabolina (oldest division of the Olenus slates).
Olenus.
Liostracus.
2. Eurycare.
Leptoplastus.
3. Peltura.
Sphærophthalmus.
Ctenopyge.
4. Cyclognathus (uppermost).
Acerocare.
[Footnote 27: Om Agnostusarterna vid Andrarum.]
It is already in the second division that we find the earliest oculate
genus in Eurycare and in the higher strata. Sphærophthalmus and
Ctenopyge with enormous hemispherical eye balls, while Olenus and
Parabolina, their earliest predecessors, probably were blind. We
have not amongst hundreds of specimens of these genera found a single
specimen showing an ocular globe covered with facets. Olenus and the
nearly related genus Parabolina are found in innumerable specimens
in the thinly laminated alumschists of Scania and other provinces of
Sweden. But rarely a perfect head shield, or nearly so, is to be found
with the free cheeks in place. If so, the semicircular scallop in the
free cheek is entirely filled up by the posterior lobe of the facial
ridge and there is no place left for any eye.--If we, again, find a
non compressed glabella and fixed cheeks likewise, the facial lobe
(-»eye-lobe») is in some elevated so much as to leave a little space
between it and the scallop of the free cheek, which space must have
been an empty lacuna if not filled up by an eye ball. But it may also
be that the free cheek has been somewhat put out of its order and that
consequently some space has been left between it and the free cheek.
It is quite as much with older genera, as Solenopleura especially,
in which the posterior lobe of the facial ridge (_vulgo_ »eye lobe»)
has attained a great development, and in which one just could expect
to find a sphærical eye resting between the elevated lobe and the
scallop. The elevated rim of this scallop does not in fact constitute
a proof for its having clasped an eye as the elevated scallop in
Sphærophthalmus did. In Paradoxides, again, in Ellipsocephalus, where
there is absolutely no trace of an eye ever having been present,
whenever you succeed to find the free cheek in juxtaposition with the
fixed cheek it is evident that the elevation of the scallop rim is
due to the impact of the posterior lobe of the facial ridge. In many
specimens of Dolichometopus and Corynexochus etc. no free cheeks have
ever been found and to judge by the shape of the facial ridge it may be
concluded that these also were deprived of eyes.
On seeing this great number of trilobites, that on account of their
organization must be considered as blind, the first suggestion that
strikes the mind, is that they must have lived in abyssal depths of the
Cambrian sea, where the most intensive darkness prevailed. Nor does
the nature of the strata contradict such an assumption as evidently
this fine sediment must have been deposited far beyond the reach of the
influence of the wave motion, a depth amounting to more than a thousand
metres as now calculated, for else it could not have preserved unbroken
such delicate parts of organisms as that free cheek of Ctenopyge pecten
with its extraordinary long and delicate horn, figured on plate III
fig. 27 and many others. But it seems incredible that such a state of
things should have prevailed during the deposition of all the Cambrian
strata, although they in Sweden amount to only 160 feet in thickness,
according to the evaluation of S. A. TULLBERG, not considering what has
been lost through denudation. The length and duration in time can in
this instance not be measured by the thickness of the beds, but by the
great changes in the faunas which there have succeeded one another. The
physical conditions, to judge by the composition of the rocks, seem in
the main to have lasted during immeasurable periods and still the fauna
has changed in no little degree. During that enormous length of time,
embracing in Sweden eight well separated periods, there must, however,
have been minor changes in the conditions of depth and consequently
in the nature of the depositions. To a certain extent the physical
agents must have influenced the organization of the animals, but not
essentially. There are sure evidences of another factor being the chief
agent and that is the evolution.
We have from the lowest Olenellus beds to the lowest Lower Silurian
strata followed the clear traces of the changes from the eyeless
Olenellidæ past the Paradoxidæ with the facial suture to the great
tribe of the Olenidæ in which the facial ridge with its lobe is so
prominent, and to the oculate genera at the top of the formation.
As stated above there seems to be evidence enough for accepting two
different lines of evolution in these the oldest trilobites. For the
oldest, Olenellus is the type and for the second Sao may be taken as
the representative.
These two evolutional lines could be represented with their phases in
chronological succession as follows:
A. =The Olenellus line.=
1. The trilobite consisting only of the head with rhachis and pleuræ,
no sutures, no ridge, no eyes. Adult forms precambrian. Corresponding
larval forms (Olenellus asaphoides) in the Lowest Cambrian. Still older
and simpler forms may be presupposed as preceding these.
2. Semilunate facial ridge on the tripartite head-shield, no sutures,
no eyes, but a hypostoma fused to the rostrum of the head and provided
with maculæ. The adult animal with thorax and pygidium in the oldest
Cambrium, the Olenellus zone. Genera: Olenellus, Holmia (with beginning
suture), Mesonacis, Schmidtia.
3. The head quinquepartite with facial suture, short semilunate ridge
in the adult, a long ridge in the young, no eyes. The genera occur
in the Paradoxides-zones of the Cambrian formation. These genera are
Paradoxides, Centropleura, Metadoxides, Hydrocephalus, which are direct
descendants of the Olenellidæ.
This line of evolution, in which the species never acquired eyes, was
extinct in the sixth zone of the Swedish Cambrian, Centropleura Lovéni
being the last.
B. =The Sao line.=
I call this so because we see in the development of Sao, as represented
by BARRANDE, most of the different' phases in evolution, which the
ridge bearing trilobites of this second group have experienced during
the Cambrian times.
I.
The body consists only of the head, neither pygidium, nor thorax yet
developed.
1. _Psilocephali_ (ψιλος, bald), primeval, precambrian, adult stage not
found, supposed to be like the head shield of Agnostus glandiformis and
A. biplicatus quite round and bare without any glabella nor sutures.
The corresponding stage in development also wanted.
2. _Glabellate._ The entire animal only head with a mesial ridge, which
at first is entire or unsegmented and later is metamorphosed into the
segmented glabella.
a. _unsegmented._, Precambrian, adult form unknown, supposed to be
like the first larval stage of Liostracus according to the figure I
of BRÖGGER and also as the head of Agnostus parvifrons LINNARSSON.
Corresponding larval stages Liostracus (I).
b. _segmented._ Probably precambrian, not represented as adult in the
Cambrian strata. Supposed to be like the larval forms II and III of
BRÖGGER'S Liostracus, consisting of the round or oval head shield and
the segmented glabella. Also BARRANDE'S first stage of Sao (BARR., fig.
1 pl. 7).
As a transition to the next phase forms may be imagined having head and
the beginning pygidium, nearly as Agnostus atavus, minus the thorax.
The corresponding larval stages are Liostracus (BRÖGGER'S figures IV,
V, VI) and Sao (BARRANDE, pl. 7 fig. 2-4 a, b). Nearly so, though the
pygidium is more developed, are the larva of Agn. bibullatus and Agn.
nudus (BARR., pl. 49) both without thorax.
II.
These have the three chief parts of the body developed.
3. _The facial ridge._ Cambrian, adult with glabella and facial ridges,
short or long, emanating from the top of the glabella, thorax and
pygidium. Several American Conocoryphæ. Corresponding larval stage,
Sao, stage 3, BARR. pl. 7 figs 4 d-9.
4. _Suture._ A suture dividing the fixed cheeks in two pair viz.
two fixed cheeks and two free cheeks. Fully developed facial ridge.
Oldest known adult forms are Ellipsocephalus, Arionellus, already in
the Lowest Cambrian, the Olenellus beds, what presupposes a long,
antecedent lineage far back in the precambrian times. Corresponding
larval stage in Sao, BARR. pl. 7 figs 10-13. The plurality of Cambrian
trilobites belong here. An intermediate stage leading to the next is
seen in such forms in which the ridge posteriorly is widened into the
»eye lobe», which rests in the scallop of the free cheek. So it is in
Liostracus and many others besides. Solenopleura possibly oculate.
III.
5. Globular eyes. Cambrian, in the youngest zone, the Olenus schists.
The oldest at present known oculate trilobite Eurycare is found in the
second division of these schists.
In the lowest Lower Silurian division, the Ceratopyge limestone, Euloma
and Ceratopyge occur as the last survivors of the blind, partially
ridge bearing genera. The Trinucleidæ and Ampyx belong to another group
of trilobites.
Even among other exclusively Lower Silurian genera, in which the
plurality of the species is oculate, there are species entirely blind.
So with Illænus, in which genus Dr HOLM has not found any eye in Ill.
Angelini, I. leptopleura and Ill. cæcus. The free cheek in these three
species is much narrow, as the facial suture lies near the margin of
the head.
=The eyes of the Trilobites.=
If, as is probable, to judge by the conformity of their cornea with
that of recent crustacea, the trilobites like these were provided with
crystalline cones beneath the corneal lenses or facets, only the latter
have been preserved in a fossil state. Although the crystalline cones
in consequence of their solid consistence might have been petrified
as well as the cornea, they must, imbedded as these tiny cones lie,
entirely wrapped up in delicate tissues, fall away and be lost, when
the dissolution of the dead body had set in. Consequently the curious
appendages on the inferior side of the lenses in Dalmanites vulgaris
(Pl. III f. 50) or Phacops quadrilineata (Pl. V fig. 38) can noways
be considered as belonging to the original structure of the eye,
apart from their great dissimilarity with anything appertaining to
the eyes of the Arthropoda. The cornea on the contrary cohered with
the integument of the body, and it has been well preserved in a great
number of trilobites.
I subdivide the trilobites in respect to the form of their eyes in the
following manner:
=I. Genera with compound eyes.=
1. _With prismatic plano-convex cornea facets._
Acidaspis, as a transitional form to the next group.
Asaphus.
Bumastus.
Cyphaspis.
Dysplanus.
Encrinurus.
Illænus.
Megalaspis.
Nileus.
Phillipsia.
Niobe.
Proetus.
Ptychopyge.
Symphysurus.
2. _With round or biconvex transversally elongate lenses._
Acerocare.
Bronteus.
Chirurus.
Ctenopyge.
Cyrtometopus.
Eurycare.
Peltura.
Sphærophthalmus.
=II. Genera with aggregate eyes of biconvex lenses.=
Acaste.
Chasmops.
Dalmanites.
Phacops.
=III. Genera with isolated eyes, one or several stemmata at the
extremity of a straight facial ridge.=
Harpes.
Harpides.
(?) Trinucleus in the larval state.
Only a few authors have before now occupied themselves with the
intimate structure of the trilobite eye. PACKARD gave in 1880, in the
»American Naturalist» a note on the structure of the eye of trilobites
(p. 503). There are some rough and inexact sketches of the eyes of
Limulus and Asaphus, and although he seems to have known the beautiful
researches of GRENACHER he still »claims that the trilobite eye was
organized on the same plan as Limulus». This statement is altogether
wrong, and as I hope to show the trilobites have had eyes entirely
different from that of Limulus and instead agreeing with those of the
Isopoda and perhaps also with a few other Crustacea. In 1889 J. M.
CLARKE published an account[28] on the »Structure and development of
the visual area in the trilobite Phacops rana GREEN». The aggregate eye
described by him are of the type forming my third group. His holochroal
division embraces my first and second groups and the schizochroal my
third.
[Footnote 28: Quart. Journ. of Morphology, vol. II p. 253.]
The latest contribution to the knowledge of these eyes is found in
EXNER'S »Physiologie der facettirten Augen von Krebsen and Insecten»,
1891, where he gives good figures of the lenses of Phacops fecundus,
pl. II figs 18, 19. He says that the palpable difference in the
structure of these eyes and those of Limulus point to a change in the
function of these eyes.
I. =Compound eyes.=
=1. Eyes with prismatic, plano-convex lenses.=
A pellucid, smooth and glossy integument, a direct continuation of the
common test of the body covers the corneal lenses, quite as is the case
in so many of the recent crustacea. In the plurality it is, however,
difficult to discern the lenses from the outside.
The lenses, as seen in a vertical section of the eye of Asaphus
expansus, (pl. I fig. 12), are columnar prisms, like the pillars of
basalt, attaining a length of 0,2 mm and at the point where the eye
joins the test of 0,3 mm. At their interior extremity they have a
breadth of 0,066 mm. On that point the surface is convex and at the
exterior surface plane. They are closely packed and in a transverse
section resemble a pavement of regular hexaeders. But they also assume
other shapes and become rhombs or even quadrates, as seen in a specimen
of Asaphus fallax (pl. I fig. 18), where the hexaeders and quadrates
lie side by side without transitional forms. As a rule the lenses
become more and more irregular in the vicinity of the surrounding
frame or near the suture, nearly blotted out, as it were, and without
any definite border line mingled with the confused, spongy mass that
like a belt or a frame surrounds they eye in Asaphus and is sharply
limited from the other part of the free cheek. This remarkable zone
which is almost only present amongst the Asaphidæ (Asaphus, Megalaspis,
Ptychopyge, Isotelus) retains in a confused manner somewhat of the
prismatic structure of the eye as shown in the section (Pl. I, fig. 11,
b). The eye of Bumastus also is environed by a similar zone, with a
structure like that of the eye (Pl. II fig 35, 41).
In an undetermined species of Asaphus the lenses, although somewhat
apart, are of an elongated hexaedric outline, which passes into a
regular circular one farther away and on the surface of the eye they
are slightly convex (Pl. I figs 27-29). In other genera belonging
to this group the shape of the lenses are like those of Asaphus, so
for instance in Illænus (I. chiron and I. Esmarki) and in Niobe. In
Dysplanus centrotus they are shorter and broad, and their interior
or lower surface strongly convex. It is likewise so in Nileus, where
Nileus armadillo has an exceedingly thick exterior integument above
the lenses. Such an integument has in a still higher degree increased
in Bumastus sulcatus so as to exceed in thickness the stratum of the
corneal prisms and it may in fact be doubted if the eyes of this
species ever were able to function as visual organs. Proetus nearly
resembles Bumastus in the thickness of the integument covering the
prismatic lenses, which are interiorly convex, with a diameter of 0,03
mm.
In all genera belonging to this group a horizontal section gives the
image of the hexaeders as in Asaphus with some change to squares or
rhombs.
In scrutinizing a horizontal, somewhat extensive section of an eye in
this group of trilobites, it will be perceived, as for instance in the
figures (Pl. V fig. 16, 22) that the regular and evidently homogenous
and intact prismatic lenses by and by have been altered and in a part
of the section, a little distant from the intact ones disintegrated
in their interior, showing various aspects of alteration. I cannot
but think that this is a destruction which has set in long after
the fossilization. It has revealed certain states of the intimate
structure, certain delicate details, that now with an astonishing
regularity come in sight and probably also lie hidden in the intact
prisms. In the specimens of Asaphus, which we have studied, the
alteration has taken the shape of a concentric stratification forming
the body of the prisms, which is well discernible in a horizontal
section, but not easy to detect in the longitudinal one (Pl. I figs
9-10, 11). It is likewise so in Niobe. In the other genera again the
decomposition makes the prisms look like empty tubes in which a few
irregular traverses and trabecular remains of their solid mass radiate
towards the interior. They thus assume the aspect of a composite coral
with its septa in the calicles (Pl. VI fig. 31). This is also evident
in Nileus palpebrosus and Dysplanus.
=2. Eyes with biconvex lenses.=