contiguous hemispherical lenses, probably once covered with a membrane,
as is still to be seen in well preserved specimens of Bronteus
laticauda. Both in Chirurus and Bronteus the lenses seen vertically are
globular and ordinated beside each other either continuous or separated
only through a faint dividing line. In a horizontal section passing
right through the point of contact they show the common hexaedral
shape and when somewhat corroded the interior radiate structure also
comes forth, the radii directed towards a little black point in the
centre. The lenses of the Brontei have the same stellate structure
as in Bumastus (Pl. II fig. 7). In Cyrtometopus the lenses are in
size the fourth of those in Chirurus and they form an extremely
thin stratum in strongest contrast with the adjoining cheek, which
surpasses them more than six times in thickness (Pl. III fig. 19). The
lenses of Cyrtometopus are more flattened and irregular than in the
former genera. The free cheek around the eyes does not form a border
zone, somewhat imitating the eye structure as in Asaphus, but is more
compact, composed of vertical elements which give to the test of the
trilobites in general a tendency to split up in vertical prisms.
Of a peculiar interest are the eyes in the oldest of all oculate
trilobites, at present with certainty known, Eurycare, Peltura,
Sphærophthalmus and Ctenopyge.
Of these genera Eurycare is the oldest (see table p. 22). Amongst the
many free and detached cheeks only a single, very little one has been
found with the eye ball fixed. It seems to be of the same structure as
in Sphærophthalmus. In Sphærophthalmus and Ctenopyge the eye globes
are enormous, considering the size of the cheek in which they are set
and occupy more than a third of the length of the free cheek (Pl. III
f. 26, 31). They are hemispheric, blackish and glossy, more so in the
former genus. The spheroidal lenses, projecting on the surface, are in
Ctenopyge larger near the facial suture and small at the opposite side
where the eye is fixed in the free cheek. For the rest, in both genera
(Pl. III fig. 34) the lenses form a thin stratum, where they in a
vertical section lie elongated, flattened and biconvex, slightly joined
with each other at the point of contact. The fine form which they
exhibit reminds of the lenses of Sphæroma.[29] They are in diameter
thrice as long as they are high. Seen in a horizontal section passing
through the point of contact they show hexaeders with a curiously
jagged outline (Pl. III fig. 33).
[Footnote 29: BELLONCI Atti dei Lincei. Memorie, vol. X, 1881,
Sphæroma, pl. II fig. 11.]
Peltura which is coeval with these, has a narrow semiglobose visual
field (Pl. III figs 35-41), the superior surface of which is quite
smooth and evenly rounded. On its interior side there stand out,
somewhat distantiated, in a low relief semiglobular facets, quite as
regular incrassations of the cornea, thus not forming free lenses, but
rather reminding of the for the rest differently formed quasi-lenses
of Limulus. In a vertical section they appear as the inferior moiety of
real ovate lenses (Pl. III f. 40-41).
The much younger Acerocare has a similar cornea. A very little
specimen, the head of scarcely more than one millim. in length, retains
both eyes, of which one shows the slightly convex lenses and the other
a cast of the interior side as in Peltura. These both genera should in
consequence of their peculiar limuloid cornea be ranged for themselves
apart from the real lenticulate genera, but any material sufficient for
doing this properly, is at present not at hand.
II. =Aggregate eyes.=
These are found solely in the family of the Phacopidæ, unless the
Lichadidæ were also provided with this sort of eyes, but we have had
no opportunity to study them. It seems, however, not likely that they
had aggregate eyes. BARRANDE has represented them quite as finely
reticulate as the eyes of any Asaphid. We have sectioned and figured
the eyes of Dalmanites vulgaris and D. obtusus from the Silurian of
Gotland and found that these have truly aggregate eyes, each consisting
of a regular biconvex lens, lying enclosed in a socket of its own and
covered by a cornea of its own. The distance between the eyes is much
variable and in a few instances they are nearly contiguous. Extremes
are seen on pl. III figures 43, 47. The lenses are comparatively large,
and have always had a covering membrane, though this in many instances
has been lost. This membrane which is an immediate continuation of
the general integument of the body covers the lenses all round their
superior moiety. In its prolongation downwards between the lenses (Pl.
VI fig. 3, 4) it is free from the contact with them and hangs alongside
and around much incrassated, so as to take in a section a lengthened
lancet like shape. It lies thus alongside the other interstitial test,
and is like this perforated by longitudinal canals. In a horizontal
section taken a little below the surface it encircles the lens as a
wall like ring (Pl. VI fig. 1, 2). In a vertical section the lenses
lie in direct contact with the cheek without any intervening zone and
the cheek has the structure so common amongst the trilobites, being
perforated by vertical tubes going straight down from the surface (Pl.
III fig. 44, Pl. VI f. 5).
In Dalm. vulgaris and also in Phacops quadrilineata there is as already
before mentioned a peculiar structure beneath the lenses, consisting
of narrow, threadlike, straight lines, twice as long as the lenses
(Pl. III 49-50, pl. V fig. 38). In a horizontal section they are found
to be irregular prisms closely packed. It can not be any structure
peculiar to the eyes or the lenses, rather some parasitic growth added
since the death of the animal. The lenses are in several specimens
composed of clear calcareous spar. In others again they have been
filled with a dark muddy calcareous rock excepting in the lower moiety
where there is left a residue of the white spar, having in all lenses
assumed a regular shape which I consider as organic (Pl. VI fig. 5).
This spar covers the whole bottom and its upper rim is incrassated and
bent inwards. In horizontal sections this residue is a whitish ring
close inside the interior ring wall (Pl. VI f. 2). I would suggest
that this curious conformation is due to the original structure of
the lens, supposing that it in these crustaceans has been built upon
the same plan as in several other Arthropoda. In Cymothoa[30] and in
Sphæroma[31] for instance the lenses are built up of thin strata, which
are parallel with the convex outside, so that on the inferior surface
of the lens they are arched downwards and on the superior side upwards,
being not strictly concentric. In the spiders they are constructed
upon this same plan[32] perhaps more evidently. If now in Phacops the
lens consisted of such semiconcentric strata and the upper moiety has
been destroyed, the rest must have taken the shape as we find it. It
is moreover peculiar that the destruction has been exactly similar in
all lenses of that specimen. Can it be due to the circumstance that the
power of resistance in the inferior strata has been greater?
[Footnote 30: BULLAR Philos. Transact. 1878, pt. II, pl. 46, fig. 12.]
[Footnote 31: BELLONCI Atti dei Lincei, Memorie X 1881, pl. II, fig.
11.]
[Footnote 32: See GRENACHER pl. II f. 18 Epeira.]
In Phacops quadrilineata the lenses are more elliptic than in the
former. On their interior surface beneath the spiny tufts mentioned
large hexaedral prisms of clear calcareous spar issue, one prism for
each lens (Pl. V fig. 38), having thus a very deceptive appearance,
but no doubt of inorganic origin with the lenses as a basis for their
crystallization, quite as in the Cystoids where the interior often is
converted into a mass of crystalline prisms, issuing from the interior
surface of the plates.
III. =Genera with stemmata and ocelli.=
In a little group that has retained larval or ancestral characters
during a great part of the palæozoic period, the genus Harpes stands
as a type. It has ranged from the oldest Lower Silurian, if we join
the related Harpides, to the middle Devonian. From near the top of
the glabella, though not so much forward as the facial ridge of the
blind trilobites (Olenus, Liostracus etc.) a straight ridge of much
varying length stands out on both sides and at its extremity two or
three globular stemmata with glossy surface lie encased. Probably this
ridge has the same origin as in the Olenidæ, the more so, as there are
indications of an extensive circulatory system. On plate IV fig. 18-19
the right hand ocelli of Harpes vittatus BARR. from Lochkow, Bohemia,
are represented. They are two, lying isolated near each other, quite
globular with circular outline, smooth and glossy as to exhibit a
shining surface. Their size is 0,4 mm in diameter. Being cut vertically
in the direction of the longitudinal axis of the head they resemble
elongated hemispheres, convex on the exterior surface, slightly concave
on the interior. The test of the head lies between them as a saddle and
covers them only partially and on the outer sides they lie with their
margins encased in the head shield. Seen in thin sections of the right
lens and magnified the whitish mass is traversed by vertical, blacker
streaks, standing somewhat radiating towards the sides and cancellate.
The other lens has a horizontal row of black dots. All this is
probably not of any structural value, only due to later changes.
The remarkable genus Harpides from the lowest Lower Silurian belongs
also to this group and has beside the fixed peduncle a peculiar
elongated ridge going from the eyes to the lateral margins of the
head[33], a ridge which is also present in some of the true Harpes.
[Footnote 33: Harpides breviceps cannot belong to this genus and is
rather related to Erinnys SALTER, as also MATTHEW holds it.]
As BEECHER has shown[34] the larva of Trinucleus possesses quite
the same transverse ridge with intumescent eyelike extremities, and
although the smallness of the specimens has not permitted to ascertain
the presence of a true eye, it may be apposite to suppose it on the
homology with the eyes of Harpes, a genus with which Trinucleus is
related. But as well known, in the adult Trinuclei there is no trace
of these ocular ridges nor of real eyes so that BEECHER in his paper
»Blind Trilobites»[35] numerates Trinucleus amongst these. Out of the
nine Scandinavian species of Trinucleus no less than seven have a well
marked little tubercle on each side of the glabella placed exactly on
the same spot where the larval Trinuclei had their much larger eyes
placed. In this case it may be allowed to suppose that the tubercles
are the direct successors of the larval eye and that they are true
ocelli. REEDE seems to be willing to regard them as possessing a visual
function.[36] BEECHER[37] holds the eye nodules in the larva and the
ocelli in the adult to be identical.
[Footnote 34: Structure and appendages of Trinucleus.]
[Footnote 35: Geol. Magazine 1898. pp. 439, 493, 552.]
[Footnote 36: l. c. p. 447.]
[Footnote 37: l. c. p. 309.]
But, as BARRANDE has shown,[38] there is a certain species of
Trinucleus, the larva of which wants a facial ridge and eyes, as there
also are several adult forms without ocelli. These have remained on
a much ancestral stage, while the larva with eyes are more highly
developed in such species, where the adult have been subject to a
retrograde development. Ampyx and Dionide, though completely blind,
evidently belong to this group, and once, as is to be hoped, larval
forms may be discovered showing their development. In a certain way
Arethusina shows characters proper for this group, in having the
straight ocular ridge, quite as in Harpes, but eyes of the reticulate
type and probably prismatic. It thus like Harpes conserved an ancestral
characteristic long periods since it had disappeared in most of the
other genera.
[Footnote 38: Sys. Sil. de Bohéme I, pl. 30, figs 41-50.]
Of the groups, in which JOH:S MÜLLER[39] long ago classified the
Crustacean eyes, his second »Hauptgruppe» (»Aggregate von einfachen
linsenhaften Augen») and the fourth »zusammengesetzten Augen ...
facettirte Hornhaut» the former corresponds with my third and the
latter with my two first divisions. In so far as the cornea and its
facets or lenses are to be regarded, there is the greatest analogy with
the Isopoda. In vertical sections of Sphæroma we have the same sort of
elongated, flattened biconvex lenses as in Sphærophthalmus and others.
Since GRENACHER and EXNER and others have published their excellent
works on the eyes of the Arthropoda, there can be no foundation for
speaking of the resemblance of the trilobitic eye with that of Limulus,
as this genus stands completely isolated amongst all Arthropoda in that
respect. There is, as stated above, a certain resemblance between the
cornea of Peltura and that of Limulus, but this is not yet ripe for a
discussion. Nor is there any evidence for correlating the eyes of the
trilobites with the eyes of the Phyllopoda. BERNARD thinks that the
so called eye of the Paradoxidæ has been formed upon the same plan as
that of Apus. There is nothing to prove this hypothesis that the facial
ridge or any part of it ever had been a visual organ, and the evidence
at hand rather tends in a contrary direction.
[Footnote 39: In Merkels Archiv 1829 p. 46 and in Treviranus
Zeitschrift für Physiologie Bd IV p. 97.]
There are signs of long physiological and anatomical efforts to prepare
the development of the eyes on the free cheek, as revealed through the
long series of blind trilobites. A system of radiating blood vessels,
similar to those described above as covering the inside of the head
in some older genera, all issuing from the scallop in the free cheek,
where later the eye had to find its place, have left their stamp,
their mark on the surface of the free cheek. They attest the great
vital activity which was so intense at the point were the eye was to
be formed. We give the figures of two such cheeks of different types.
One from Parabolina spinulosa (pl. V fig. 31) is the more common, where
six or more isolated trunks radiate from the semilunar ridge round the
indenture and subdivide in branchlets which cease near the lateral
margin of the cheek. It may be that it is an annular vessel near the
indenture that feeds them all and that this probably is in connection
with the great central circulatory system. In Olenus (pl. V fig. 29)
the vessels are partly anastomosing and form a reticulate system and
they are studded with minute wartlets. Another sign, which may be taken
as a preparation, is the elevated rim around the scallop, which is so
prominent in several of the Cambrian genera, but which does not embrace
any facet bearing cornea.
From what has been stated above the following conclusions have been
arrived at.
1. The plurality of the genera living during the Cambrian period were
blind and it was first at the close of that period, in the Olenus
schists, that genera with real visual organs appeared. There may have
been oculate trilobites earlier, as Solenopleura, but we know nothing
of their eyes.
2. The primordial glabellar pleuron which was metamorphosed into a
facial ridge is no visual organ. It is in the Olenidæ nothing but the
elevated line made in the test by the subjacent main trunk of the
circulatory system. It swells out in a node, »palpebral lobe», but not
before the facial suture has been formed. In the genera where there is
no facial suture, there is no node. In the Paradoxidæ where the ridge
is of a different origin, there is no node, though there is a suture.
The four types of eyes in the trilobites have probably succeeded one
another in the following chronological order:
1) with stemmata or ocelli; 2) biconvex or lentiform; 3) prismatic;
4) aggregate. The oldest known representatives for each type are for
1) Harpides rugosus in the Ceratopyge limestone of the Lower Silur.,
for 2) Eurycare, in the Cambrian Olenid schists, division 2, for 3)
Megalaspis, in the Ceratopyge limestone of the Lower Silurian, for
4) Phacops in the Lower gray Orthoceratite limestone. The eyes of
the trilobites show the greatest conformity with those of the recent
Isopoda.
The most perfect eyes amongst all the trilobitic eyes may be those of
the Phacopidæ, which are also geologically the youngest, the least
developed again those of the Proetidæ or rather of the Bumasti. The
great thickness of the cornea in these must have weakened their power
of vision and they had probably only a faint perception of light.
=On the maculæ of the hypostoma.=
We shall now turn our attention to the visual organs which LILJEVALL
discovered on the hypostoma of Bronteus. In doing this, we may bear
in mind, that the genera in which we really have found lenses on the
maculæ are relatively few, but that we shall review the maculæ in all
genera, which we have been able to examine, and try to show that even
most of these may, although in an inferior degree, have acted as visual
organs. At the same time some more details shall be given about the
cephalic eyes for comparing them with the hypostomic ones.
=Acidaspis= MURCH.
The hypostoma is of a peculiar type, deviating from that of the other
genera, squarish or rectangularly transverse, entirely without terrace
lines and no maculæ proper. BARRANDE has given no less than nine
different samples of these hypostomas.
=Acidaspis crenata= EMMR.
(Pl. I figs 1-6.)
There are certainly no maculæ of the same sort as in so many other
trilobites and the interior side of the hypostoma does not bear the
least traces of impressions which might be taken as the reverse of the
maculæ and still more less as muscular impressions. In fact, the total
absence of such in Acidaspis militates against the interpretation of
the macula impressions as muscular scars. When the maculæ fail, also
the so called muscular impressions fail. Moreover we are able to see
still clearer in this matter through what we know about the structure
and position of the extremely similar hypostoma of Apus, of which we
have given a description in the end of this memoir. This skeletal part
of Apus is without any connection with the surroundings, excepting
at its anterior margin, and is consequently movable in a direction
outwards and upwards. Along that anterior margin it is fixed to the
outside of the ventricle through three pair of muscles, three muscles
on each side of the margin. ZADDACH De Apodis cancriformis anatome (pl.
II fig. XIV p. 68).
For the rest the marks of the attachment of the muscles are as a rule
in the Crustacea, at least those of the head, elevated small platforms,
so in the Trilobites, of which I have excellent specimens in a Bumastus
and others. On the inside of the head of Limulus they are faintly
elevated patches. The small hollows on the inside of the hypostoma
formed by the maculæ are indeed the sockets in which the soft parts
of these more or less developed hypostomic eyes were sheltered. But
there is still a feature in the hypostoma of Acidaspis which merits our
attention and which perhaps may have a significance akin to that of the
maculæ.
The hypostoma is square, with two short pointed wings, one on each side
of the slightly bent anterior margin and likewise two smaller ones
at the corners of the posterior margin. A groove follows on a short
distance the lateral margins and the posterior margin and disappears a
little below the anterior one. In the same direction, distally, though
a little more inside and unconnected with them there are two small
grooves, the bottom of which consists of a shell substance of different
colour and structure than the other parts. Having been a little ground
and seen in transmitted light it exhibits the shape of a club and a
homogenous yellow spot, tapering posteriorly and swelling out distally
(Pl. I fig. 4). It must be left an open question whether these maculæ
share in the nature of visual organs as the quite different maculæ of
the other trilobites, but it may be possible that it is so. It must,
however, be remarked that there are two types of hypostoma in the genus
Acidaspis as shown by the illustrations of BARRANDE. One has the small
grooves, possibly all sheltering the claviform maculæ, disposed as in
the now described A. crenata. This group embraces five species of the
Bohemian Silurian formation. The other group of three species again has
the hypostoma of the same quadratic or rectangular shape, but the two
short grooves, which may be expected to contain the maculæ, are placed
midways between the anterior and the posterior margins, nearly as the
maculæ bearing grooves of other trilobites. We have however not had
material for pursuing our researches in this genus, the other species
of the Swedish Acidaspidæ being unknown as to their hypostoma.
It may here be added an observation concerning the ornamentation of
the exterior surface of the hypostoma of Ac. crenata. It is covered
by a great number of diminutive circular or oblong wartlets occupying
the whole surface excepting the lower third of the central field just
above the posterior groove which is smooth. These wartlets seen through
transmitted light (Pl. I fig. 5) show in their interior something like
a peculiar black spiculum rising from a bifid rootlet and confined
within the wartlet and not extruding from it. In a longitudinal section
(Pl. I fig. 6) the spicula perforate the wartlets reaching through
their whole length. As in Calymmene these interior pseudo-spicula are
tubes, filled with iron-pyrites. It is probable that these tubes were
once bearing setæ and quite as in Apus formed a fur of bristles.
The structure of the cephalic eyes (Pl. 1 f. 1-2) is prismatic, but the
separate prisms are rather short and broad. Their lower or interior
end is convex. The separating lines between the single prisms are not
always distinct.
=Agnostus= DALM.
Pl. 1 fig. 7.
Agnostus glandiformis ANG. Although there is not the slightest evidence
of eyes in this the largest of its genus, nor any free hypostoma
hitherto has been found, we may here give a little account of our
researches into this species. The scantiness of material forms a chief
obstacle to our knowledge. Only three entire, rolled up specimens have
been found, and it is by sectioning and preparing such that any hope
can be entertained to gain reliable results.
A rolled up specimen from Andrarum Scania was sectioned lengthwise.
The tail-piece closed tight against the head-shield, so there had
been little chance for foreign matter to penetrate into the interior
which, however, is filled. Close below the cephalic shield there is a
remarkable structure, mostly resembling an elongated intestine with
swellings joined by more narrow ducts and anteriorly the coherence is
interrupted. As in crustaceans and Arthropods in general the stomach
and the intestine are situated on the dorsal side of the body, there is
nothing unlikely in assuming that this in reality may be the remnants
of the intestine. This may also be compared with the observation
made by VOLLBORTH in his memoir »Ueber die mit glatten Rumpfgliedern
versehenen Trilobiten», 1863, p. 46, tab. 1 fig. 12 where the
heart-tube probably is delineated. BARRANDE has also given figures of
what he considers as the intestine in Trinucleus.
Below this organ in Agnostus, there lies a section of a vaulted
calcareous plate with its convexity turned against the dorsal side of
the head shield, that is to say quite the reverse what might have been
expected if it had been the hypostoma in its true position. This may,
however, not be any objection against considering it as a sectioned
hypostoma, loosened from its connection with the cephalic shield,
disturbed in its original position and turned round, when the shell
became filled with mud. At the distal end there is a much distinct
duplicature.
=Asaphus= BRONGN.
The wellknown hypostoma of this genus has the same characteristic
shape, though more pronounced, as in Ptychopyge, with its posterior
margin deeply indented, so as to form two large, pointed lobes. There
is properly only one median field, surrounded by flat, lateral borders,
continuing down into the posterior lobes, from which it is separated
through a shallow groove. The macula are situated in this groove, on
each side of the inferior border of the central field. They are more
or less prominent, but whatever their form may be, their surface is
always entirely smooth, lying well circumscribed amidst the surrounding
terrace lines.