a simple biramous type. These modifications usually take the form of
reduction by the loss or fusion of segments and quite generally either
the entire endopodite or exopodite is lacking. Modification by
addition frequently occurs in the growth of epipodites, "endites," and
"exites" from the coxopodite, basipodite, or both. A protopodite is
generally formed by the fusion of coxopodite and basipodite,
accompanied by a transference of the proximal end of the exopodite to
the distal end of the basipodite. A new segment, not known in the
trilobites (precoxal), is sometimes added at the inner end.
Among modern Crustacea, the anterior cephalic appendages and thoracic
appendages of the Copepoda and the thoracic appendages of certain
Malacostraca, Syncarida especially, are most nearly like those of the
trilobite. The exact homology, segment for segment, between the
walking legs of the trilobite and those of many of the Malacostraca,
even the Decapoda, is a striking instance of retention of primitive
characteristics in a specialized group, comparable to the retention of
primitive appendages in man.
NUMBER OF SEGMENTS IN THE TRUNK.
Various attempts have been made to show that despite the great
variability, trilobites do show a tendency toward a definite number of
segments in the body.
Emmrich (1839), noting that those trilobites which had a long thorax
usually had a short pygidium, and that the reverse also held true,
formulated the law that the number of segments in the trunk was
constant (20 + 1) Very numerous exceptions to this law were, however,
soon discovered, and while the condition of those with less than
twenty-one segments was easily explained, the increasing number of
those with more than twenty-one soon brought the idea into total
disrepute.
Quenstedt (1837) had considered the number of segments of at least
specific importance, and both he and Burmeister (1843) considered that
the number of segments in the thorax must be the same for all members
of a genus. As first shown by Barrande (1852. p. 191 et seq.), there
are very many genera in which there is considerable variation in the
number of thoracic segments, and a few examples can be cited in which
there is variation within a species, or at least in very closely
related species.
Carpenter (1903, p. 333) has tabulated the number of trunk segments of
such trilobites as were listed by Zittel in 1887 and finds a steady
increase throughout the Palæozoic. His table, which follows, is,
however, based upon very few genera.
Period No. of Genera Average No. of
body-segments
===============================================
Cambrian 12 17.66
Ordovician 23 18.58
Silurian 16 19.34
Devonian 10 20.70
Carboniferous 2 20.75
Due chiefly to the efforts of Walcott, an increasingly large number of
Cambrian genera are now represented by entire specimens, and since
these most ancient genera are of greatest importance, a few comments
on them may be offered.
The total number of segments can be fairly accurately determined in at
least nineteen genera of trilobites from the Lower Cambrian. These
include eight genera of the Mesonacidæ (_Olenellus_ was excluded)
and _Eodiscus_, _Goniodiscus_, _Protypus_, _Bathynotus_, _Atops_,
_Olenopsis_, _Crepicephalus_, _Vanuxemella_, _Corynexochus_,
_Bathyuriscus_, and _Poliella_. The extremes of range in total
segments of the trunk is seen in _Eodiscus_ (9) and _Pædeumias_ (45+),
and these same genera show the extremes in the number of thoracic
segments, there being 3 in the one and 44+ in the other. _Pædeumias_
probably shows the greatest variation of any one genus of trilobites,
various species showing from 19 to 44+ thoracic segments. The average
for the nineteen genera is 13.9 segments in the thorax, 3.7 segments
in the pygidium, or a total average of 17.6 segments in the trunk.
_Crepicephalus_ with 12-14 segments in the thorax and 4-6 in the
pygidium, and _Protypus_, with 13 in the thorax and 4-6 in the
pygidium, are the only genera which approach the average. All of the
Mesonacidæ, except one, _Olenelloides_, have far more thoracic and
fewer pygidial segments than the average, while the reverse is true of
the Eodiscidæ, _Vanuxemella_, _Corynexochus_, _Bathyuriscus_, and
Poliella.
The eight genera of the Mesonacidæ, _Nevadia_, _Mesonacis_,
_Elliptocephala_, _Callavia_, _Holmia_, _Wanneria_, _Pædeumias_, and
_Olenelloides_, have an average of 20.25 segments in the thorax and
1.5 in the pygidium, a total of 21.75. If, however, the curious little
_Olenelloides_ be omitted, the average for the thorax rises to 22.14
and the total to 23.84. _Olenelloides_ is, in fact, very probably the
young of an _Olenellus_. Specimens are only 4.5 to 11 mm. long, and
occur in the same strata with _Olenellus_ (see Beecher 1897 A, p.
191).
Thirty-three genera from the Middle Cambrian afford data as to the
number of segments, the Agnostidæ being excluded. The extreme of
variation there is smaller than in the Lower Cambrian. The number of
thoracic segments varies from 2 in Pagetia to 25 in _Acrocephalites_,
and these same genera show the greatest range in total number of trunk
segments, 8 and 29 respectively.
The average of thoracic segments for the entire thirty-three genera is
10.5, of pygidial segments 5.9, a total average of 16.4. It will be
noted that the thorax shows on the average less and the pygidium more
segments than in the Lower Cambrian. If the Agnostidæ could be
included, this result would doubtless be still more striking. Of the
genera considered, _Asaphiscus_ with 7-11 thoracic and 5-8 pygidial
segments, _Blainia_ with 9 thoracic and 6-11 pygidial, _Zacanthoides_
with 9 thoracic and 5 pygidial, and _Anomocare_ with 11 thoracic
and 7-8 pygidial segments came nearest to the average. Only a few
departed widely from it. The genera tabulated were _Acrocephalites_,
_Alokistocare_, _Crepicephalus_, _Karlia_, _Hamburgia_,
_Corynexochus_, _Bathyuriscus_, Poliella, _Agraulos_,
_Dolichometopus_, _Ogygopsis_, _Orria_, _Asaphiscus_, _Neolenus_,
_Burlingia_, _Blainia_, _Blountia_, _Marjumia_, _Pagetia_, _Eodiscus_,
_Goniodiscus_, _Albertella_, _Oryctocara_, _Zacanthoides_,
_Anomocare_, _Anomocarella_, _Coosia_, _Conocoryphe_, _Ctenocephalus_,
_Paradoxides_, _Ptychoparia_, _Sao_, and _Ellipsocephalus_.
Enough genera of Upper Cambrian trilobites are not known from entire
specimens to furnish satisfactory data. Excluding from the list the
Proparia recently described by Walcott, the average total trunk
segments in ten genera is 18, but as most of the genera are Olenidæ or
olenid-like, not much weight can be attached to these figures.
For the Cambrian as a whole, the average for sixty-two genera is
between 17 and 18 trunk segments, which is surprisingly like the
result obtained by Carpenter from only twelve genera, and tends to
indicate that it must be somewhere near the real average. If the 5 or
6 segments of the head be added, it appears that the "average" number
of segments is very close to the malacostracan number 21. Genera with
16 to 18 trunk segments are Callavia, _Protypus_, _Bathynotus_,
_Crepicephalus_, _Bathyuriscus_, _Ogygopsis_, _Burlingia_, _Orria_,
_Asaphiscus_, _Blainia_, _Zacanthoides_, _Neolenus_, _Anomocare_,
_Conocoryphe_, _Saukia_, _Olenus_, and _Eurycare_.
The order Proparia originated in the Cambrian, and Walcott has
described four genera, one from the Middle, and three from the Upper.
The number of segments in these genera is of interest. _Burlingia_,
the oldest, has 14 segments in the thorax and 1 in the pygidium. Of
the three genera in the Upper Cambrian, _Norwoodia_ has 8-9 segments
in the thorax and 3-4 in the pygidium; _Millardia_ 23 in thorax and
3-4 in pygidium; and _Menomonia_ 42 in thorax and 3-4 in pygidium. It
is of considerable interest and importance to note that the very
elongate ones are not from the Middle but from the Upper Cambrian.
Forty genera of Ordovician trilobites known from entire specimens were
tabulated, and it was found that the range in the number of segments
in the thorax and pygidium was surprisingly large. _Agnostus_, which
was not included in the table, has the fewest, and _Eoharpes_, with
29, the most. While the range in number of segments in the thorax is 2
to 29, the range of the number in the pygidium, 2 to 26, is almost as
great. A species of _Dionide_ has 26 in the pygidium, while
_Remopleurides_ and _Glaphurus_ have evidence of only 2. The average
number of segments in the thorax for the forty genera was 10.15, in
the pygidium 8.81, and the average number for the trunk 19.
Genera with just 19 segments in the trunk appear to be rare in the
Ordovician, a species of _Ampyx_ being the only one I have happened to
notice. _Calymene_, _Tretaspis_, _Triarthrus_, _Asaphus_, _Ogygites_,
and _Goldius_ come with the range of 18 to 20. _Goldius_, with 10
segments in the thorax and (apparently) 8 in the pygidium, comes
nearest to the averages for these two parts of the trunk. _Goldius_,
_Amphilichas_, _Bumastus_, _Acidaspis_, _Actinopeltis_, and
_Sphærexochus_ are among the genera having 10 segments in the thorax,
and there are many genera which have only one or two segments more or
less than 10.
In most Ordovician genera, thirty-five out of the forty tabulated, the
number of segments in the thorax is fixed, and the variation is in any
case small. In four of the five genera where it was not fixed, there
was a variation of only one segment, and the greatest variation was in
_Pliomerops_, where the number is from 15 to 19. This of course
indicates that the number of segments in the thorax tends to become
fixed in Ordovician time. The variation in the number of segments in
the pygidium is, however, considerable. It is difficult in many cases
to tell how many segments are actually present in this shield, as it
is more or less smooth in a considerable number of genera. Extreme
cases of variation within a genus are found in _Encrinurus_, species
of which have from 7 to 22 segments in the pygidium, _Cybeloides_ with
10 to 20, and _Dionide_ with 10 to 26. As the number in the thorax
became settled, the number in the pygidium became more unstable, so
that not even in the Ordovician can the total number of segments in
the trunk be said to show any tendency to become fixed.
The genera used in this tabulation were: _Eoharpes_, _Cryptolithus_,
_Tretaspis_, _Trinucleus_, _Dionide_, _Raphiophorus_, _Ampyx_,
_Endymionia_, _Anisonotus_, _Triarthrus_, _Remopleurides_,
_Bathyurus_, _Bathyurellus_, _Ogygiocaris_, _Asaphus_, _Ogygites_,
_Isotelus_, _Goldius_, _Cyclopyge_, _Amphilichas_, _Odontopleura_,
_Acidaspis_, _Glaphurus_, _Encrinurus_, _Cybele_, _Cybeloides_,
_Ectenonotus_, _Calymene_, _Ceraurus_, _Pliomera_, _Pliomerops_,
_Pterygometopus_, _Chasmops_, _Eccoptochile_, _Actinopeltis_,
_Sphærexochus_, _Placoparia_, _Pilekia_, _Selenopeltis_, and
_Calocalymene_.
Only sixteen genera of Devonian trilobites were available for
tabulation, and it is not always possible to ascertain the exact
number of segments in the pygidium, although genera with smooth caudal
shields had nearly all disappeared. The number of segments in the
thorax had become pretty well fixed by the beginning of the Devonian,
_Cyphaspis_ with a range of from 10 to 17 furnishing the only notable
exception. The range for the sixteen genera is from 8 to 17, the
average 11, the number exhibited by the Phacopidæ which form so large
a part of the trilobites of the Devonian. The greater part of the
species have large pygidia, and while the range is from 3 to 23, the
average is 11.2. _Probolium_, with 11 in the thorax and 11-13 in the
pygidium, and _Phacops_, with 11 in the thorax and 9-12 in the
pygidium, approach very closely to the "average" trilobite, and
various species of other genera of the Phacopidæ have the same number
of segments as the norm. In every genus, however, the number of
segments in the pygidium is variable, the greatest variation being in
_Dalmanites_, with a range of from 9 to 23. The number of segments in
the pygidium was therefore not fixed and was on the average higher
than in earlier periods.
The genera used in the tabulation were: _Calymene_, _Dipleura_,
_Goldius_, _Proëtus_, _Cyphaspis_, _Acidaspis_, _Phacops_,
_Hausmania_, _Coronura_, _Odontochile_, _Pleuracanthus_, _Calmonia_,
_Pennaia_, _Dalmanites_, _Probolium_, and _Cordania_.
The trilobites of the late Palæozoic (Mississippian to Permian)
belong, with two possible exceptions, to the Pröetidæ, and only three
genera, _Proëtus_, _Phillipsia_, and _Griffithides_, appear to be
known from all the parts. I am, however, assuming that both
_Brachymetopus_ and _Anisopyge_ have 9 segments in the thorax, and so
have tabulated five genera. The range in the number of segments in the
pygidium is large, from 10 in some species of _Proëtus_ to 30 in
_Anisopyge_, and the average, 17.3, is high, as is the average for
total number in the trunk, 26.3. _Anisopyge_, a late Permian trilobite
described by Girty from Texas, is perhaps the last survivor of the
group. It seems to have had 39 segments in the trunk, making it, next
to the Cambrian _Pædeumias_ and _Menomonia_, the most numerously
segmented of all the trilobites.
The above data may be summarized in the following table:
Period No. of Av. No. of Av. No. of Av. No.
genera segments in segments in of trunk
thorax pygidium segments
==========================================================
Lower Cambrian 19 13.9 3.7 17.6
Middle Cambrian 33 10.5 5.9 16.4
Entire Cambrian 62 ... ... 17-19
Ordovician 40 10.15 8.81 18.96
Devonian 16 11 11.2 22.2
Late Palæozoic 5 9 17.3 26.3
This table confirms that made up by Carpenter, and shows even more
strikingly the progressive increase in the average number of segments
in the trunk throughout the Palæozoic.
While the two trilobites with the greatest number of segments are
Cambrian, yet on the average, the last of the trilobites had the more
numerously segmented bodies. The multisegmented trilobites are:
Period Genus Av. No. of Av. No. of Av. No.
segments in segments in of trunk
thorax pygidium segments
================================================================
Lower Cambrian _Pædeumias_ 44+ 1 45+
Upper Cambrian _Menomonia_ 42 4 46
_Ectenonotus_ 12 22 34
Ordovician _Encrinurus_ 11 22 33
_Dionide_ 6 26 32
Silurian _Harpes_ 29 3 32
Devonian _Coronura_ 11 23 34
_Dalmanites_ 11 23 34
Permian _Anisopyge_ 7+(9?) 30 39?
_Anisopyge_, the last of the trilobites, stands third on the list of
those having great numbers of segments, and in each period there are a
few which have considerably more than the average number. It may be of
some significance that of these nine genera only _Pædeumias_ and
_Anisopyge_ belong to the Opisthoparia, the great central group, and
that five are members of the Proparia, the latest and most specialized
order.
FORM OF THE SIMPLEST PROTASPIS.
It would naturally be expected that the young of the Cambrian
trilobites should be more primitive than the young of species from
later formations, and Beecher (1895 C) has shown that this is the
case. He had reference, however, chiefly to the eyes, free cheeks, and
spines, and by comparison of ontogeny and phylogeny, demonstrated the
greater simplicity of the protaspis which lacked these organs. It
remains to inquire which among the other characteristics are most
fundamental.
Among the trilobites of the Lower Cambrian, no very young have been
seen except of Mesonacidæ. Of these, the ontogeny of _Elliptocephala
asaphoides_ Emmons is best known, thanks to Ford, Walcott, and
Beecher, but, as the last-named has pointed out, the actual protaspis
or earliest shield has not yet been found. The youngest specimen is
the one roughly figured by Beecher (1895 C, p. 175, fig. 6). It lacks
the pygidium, but if completed by a line which is the counterpart of
the outline of the cephalon, it would have been 0.766 mm. long. The
pygidium would have been 0.183 mm. long, or 23 per cent of the whole
length. The axial lobe was narrow, of uniform width along the
cephalon, showed a neck-ring and four indistinct annulations, but did
not reach quite to the anterior end, there being a margin in front of
the glabella about 0.1 mm. wide. The greatest width of the cephalon
was 0.66 mm., and of the glabella 0.233 mm., or practically 35 per
cent of the total width. Other young _Elliptocephala_ up to a length
of 1 mm., and young _Pædeumias_, _Mesonacis_, and _Holmia_ (see Kiær,
Videnskaps, Skrifter, 1 Mat.-Naturv. Klasse, 1917, No. 10) show about
the same characteristics, but all these have large compound eyes on
the dorsal surface and specimens in still younger stages are expected.
It may be pointed out, however, that in these specimens the pygidium
is proportionately larger than in the adult. Walcott cites one adult
126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5
per cent of the total length, while in the incomplete specimen
described above, it was apparently 23 per cent. In a specimen 1 mm.
long figured by Walcott, the pygidium is 0.15 mm. long, or 15 per cent
of the whole length.
The development of several species of trilobites from the Middle
Cambrian is known. Barrande (1852) described the protaspis of _Sao
hirsuta_, _Peronopsis integer_, _Phalacroma bibullatum_, _P. nudum_,
and _Condylopyge rex_. Broegger figured that of a _Liostracus_ (Geol.
For. Förhandl., 1875, pl. 25, figs. 1-3) and Lindstroem (1901, p. 21)
has reproduced the same. Matthew (Trans. Roy. Soc. Canada, vol. 5,
1888, pl. 4, pls. 1, 2) has described the protaspis of a _Liostracus_,
_Ptychoparia linnarssoni_ Broegger, and _Solenopleura robbi_ Hartt.
Beecher (1895 C, pl. 8) has figured the protaspis of _Ptychoparia
kingi_ Meek, and the writer that of a Paradoxides (Bull. Mus. Comp.
Zool., vol. 58, No. 4, 1914, pl. i).
_Sao_, _Liostracus_, _Ptychoparia_, and _Solenopleura_ all have the
same sort of protaspis. In all, the axial lobe reaches the anterior
margin and is somewhat expanded at that end; in all, the glabella
shows but slight trace of segmentation; and in all, the pygidium
occupies from one fifth to one fourth the total length. There is
considerable variation in the width of the axial lobe. It is narrowest
in _Ptychoparia_, where in the middle it is only 14 per cent of the
whole width, and widest in _Solenopleura_, where it is 28 per cent. In
_Ptychoparia_ the pygidium of the protaspis occupies from 18 to 22 per
cent of the whole length. In the adult it occupies 10 to 12 per cent.
In _Solenopleura_ it makes up about 26 per cent of the protaspis, and
in the adult about 8 per cent.
In the youngest stages of all these trilobites, the pygidium is
incompletely separated from the cephalon. The first sign of
segmentation is a transverse crack which begins to separate the
cephalon and pygidium, and by the time this has extended across the
full width the neck segment has become rather well defined. In this
stage the animal is prepared to swim by means of the pygidium, and
first becomes active. The coincident development of the free pygidium
and the neck-ring strongly suggests that the dorsal longitudinal
muscles are attached beneath the neck-fur row.
The single protaspis of _Paradoxides_ now known, while only 1 mm.
long, is not in the youngest stage of development. It is like the
protaspis of _Olenellus_ in having large eyes on the dorsal surface
and a narrow brim in front of the glabella. The glabella is narrower
than in the adult.
The initial test of no agnostid has probably as yet been seen, as
all the young now known show the cephalon and pygidium distinctly
separated. _Phalacroma bibullatum_ and _P. nudum_ are both practically
smooth and isopygous when 1.5 mm. long. _P. bibullatum_ shows no axial
lobe at this stage, but a wide glabella and median tubercle develop
later, and when the glabella first appears, it extends to the anterior
margin. In _Peronopsis integer_ and _Condylopyge rex_, the axial lobe
is outlined on each of the equal shields in specimens about 1 mm.
long, but is without furrows and reaches neither anterior nor
posterior margin.
From the foregoing brief description it appears that the pygidium of
the protaspis varies in different groups from as little as 15 per cent
of the total length in the Mesonacidæ to as much as 50 per cent in the
Agnostidæ; that the axial lobe varies from as little as 14 per cent of
the total width in one _Ptychoparia_ to as much as 50 per cent in
_Phalacroma nudum_; that the glabella reaches the anterior margin in
the Olenidæ, Solenopleuridæ, and _Phalacroma bibullatum_, while there
is a brim in front of it in the Olenellidæ, Paradoxidæ, and three of
the species of the Agnostidæ. The decision as to which of these
conditions are primitive may be settled quite satisfactorily by study
of the ontogeny of the various species.
ORIGIN OF THE PYGIDIUM.
Taking first the pygidium, it has already been pointed out that in
each case the pygidium of the adult is proportionally considerably
smaller than the pygidium of the protaspis. The stages in the growth
of the pygidium are better known in Sao hirsuta than in any other
trilobite, and a review of Barrande's description will be
advantageous.
Barrande recognized twenty stages in the development of this species,
but there was evidently a still simpler protaspis in his hands than
the smallest he figured, for he says, after describing the specimen in
the first stage: "We possess one specimen on which the head extends
from one border to the other of the disk, but as this individual is
unique we have not thought it sufficient to establish a separate
stage." This specimen is important as indicating a stage in which
there was not even a suggestion of division between cephalon and
pygidium.
In the first stage described by Barrande, the form is circular, the
length is about 0.66 mm., and the glabella is narrow with parallel
sides and no indications of lateral furrows. The neck segment is
indicated by a slight prominence on the axial lobe, and back of it a
constriction divides the axial lobe of the pygidium into two nodes,
but does not cross the pleural lobes. The position of the nuchal
segment permits a measurement of the part which is to form the
pygidium, and shows that that shield made up 30 per cent of the entire
length.
In the second stage, when the test is 0.75 mm. long, the cephalon and
pygidium become distinctly separated, and the latter shield shows
three annulations on the axial and two pairs of ribs on the pleural
lobes. It now occupies 33-1/3 per cent of the total length.
In the third stage, when the total length is about 1 mm., the pygidium
has continued to grow. It now shows five annulations on the axial
lobe, and is 46 per cent of the total length.
In the fourth stage, two segments of the axial lobe have been set free
from the front of the pygidium. The length is now 1.5 mm. and the
pygidium makes up 32 per cent of the whole. From this time the
pygidium continues to decrease in size in proportion to the total
length, as shown in the following table.
Stage Length in Percentage Segments in Segments in
mm. of pygidium thorax pygidium
========================================================
1 0.66 30 0 2
2 0.75 33-1/3 0 3
3 1.00 46 0 5
4 1.50 32 2 5-6
5 1.50 25 3 4
6 1.75 23 4 4
7 1.80 21 5 3
8 2.00 17 6 3
9 2.50 13 7 3
10 3.00 12 8 3
11 3.50 11 9 3-4
12 4.00 11 10 3-4
13 5.00 10 11 3
14 5.50 9 12 2-4
15 6.00 8 13 3-4
16 6.50 8 14 3
17 7.00 7 15 3
18 7.50 7 16 3
19 7.50 6 17 2
20 10.25 6 17 2
This table shows the rapid increase in the length of the pygidium till
the time when the thorax began to be freed, the very rapid decrease
during the earlier part of its formation until six segments had been
set free, and then a more gradual decrease until the entire seventeen
segments had been acquired, after which time the relative length
remained constant. From an initial proportion of 30 per cent, it rose
to nearly one half the whole length, and then dwindled to a mere 6 per
cent, showing conclusively that the thorax grew at the expense of the
pygidium.
If this conclusion can be sustained by other trilobites, it indicates
that the large pygidium is a more primitive characteristic of a
protaspis than is a small one. I have already shown that the pygidium
is proportionately larger in the protaspis in the Mesonacidæ,
Solenopleuridæ, and Olenidæ, and a glance at Barrande's figures of
_"Hydrocephalus" carens_ and _"H." saturnoides_, both young of
_Paradoxides_ will show that the same process of development goes
on in that genus as in _Sao_. There is first an enlargement of the
pygidium to a maximum, a rise from 20 per cent to 33 per cent in
the case of _H. carens_ and then, with the introduction of thoracic
segments, a very rapid falling off. All of these are, however,
trilobites with small pygidia, and it has been a sort of axiom among
palæontologists that large pygidia were made up of a number of
coalesced segments. While not definitely so stated, it has generally
been taken to mean the joining together of segments once free. The
asaphid, for instance, has been thought of as descended from some
trilobite with rich segmentation, and a body-form like that of a
_Mesonacis_ or _Paradoxides_.
The appeal to the ontogeny does not give as full an answer to this
question as could be wished, for the complete life-history of no
trilobite with a large pygidium is yet known. While the answer is not
complete, enough can be gained from the study of the ontogeny of
_Dalmanites_ and _Cyclopyge_ to show that in these genera also the
thorax grows by the breaking down of the pygidium and that no segment
is ever added from the thorax to the pygidium. The case of _Dalmanites
socialis_ as described by Barrande (1852, p. 552, pl. 26) will be
taken up first, as the more complete. The youngest specimen of this
species yet found is 0.75 mm. long, the pygidium is distinctly
separated from the cephalon, and makes up 25 per cent of the length.
This is probably not the form of the shell as it leaves the egg. At
this stage there are two segments in the pygidium, but they increase
to four when the test is 1 mm. long. The cephalon has also increased
in length, however, so that the proportional length is the same. The
subjoined table, which is that compiled by Barrande with the
proportional length of the pygidium added, is not as complete as could
be desired, but affords a very interesting history of the growth of
the caudal shield. The maximum proportional length is reached before
the introduction of thoracic segments, and during the appearance of
the first five segments the size of the pygidium drops from 25 to 15
per cent. Several stages are missing at the critical time between
stages 8 and 9 when the pygidium had added three segments to itself
and has supplied only one to the thorax. This would appear to have
been a sort of resting or recuperative stage for the pygidium, for it
increased its own length to 20 per cent, but from this stage up to
stage 12 it continued to give up segments to the thorax and lose in
length itself. After stage 12, when the specimens were 8 mm. long, no
more thoracic segments were added, but new ones were introduced into
the pygidium, until it reached a size equal to one fifth the entire
length, as compared with one fourth in the protaspis.
Stage Length Percentage Segments in Segments in
in mm. of pygidium thorax pygidium
====================================================
1 0.75 25 0 2
2 0.75 25 0 3
3 1.00 25 0 4
4 1.00 22 1 3
5 1.25 20 2 3
6 1.25 18 3 3
7 1.60 15 4 3
8 1.60 15 5 3
9 3.00 20 6 6
10 3.50 20 7 6
11 8.00 18 9 7
12 8.00 16 11 5
13 12.00 16 11 7
14 19.00 18 11 9
15 95.00 20 11 11
Since the above was written, Troedsson (1918, p. 57) has described the
development of _Dalmanites eucentrus_, a species found in the
Brachiopod shales (Upper Ordovician) of southern Sweden. This species
follows a course similar to that of _D. socialis_, so that the full
series of stages need not be described. The pygidium is, however, of
especial interest, for there is a stage in which it shows two more
segments than in the adult. Troedsson figures a pygidium 1.28 mm. long
which has eight pairs of pleural ribs, while the adult has only six
pairs. The ends of all these ribs are free spines, and were the
development not known one would say that this was a case of incipient
fusion, while as a matter of fact, it is incipient freedom.
A further interest attaches to this case, because of the close
relationship between _D. eucentrus_ and _D. mucronatus_. The latter
species appears first in the _Staurocephalus_ beds which underlie the
Brachiopod shales, so that in its first appearance it is somewhat the
older. The pygidium of the adult _D. mucronatus_ is larger than that
of _D. eucentrus_, having eight pairs of pleural ribs, the same number
as in the young of the latter. In short, _D. eucentrus_ is probably
descended from _D. mucronatus_, and in its youth passes through a
stage in which it has a large pygidium like that species. Once more it
appears that the small pygidium is more specialized than the large
one.
The full ontogeny of _Cyclopyge_ is not known, but young specimens
show conclusively that segments are not transferred from the thorax to
the pygidium, but that the opposite occurs. As shown by Barrande
(1852) and corroborated by specimens in the Museum of Comparative
Zoology, the process is as follows: The third segment of the adult of
this species, that is, the fourth from the pygidium, bears a pair of
conspicuous cavities on the axial portion. In a young specimen, 7 mm.
long, the second segment bears these cavities, but as the thorax has
only four segments, this segment is also the second instead of the
fourth ahead of the pygidium. The pygidium itself, instead of being
entirely smooth, as in the adult state, is smooth on the posterior
half, but on the anterior portion has two well formed but still
connected segments, the anterior one being more perfect than the
other. These are evidently the two missing segments of the thorax, and
instead of being in the process of being incorporated in the pygidium,
they are in fact about to be cast off from it to become free thoracic
segments. In other words, the thorax grows through the degeneration
of the pygidium. That the thorax grows at actual expense to the
pygidium is shown by the proportions of this specimen. In an adult of
this species the pygidium, thorax, and cephalon are to each other as
9:11:13. In the young specimen they are as 10:6:12, the pygidium being
longer in proportion both to the thorax and to the cephalon than it
would be in the adult.
This conception of the breaking down of the pygidium to form the
thorax will be very helpful in explaining many things which have
hitherto seemed anomalous. For instance, it indicates that the
Agnostidæ, whose subequal shields in early stages have been a puzzle,
are really primitive forms whose pygidia do not degenerate; likewise
the Eodiscidæ, which, however, show within the family a tendency to
free some of the segments. The annelidan Mesonacidæ may not be so
primitive after all, and their specialized cephala may be more truly
indicative of their status than has previously been supposed.
The facts of ontogeny of trilobites with both small and large pygidia
do show that there is a reduction of the relative size of the caudal
shield during the growth-stages, and therefore that the large pygidium
in the protaspis is probably primitive. The same study also shows that
the large pygidium is made up of "coalesced segments" only to the
extent that they are potentially free, and not in the sense of fused
segments.
WIDTH OF THE AXIAL LOBE.
That the narrow type of axial lobe is more primitive than the wide one
has already been demonstrated by the ontogeny of various species, and
space need not be taken here to discuss the question. Most Cambrian
trilobites have narrow axial lobes even in the adult so that their
development does not bring this out very strikingly, though it can be
seen in Sao, Ptychoparia, etc., but in Ordovician trilobites such as
Triarthrus and especially Isotelus, it is a conspicuous feature.
PRESENCE OR ABSENCE OF A "BRIM."
That the extension of the glabella to the front of the cephalon is a
primitive feature is well shown by the development of Sao (Barrande,
1852, pl. 7), Ptychoparia (Beecher, 1895 C, pl. 8), and Paradoxides
(Raymond, Bull. Mus. Comp. Zool., vol. 57, 1914), although in the last
genus the protaspis has a very narrow brim, the larva during the
stages of introduction of new segments a fairly wide one, and most
adults a narrow one.
The brim of Sao seems to be formed partly by new growth and partly at
the expense of the frontal lobe, for that lobe is proportionately
shorter in the adult than in the protaspis. In _Cryptolithus_ and
probably in _Harpes_, _Harpides_, etc., the brim is quite obviously new
growth and has nothing to do with the vital organs. Its presence or
absence may not have any great significance, but when the glabella
extends to the frontal margin, it certainly suggests a more anterior
position of certain organs. In _Sao_, the only trilobite in which
anything is known of the position of the hypostoma in the young, the
posterior end is considerably further forward in a specimen a. 5 mm.
long than in one 4 mm. long, thus indicating a backward movement of
the mouth during growth, comparable to the backward movement of the
eyes.
SEGMENTATION OF THE GLABELLA.
The very smallest specimens of _Sao_ show a simple, unsegmented axial
lobe, and the same simplicity has been noted in the young of other
genera. Beecher considered this as due to imperfect preservation of
the exceedingly small shells, which practically always occur as moulds
or casts in soft shale. There is, however, a very general increase in
the strength of glabellar segmentation in the early part of the
ontogeny of all trilobites whose life history is known, and in some
genera, like the Agnostidæ, there is no question of the comparatively
late acquisition of glabellar furrows. Even in _Paradoxides_, the
furrows appear late in the ontogeny.
_Summary._
If absence of eyes on the dorsal surface be primitive, as Beecher
has shown, and if the large pygidium, narrow axial lobe, and long
unsegmented glabella be primitive, then the known protaspis of the
Mesonacidæ and Paradoxidæ is not primitive, that of the Olenidæ is
very primitive, and that of the Agnostidæ is primitive except that in
one group the axial lobe, when it appears, is rather wide, and in the
other a brim is present.
[Illustration: Fig. 35.--A specimen of _Weymouthia nobilis_ (Ford),
collected by Mr. Thomas H. Clark at North Weymouth, Mass. Note the
broad smooth shields of this Lower Cambrian eodiscid. × 6.]
Subsequent development from the simple unsegmented protaspis would
appear to show, first, an adaptation to swimming by the use of the
pygidium; next, the invagination of the appendifers as shown in the
segmentation of the axial lobe indicates the functioning of the
appendages as swimming legs; then with the introduction of thoracic
segments the assumption of a bottom-crawling habit is indicated. Some
trilobites were fully adapted for bottom life, and the pygidium became
reduced to a mere vestige in the production of a worm-like body. Other
trilobites retained their swimming habits, coupled with the crawling
mode of life, and kept or even increased (_Isotelus_) the large
pygidium.
The Simplest Trilobite.
In the discussion above I have placed great emphasis on the large size
of the primitive pygidium, because, although there is nothing new in
the idea, its significance seems to have been overlooked.
If the large pygidium is primitive, then multisegmentation in
trilobites can not be primitive but is the result of adaptation to a
crawling life. It is annelid-like, but is not in itself to be relied
upon as showing relationship to the Chætopoda. Simple trilobites with
few segments, like the Agnostidæ, Eodiscidæ etc., were, therefore,
properly placed by Beecher at the base of his classification, and
there is now less chance than ever that they can be called degenerate
animals.
From the phylogeny of certain groups, such as the Asaphidæ, it is
learned that the geologically older members of the family have more
strongly segmented anterior and posterior shields than the later ones.
That there has been a "smoothing out" is demonstrated by a study of
the ontogeny of the later forms. From such examples it has come to
be thought that all smooth trilobites are specialized and occupy a
terminal position in their genealogical line. This has caused some
wonder that smooth agnostids like _Phalacroma bibullatum_ and _P.
nudum_ should be found in strata so old as the Middle Cambrian, and
was a source of great perplexity to me in the case of _Weymouthia_
(Ottawa Nat., vol. 27, 1913) (fig. 35). This is a smooth member of the
Eodiscidæ, and, in fact, one of the simplest trilobites known, for
while it has three thoracic segments, it shows almost no trace of
dorsal furrows or segmentation on cephalon or pygidium, and, of
course, no eyes. Following the general rule, I took this to be a
smooth-out eodiscid, and was surprised that it should come from the
Lower Cambrian, where it is associated with _Elliptocephala_ at Troy,
New York, and with _Callavia_ at North Weymouth, Massachusetts, and
where it has lately been found by Kiær associated with _Holmia_ and
_Kjerulfia_ at Tømten, Norway. It now appears it is really in its
proper zone, and instead of being the most specialized, is the
simplest of the Eodiscidæ.
What appears to be a still simpler trilobite is the form described by
Walcott as Naraoia.
=Naraoia compacta= Walcott.
(Text fig. 36.)
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 175,
pl. 28, figs. 3, 4.--Cleland, Geology, Physical and Historical, New
York, 1916, p. 412, fig. 382 F (somewhat restored).
This very imperfectly known form is referred by Walcott to the
Notostraca on what appear to be wholly inadequate grounds, and while I
do not insist on my interpretation, I can not refrain from calling
attention to the fact that it _can_ be explained as the most primitive
of all trilobites. It consists of two subequal shields, the anterior
of which shows slight, and the posterior considerable evidence of
segmentation. It has no eyes, no glabella, and no thorax, and is
directly comparable to a very young _Phalacroma bibullatum_ (see
Barrande 1852, pl. 49, figs. a, b). Walcott states that there is
nothing to show how many segments there are in the cephalic shield,
but that on one specimen fourteen were faintly indicated on the
abdominal covering. The appendages are imperfectly unknown, as no
specimen showing the ventral side has yet been described. The possible
presence of antennas and three other appendages belonging to the
cephalic shield is mentioned, and there are tips of fourteen legs
projecting from beneath the side of one specimen. As figured, some of
the appendages have the form of exopodites, others of endopodites,
indicating that they were biramous.
_Naraoia_ is, so far as now known, possessed of no characteristics
which would prevent its reference to the Trilobita, while the
presence of a large abdominal as well as a cephalic shield would make
it difficult to place in even so highly variable a group as the
Branchiopoda. On the other hand, its only exceptional feature as a
trilobite is the lack of thorax, and all study of the ontogeny of the
group has led us to expect just that sort of a trilobite to be found
some day in the most ancient fossiliferous rocks. _Naraoia_ can, I
think, be best explained as a trilobite which grew to the adult state
without losing its protaspian form. It was found in the Middle
Cambrian of British Columbia.
Even if _Naraoia_ should eventually prove to possess characteristics
which preclude the possibility of its being a primitive trilobite, it
at least represents what I should expect a pre-Cambrian trilobite to
look like. What the ancestry of the nektonic primitive trilobite may
have been is not yet clear, but all the evidence from the morphology
of cephalon, pygidium, and appendages indicates that it was a
descendant of a swimming and not a crawling organism.
Since the above was written, the Museum of Comparative Zoology has
purchased a specimen of this species obtained from the original
locality. The shields are subequal, the posterior one slightly the
larger, and the axial lobes are definitely outlined on both. The
glabella is about one third the total width, nearly parallel-sided,
somewhat pointed at the front. There are no traces of glabellar
furrows. The axial lobe of the pygidium is also about one third the
total width, extends nearly to the posterior margin, and has a rounded
posterior end. The measurements are as follows: Length, 33 mm.; length
of cephalon, 16 mm., width, 15 mm.; length of glabella, 11.5 mm.,
width, 5.5 mm.; length of pygidium, 17 mm., width, 15 mm.; length of
axial lobe, 14 mm., width, 5.5 mm.
The species is decidedly _Agnostus_-like in both cephalon and
pygidium, and were it not so large, might be taken for the young of
such a trilobite. The pointed glabella is comparable to the axial
lobes of the so-called pygidia of the young of _Condylopyge rex_ and
_Peronopsis integer_ (Barrande, Syst. Sil., vol. 1, pl. 49).
The Ancestor of the Trilobites, and the Descent of the Arthropoda.
The "annelid" theory of the origin of the Crustacea and therefore of
the trilobites, originating with Hatschek (1877) and so ably
championed by Bernard (1892), has now been a fundamental working
hypothesis for some years, and has had a profound influence in
shaping thought about trilobites. This hypothesis has, however,
its weak points, the principal one being its total inhibition of
the workings of that great talisman of the palæontologist, the law of
recapitulation. Its acceptance has forced the zoologist to look upon
the nauplius as a specially adapted larva, and has caused more than
one forced explanation of the protaspis of the trilobite. When so keen
a student as Calman says that the nauplius must point in some way to
the ancestor of the Crustacea (1909, p. 26), it is time to reëxamine
some of the fundamentals. This has been done in the preceding pages
and evidence adduced to show that the primitive features of a
trilobite indicate a swimming animal, and that the adaptations are
those which enabled it to assume a crawling mode of existence. It has
also been pointed out that in Naraoia there is preserved down to
Middle Cambrian times an animal like that to which ontogeny points as
a possible ancestor of the trilobites. _Naraoia_ is not the simplest
conceivable animal of its own type, however, for it has built up a
pygidium of fourteen or fifteen somites. One would expect to find in
Proterozoic sediments remains of similar animals with pygidia composed
of only one or two somites, with five pairs of appendages on the
cephalon, one or two pairs on the pygidium, a ventral mouth, and a
short hypostoma. Anything simpler than this could not, in my opinion,
be classed as a trilobite.
What the ancestor of this animal was is mere surmise. It probably had
no test, and it may be noted in this connection that _Naraoia_ had a
very thin shell, as shown by its state of preservation, and was in
that respect intermediate between the trilobite and the theoretical
ancestor. Every analysis of the cephalon of the trilobite shows that
it is made up of several segments, certainly five, probably six,
possibly seven. Every study of the trilobite, whether of adult, young,
or protaspis, indicates the primitiveness of the lateral extensions or
pleural lobes. The same studies indicate as clearly the location of
the vital organs along the median lobe. These suggestions all point to
a soft-bodied, depressed animal composed of few segments, probably
with simple marginal eyes, a mouth beneath the anterior margin,
tactile organs at one or both ends, with an oval shape, and a straight
narrow gut running from anterior mouth to terminal anus. The broad
flat shape gives great buoyancy and is frequently developed in the
plankton. Inherited by the trilobites, it proved of great use to the
swimmers among them.
The known animal which most nearly approaches the form which I should
expect the remote ancestor of the trilobites to have had is _Amiskwia
sagittiformis_ Walcott (Smithson. Misc. Coll., vol. 57, 1911, p. 112,
pl. 22, figs. 3, 4). This "worm" from the Middle Cambrian is similar
in outline to the recent _Spadella_, and is referred by Walcott to the
Chætognatha. It has a pair of lateral expansions and a flattened
caudal fin, a narrow median alimentary canal, and a pair of rather
long simple tentacles. With the exception of a thin septum back of the
head, no traces of segmentation are shown.
Some time in the late pre-Cambrian, the pre-trilobite, which probably
swam by rhythmic undulations of the body, began to come into
occasional contact with a substratum, and two things happened:
symmetrically placed, i. e., paired, appendages began to develop on
the contact surface, and a test on the dorsal side. The first use of
the appendages may have been in pushing food forward to the mouth,
and for the greater convenience in catching such material, a fold
in front of the mouth may have elongated to form the prototype of the
hypostoma. At this time the substratum may not have been the ocean
bottom at all, but the animals, still free swimmers, may have alighted
at feeding time on floating algæ from the surface of which they
collected their food. While the dorsal test was originally jointed at
every segment, the undulatory mode of swimming seems to have given way
to the method of sculling by means of the posterior end only, or by
the use of the appendages, and the anterior segments early became
fused together.
The result of the hardening of the dorsal test was of course to reduce
to that extent the area available for respiration, and this function
was now transferred in part to the limbs, which bifurcated, one branch
continuing the food-gathering process and the other becoming a gill.
The next step may have been the "discovery" of the ocean bottom and
the tapping of an hitherto unexploited supply of food. Upon this,
there set in those adaptations to a crawling mode of existence which
are so well shown in the trilobite. The crawling legs became
lengthened and took on a hardened test, the hypostoma was greatly
elongated, pushing the mouth backward, and new segments were added to
produce a long worm-like form which could adapt itself to the
inequalities of the bottom. That the test of the appendages became
hardened later than that of the body is shown by the specimens of
Neolenus, in which the dorsal shell as preserved in the shale is thick
and solid, while the test of the appendages is a mere film.
The late Proterozoic or very earliest Cambrian was probably the time
of the great splitting up into groups. The first development seems to
have been among the trilobites themselves, the Hypoparia giving rise
to two groups with compound eyes, first the Opisthoparia and later the
Proparia. About this same time the Copepoda may have split off from
the Hypoparia, continuing in the pelagic habitat. At first, most of
the trilobites seem to have led a crawling existence, but about Middle
Cambrian time they began to go back partially to the ancestral
swimming habits, and retained some of the trunk segments to form a
larger pygidium. The functional importance of the pygidium explains
why it can not be used successfully in making major divisions in
classification. Nearly related trilobites may be adapted to diverse
methods of life.
EVOLUTION WITHIN THE CRUSTACEA.
The question naturally arises as to whether the higher Crustacea were
derived from some one trilobite, or whether the different groups have
been developed independently from different stocks. The opinion that
all other crustaceans could have been derived from an _Apus_-like form
has been rather generally held in recent years, but Carpenter (1903,
p. 334) has shown that the leptostracan, _Nebalia_, is really a more
primitive animal than _Apus_. He has pointed out that in Leptostraca
the thorax bears eight pairs of simple limbs with lamelliform
exopodites and segmented endopodites, while the abdomen of eight
segments has six pairs of pleopods and a pair of furcal processes,
so that only one segment is limbless. Contrasted with this are the
crowded and complicated limbs of the anterior part of the trunk of
_Apus_, and the appendage-less condition of the hinder portion.
Further, a comparison between the appendages of the head of _Nebalia_
and those of _Apus_ shows that the former are the more primitive. The
antennules of Nebalia are elongate, those of _Apus_ greatly reduced;
the mandible of _Nebalia_ has a long endopodite, and Carpenter points
out that from it either the malacostracan mandible with a reduced
endopodite or the branchiopodan mandible with none could be derived,
but that the former could not have arisen from the latter. The maxillæ
of _Apus_ are also much the more specialized and reduced.
_Nebalia_ being in all else more primitive than _Apus_, it follows
that the numerous abdominal segments of the latter may well have
arisen by the multiplication of an originally moderate number, and the
last trace of primitiveness disappears.
It is now possible to add to the results obtained from comparative
morphology the testimony of palæontology, already outlined above, and
since the two are in agreement, it must be admitted that the modern
Branchiopoda are really highly specialized.
As has already been pointed out, _Hymenocaris_, the leptostracan of
the Middle Cambrian, has very much the same sort of appendages as the
Branchiopoda of the same age, both being of the trilobite type. Which
is the more primitive, and was one derived from the other?
The Branchiopoda were much more abundant and much more highly
diversified in Cambrian times than were the Leptostraca, and,
therefore, are probably older. Some of the Cambrian branchiopods were
without a carapace, and some were sessile-eyed. These were more
trilobite-like than Hymenocaris. Many of the Cambrian branchiopods had
developed a bivalved carapace, though not so large a one as that of
the primitive Leptostraca. The present indications are, therefore,
that the Branchiopoda are really older than the Leptostraca, and also
that the latter were derived from them. It seems very generally agreed
that the Malacostraca are descended from the Leptostraca, and the
fossils of the Pennsylvanian supply a number of links in the chain of
descent. Thus, _Pygocephalus cooperi_, with its brood pouches, is
believed by Calman (1909, p. 181) to stand at the base of the
Peracaridan series of orders, and _Uronectes_, _Palæocaris_, and
the like are Palæozoic representatives of the Syncarida. Others
of the Pennsylvanian species appear to tend in the direction of
the Stomatopoda, whose true representatives have been found in the
Jurassic. The Isopoda seem to be the only group of Malacostraca not
readily connected up with the Leptostraca. Their depressed form, their
sessile-eyes, and their antiquity all combine to indicate a separate
origin for the group, and it has already been pointed out how readily
they can be derived directly from the trilobite.
While the Copepoda seem to have been derived directly from the
Hypoparia, the remainder of the Crustacea apparently branched off
after the compound eyes became fully developed, unless, as seems
entirely possible, compound eyes have been developed independently in
various groups. Most Crustacea were derived from crawling trilobites
(Lower Cambrian or pre-Cambrian Opisthoparia), for they lost the large
pygidium, and also the major part of the pleural lobes. In all
Crustacea, too, other than the Copepoda and Ostracoda, there is a
tendency to lose the exopodites of the antennæ.
These modifications, which produced a considerable difference in the
general appearance of the animal, are easily understood. As has been
shown in previous pages, the trilobites themselves exhibit the
degenerative effect on the anterior appendages of the backward
movement of the mouth, and the transformation of a biramous appendage
with an endobase into a uniramous antenna is a simple result of such
a process. The feeding habits of the trilobites were peculiar and
specialized, and it is natural that some members of the group should
have broken away from them. In any progressive mode of browsing
the hypostoma was a hindrance, so was soon gotten rid of, and the
endobases not grouped around the mouth likewise became functionless.
The chief factor in the development of the higher Crustacea seems to
have been the pinching claw, by means of which food could be conveyed
to the mouth. It had the same place in crustacean development that the
opposable thumb is believed to have had in that of man.
An intermediate stage between the Trilobita and the higher Crustacea
is at last exhibited to us by the wonderful, but unfortunately rather
specialized _Marrella_, already described. It retains the hypostoma
and the undifferentiated biramous appendages of the trilobite, but has
uniramous antennæ, there are no endobases on the coxopodites of the
thoracic appendages, the pygidium is reduced to a single segment, and
the lateral lobes of the thorax are also much reduced. _Marrella_ is
far from being the simplest of its group, but is the only example
which survived even down to Middle Cambrian times of what was probably
once an important series of species transitional between the
trilobites and the higher Crustacea.
In this theory of the origin of the Crustacea from the Trilobita, the
nauplius becomes explicable and points very definitely to the
ancestor. According to Calman (1909, p. 23):
The typical nauplius has an oval unsegmented body and three pairs
of limbs, corresponding to the antennules, antennas, and mandibles
of the adult. The antennules are uniramous, the others biramous,
and all three pairs are used in swimming. The antennæ may have a
spiniform or hooked masticatory process at the base, and share with
the mandibles which have a similar process, the function of seizing
and masticating the food. The mouth is overhung by a large labrum
or upper lip and the integument of the dorsal surface of the body
forms a more or less definite dorsal shield. The paired eyes are as
yet wanting, but the median eye is large and conspicuous.
The large labrum or hypostoma, the biramous character of the
appendages, especially of the antennæ, the functional gnathobases on
the second and third appendages, and the oval unsegmented shield are
all characteristics of the trilobites, and it is interesting to note
that all nauplii have the free-swimming habit.
The effect of inheritance and modification through millions of
generations is also shown in the nauplius, but rather less than would
be expected. The most important modification is the temporary
suppression of the posterior pairs of appendages of the head, so that
they are generally developed later than the thoracic limbs. The median
or nauplius eye has not yet been found in trilobites, and if it is, as
it appears to be, a specialized eye, it has probably arisen since the
later Crustacea passed the trilobite stage in their phylogeny.
The oldest Crustacea, other than trilobites, so far known are the
Branchiopoda and Phyllocarida described by Walcott and discussed
above. It is important to note that while the former have already
achieved such modified characteristics that they have been referred to
modern orders, they retain the trilobite-like limbs and some of them
still have well developed pleural lobes.
Calman (1909, p. 101) says of the Copepoda:
On the hypothesis that the nauplius represents the ancestral type
of the Crustacea, the Eucopepoda would be regarded as the most
primitive existing members of the class, retaining as they do,
naupliar characters in the form of the first three pairs of
appendages and in the absence of paired eyes and of a shell-fold.
As already indicated, however, it is much more probable that they
are to be regarded as a specialized and in some respects degenerate
group which, while retaining, in some cases, a very primitive
structure of the cephalic appendages, has diverged from the
ancestral stock in the reduction of the number of somites, the loss
of the paired eyes and the shell-fold, and the simplified form of
the trunk-limbs.
If the Eucopepoda be viewed in the light of the theory of descent here
suggested, it is at once seen that while they are modified and
specialized, they more nearly approximate the hypothetical ancestor
than any other living Crustacea. Compound eyes are absent, and it can
not be proved that they were ever present, although Grobben is said to
have observed rudiments of them in the development of _Calanus_. The
"simplified limbs" are the simple limbs of the trilobite, somewhat
modified. The absence of the shell-fold and carapace is certainly a
primitive characteristic. Add to this the direct development of the
small number of segments, and the infolded pleural lobes, and it must
be admitted that the group presents more trilobite-like
characteristics than any other. It seems very likely that the
primitive features were retained because of the pelagic habitat of a
large part of the group.
Ruedemann (Proc. Nat. Acad. Sci., vol. 4, 1918, p. 382, pl.) has
recently outlined a possible method of derivation of the acorn
barnacles from the phyllocarids. Starting from a recent _Balanus_ with
rostrum and carina separated by two pairs of lateralia, he traces back
through _Calophragmus_ with three pairs of lateralia to _Protobalanus_
of the Devonian with five pairs. Still older is the newly discovered
_Eobalanus_ of the upper Ordovician, which also has five pairs of
lateralia but the middle pair is reversed, so that when the lateralia
of each side are fitted together, they form a pair of shields like
those of _Rhinocaris_, separated by the rostrum and carina, which are
supposed to be homologous with the rostrum and dorsal plate of the
Phyllocarida. Ruedemann suggests that the ancestral phyllocarid
attached itself by the head, dorsal side downward, and the lateralia
were developed from the two valves of the carapace during its upward
migration, to protect the ventral side exposed in the new position.
This theory is very ingenious, but has not been fully published at the
time of writing, and it seems very doubtful if it can be sustained.
_Summary._
The salient points in the preceding discussion should be disentangled
from their setting and put forward in a brief summary.
It is argued that the ancestral arthropod was a short and wide pelagic
animal of few segments, which so far changed its habits as to settle
upon a substratum. As a result of change in feeding habits, appendages
were developed, and, due perhaps to physiological change induced by
changed food, a shell was secreted on the dorsal surface, covering
the whole body. Such a shell need not have been segmented, and, in
fact, the stiffer the shell, the more reason for development of the
appendages. Activity as a swimming and crawling animal tended to break
up the dorsal test into segments corresponding to those of the soft
parts, and, by adaptation, a floating animal became a crawling one,
with consequent change from a form like that of _Naraoia_ to one like
_Pædeumias_. (See figs. 36-40.) A continuation of this line of
development by breaking up and loss of the dorsal test led through
forms similar to _Marrella_ to the Branchiopoda of the Cambrian, in
which not only is there great reduction in the test, but also loss of
appendages. The origin of the carapace is still obscure, but Bernard
(1892, p. 214, fig. 48) has already pointed out that some trilobites,
Acidaspidæ particularly, have backward projecting spines on the
posterior margin of the cephalon, which suggest the possibility of the
production of such a shield, and in _Marrella_ such spines are so
extravagantly developed as almost to confirm the probability of such
origin. In this line of development two pairs of tactile antennæ were
produced, while the anomomeristic character of the trilobite was
retained. From similar opisthoparian ancestors there were, however,
derived primitive Malacostraca retaining biramous antennæ, but with a
carapace and reduced pleural lobes and pygidium. From this offshoot
were probably derived the Ostracoda, the Cirripedia, and the various
orders of the Malacostraca, with the possible exception of the
Isopoda. I have suggested independent origins of the Copepoda and
Isopoda, but realize the weighty arguments which can be adduced
against such an interpretation.
[Illustration: Fig. 36.--_Naraoia compacta_ Walcott. An outline of
the test, after Walcott. Natural size.]
[Illustration: Fig. 37.--_Pagetia clytia_ Walcott. An eodiscid with
compound eyes. After Walcott. × 5.]
[Illustration: Fig. 38.--_Asaphiscus wheeleri_ Meek. A representative
trilobite of the Middle Cambrian of the Pacific province. After Meek.
× 1/2.]
[Illustration: Fig. 39.--_Pædeumias robsonensis_ Burling. Restored
from a photograph published by Burling. × 1/4.]
[Illustration: Fig. 40.--_Robergia_ sp. Restored from fragments found
in the Athens shale (Lower Middle Ordovician), at Saltville, Va.
Natural size.]
It is customary to speak of the Crustacea and Trilobita as having had
a common ancestry, rather than the former being in direct line of
descent from the latter, but when it can be shown that the higher
Crustacea are all derivable from the Trilobita, and that they possess
no characteristics which need have been inherited from any other
source than that group, it seems needless to postulate the evolution
of the same organs along two lines of development.
I can not go into the question of which are more primitive, sessile or
stalked eyes, but considering the various types found among the
trilobites, one can but feel that the stalked eyes are not the most
simple. While no trilobite had movable stalked eyes, it is possible to
homologize free cheeks with such structures. They always bear the
visual surface, and, in certain trilobites (_Cyclopyge_), the entire
cheek is broken up into lenses. Since a free cheek is a separate
entity, it is conceivable that it might lie modified into a movable
organ.
EVOLUTION OF THE MEROSTOMATA.
It has been pointed out above that the Limulava (_Sidneyia_,
_Amiella_, _Emeraldella_) have certain characteristics in common with
the trilobites on the one hand and the Eurypterida on the other. These
relationships have been emphasized by Walcott, who derives the
Eurypterida through the Limulava and the Aglaspina from the Trilobita.
The Limulava may be derived from the Trilobita, but indicate a line
somewhat different from that of the remainder of the Crustacea. In
this line the second cephalic appendages do not become antennæ and
the axial lobe seems to broaden out, so that the pleural lobes become
an integral part of the body. As in the modern Crustacea, the pygidium
is reduced to the anal plate, and this grows out into a spine-like
telson.
From the Limulava to the Eurypterida is a long leap, and before it can
be made without danger, many intermediate steps must be placed in
position. The direct ancestor of the Eurypterida is certainly not to
be seen in the highly specialized _Sidneyia_, and probably not in
_Emeraldella_, but it might be sought in a related form with a few
more segments. The few species now known do suggest the beginning of a
grouping of appendages about the mouth, a suppression of appendages on
the abdomen, and a development of gills on the thorax only. Further
than that the route is uncertain.
Clarke and Ruedemann, whose recent extensive studies give their
opinion much weight, seem fully convinced that the Merostomata could
not have been derived from the Trilobita, but are rather inclined to
agree with Bernard that the arachnids and the crustaceans were derived
independently from similar chætopod annelids (1912, p. 148).
The greater part of their work was, however, finished before 1910, and
although they refer to Walcott's description of the Limulava (1911),
they did not have the advantage of studying the wonderful series of
Crustacea described by him in 1912. While the evidence is far from
clear, it would appear that the discovery of animals with the form of
Limiting and the eurypterids and the appendages of trilobites means
something more than descent from similar ancestors. Biramous limbs of
the type found in the trilobites would probably not be evolved
independently on two lines, even if the ancestral stocks were of the
same blood.
The Aglaspidæ, as represented by _Molaria_ and _Habelia_ in the Middle
Cambrian, are quite obvious closely related to the trilobites easily
derived from them, and retain numerous of their characteristics. That
they are not trilobites is, however, shown by the presence of two
pairs of antennæ, the absence of facial sutures, and the possession of
a spine-like telson.
The Aglaspidæ have always been placed in the Merostomata, and nearer
the Limulidæ than the Eurypterida. The discovery of appendages does
not at all tend to strengthen that view, but indicates rather that
they are true Crustacea which have not given rise to any group now
known. The exterior form is, however, _Limulus_-like, and since it is
known from ontogeny that the ancestor of that genus was an animal with
free body segments, there is still a temptation to try to see in the
Aglaspidæ the progenitors of the limulids.
The oldest known _Limulus_-like animal other than the Aglaspidæ is
_Neolimulus falcatus_ Woodward (Geol. Mag., dec. 1, vol. 5, 1868,
p. 1, pl. 1, fig. 1). The structure of the head of this animal is
typically limuloid, with simple and compound eyes and even the
ophthalmic ridges. Yet, curiously enough, it shows what in a trilobite
would be considered the posterior half of the facial suture, running
from the eye to the genal angle. The body is composed of eight free
segments with the posterior end missing. _Belinurus_, from the
Mississippian and Pennsylvanian, has a sort of pygidium, the posterior
three segments being fused together, and _Prestwichia_ of the
Pennsylvanian has all the segments of the abdomen fused together. So
far as form goes, a very good series of stages can be selected, from
the Aglaspidæ of the Cambrian through _Neolimulus_ to the Belinuridæ
of the late Palæozoic and the Limulidæ of the Mesozoic to recent.
Without much more knowledge of the appendages than is now available,
it would be quite impossible to defend such a line. It is, however,
suggestive.
EVOLUTION OF THE "TRACHEATA."
The trilobites were such abundant and highly variable animals,
adapting themselves to various methods of life in the sea, that it
appears highly probably that some of them may have become adapted to
life on the land. The ancestors of the Chilopoda, Diplopoda, and
Insecta appear to have been air-breathing animals as early as the
Cambrian, or at latest, the Ordovician. Since absolutely nothing is
yet known of the land or even of the fresh-water life of those
periods, nothing can now be proved.
In discussing the relationship of the trilobites to the various
tracheate animals, I have pointed out such palæontologic evidence
as I have been able to gather. Studies in the field of comparative
morphology do not fall within my province. I only hope to have made
the structure of the trilobite a little more accessible to the student
of phylogenies.
SUMMARY ON LINES OF DESCENT.
In order to put into graphic and concise form the suggestions made
above, it is necessary to define and give names to some of the groups
outlined. The hypothetical ancestor need not be included in the
classification and for reasons of convenience may be referred to
merely as the Protostracean.
The group of free-swimming trilobites without thoracic segments was
probably a large one, and within it there were doubtless considerable
variations and numerous adaptations. While the only known animal which
could possibly be referred to this group, _Naraoia_, is blind, it is
entirely possible that other species had eyes, and that the cephala
and pygidia were variously modified. For this reason and because of
the lack of all thoracic segments, it seems better to erect a new
order rather than merely a family for the group, and _Nektaspia_
(swimming shields) may be suggested. The only known family is Naraoidæ
Walcott, which must be redefined.
_Marrella_ and _Habelia_ are types of Crustacea which can neither be
placed in the Trilobita nor in any of the established subclasses of
the Eucrustacea. They represent a transitional group, the members of
which are, so far as known, adapted to the crawling mode of life,
though it may prove that there are also swimmers which can be
classified with them. To this subclass the name _Haplopoda_ may be
applied, the feet being simple.
The two known families, Marrellidæ Walcott and Aglaspidæ Clarke,
belong to different orders, the second having already the name
Aglaspina Walcott. The name _Marrellina_ may therefore be used for the
other.
For _Sidneyia_, Walcott proposed the new subordinal name Limulava,
placing it under the Eurypterida. While _Sidneyia_, _Emeraldella_, and
_Amiella_ may belong to the group that gave rise to the Eurypterida,
they are themselves Crustacea, and a place must be found for them in
that group. The possession of only one pair of antennæ prevents their
reception by the Haplopoda, and allies them to the Trilobita, but the
modifications of the trunk and its appendages keep them out of that
subclass, and a new one has to be erected for them. This may be known
as the _Xenopoda_, in allusion to the strange appendages of
_Sidneyia_.
_Synopsis._
Class Crustacea.
Subclass Trilobita Walch.
Crustacea with one pair of uniramous antennæ, and possessing facial
sutures.
Order Nektaspia nov.
Trilobita without thoracic segments. Cephala and pygidia simple.
Family Naraoidæ Walcott.
Cephalon and pygidium large, both shields nearly smooth. Eyes absent.
A single species: _Naraoia compacta_ Walcott, Middle Cambrian, British
Columbia.
Subclass Haplopoda nov.
Crustacea with trilobate form, two pairs of uniramous antennæ, no
facial sutures, sessile compound eyes present or absent, pygidium and
pleural lobes generally reduced, large labrum present, appendages of
the trunk biramous.
Order Marrellina nov.
Form trilobite-like, pleural lobes reduced, endobases absent from
coxopodites of body, pygidium a small plate.
Family Marrellidæ Walcott.
Cephalon with long genal and nuchal spines. Eyes marginal. A single
species: _Marrella splendens_ Walcott, Middle Cambrian, British
Columbia.
Order Aglaspina Walcott.
Body trilobite-like, with few thoracic segments, and a spine-like
telson. Appendages biramous.
Family Aglaspidæ Clarke.
Cephalon trilobate, with or without compound eyes, seven or eight
segments in the thorax.
Genus _Aglaspis_ Hall.
Compound eyes present, seven segments in thorax. Upper Cambrian,
Wisconsin.
Genus _Molaria_ Walcott.
Compound eyes absent, eight segments in thorax. Middle Cambrian,
British Columbia.
Genus _Habelia_ Walcott.
Compound eyes absent. Not yet fully described. Middle Cambrian,
British Columbia.
Subclass Xenopoda nov.
Crustacea with more or less eurypterid-like form, one pair of
uniramous antennæ, biramous appendages on anterior part of trunk,
modified endopodites on cephalon.
Order Limulava Walcott.
Cephalon with lateral or marginal eyes and large epistoma. Body with
eleven free segments and a telson. Cephalic appendages grouped about
the mouth.
Family Sidneyidæ Walcott.
Trunk probably with exopodites only, and without appendages on the
last two segments. Telson with a pair of lateral swimmerets.
Genus _Sidneyia_ Walcott.
Third cephalic appendage a large compound claw. Gnathobases forming
strong jaws. Middle Cambrian, British Columbia.
Genus _Amiella_ Walcott.
Middle Cambrian, British Columbia.
Family Emeraldellidæ nov.
Trunk with biramous appendages in anterior part, and appendages on all
segments except possibly the spine-like telson.
Genus _Emeraldella_ Walcott.
Cephalic appendages simple spiniferous endopodites. Eyes unknown.
Middle Cambrian, British Columbia.
[Illustration: Fig. 41.--A diagram showing possible lines of descent
of the other Arthropoda from the Trilobita. The three recognized
orders of the latter are shown separately. The known geological range
is indicated in solid black, the hypothetical range and connections
stippled. The short branch beside the Opisthoparia represents the
range of the Haplopoda. The term Arachnida is used for all arachnids
other than Merostomata, merely as a convenient inclusive name for the
groups not especially studied.]
Final Summary.
It is generally believed that the Arthropoda constitute a natural,
monophyletic group. The data assembled in the preceding pages indicate
that the other Arthropoda were derived directly or indirectly from the
Trilobita because:
(1) the trilobites are the oldest known arthropods;
(2) the trilobites of all formations show great variation in the
number of trunk segments, but with a tendency for the number to become
fixed in each genus;
(3) the trilobites have a constant number of segments in the head;
(4) the position of the mouth is variable, so that either the
Crustacea or the Arachnida could be derived from the trilobites;
(5) the trilobite type of appendage is found, in vestigial form at
least, throughout the Arthropoda;
(6) the appendages of all other Arthropoda are of forms which could
have been derived from those of trilobites;
(7) the appendages of trilobites are the simplest known among the
Arthropoda;
(8) the trilobites show practically all known kinds of sessile
arthropodan eyes, simple, compound, and aggregate;
(9) the apparent specializations of trilobites, large pleural lobes
and pygidia, are primitive, and both suffer reduction within the
group.
The ancestor of the trilobite is believed to have been a soft-bodied,
free-swimming, flat, blind or nearly blind animal of few segments,
because:
(a) the form of both adult and embryo is of a type more adapted for
floating than crawling;
(b) the large pygidium is shown by ontogeny to be primitive, and the
elongate worm-like form secondary;
(c) the history of the trilobites shows a considerable increase in the
average number of segments in successive periods from the Cambrian to
the Permian;
(d) the simplest trilobites are nearly or quite blind.