The Appendages, Anatomy, and Relationships of Trilobites Part 21

The oldest chilopods are species described by Scudder (Mem. Boston Soc. Nat. Hist., vol. 4, 1890, p. 417, pl. 38) from the Pennsylvania!!

at Mazon Creek, Grundy County, Illinois. Only one of these, _Latzelia primordialis_ Scudder (pl. 38 fig. 3), is at all well preserved. This little animal, less than an inch long, had a depressed body, with a median carina, exceedingly long slender legs, and about nineteen segments. The head is very nearly obliterated.

Diplopoda.

The diplopods, especially the polydesmids with their lateral outgrowths, often have a general appearance somewhat like that of a trilobite, but on closer examination few likenesses are seen. The most striking single feature of the group, the possession by each segment of two pairs of appendages, is not in any way foreshadowed in the trilobites, none of which shows any tendency toward a fusion of pairs of adjacent segments. The antennules are short, antennae absent, mandibles and maxillulae much modified, the latter possibly biramous, and the maxillae absent. The trunk appendages are very similar to those of chilopods, and could readily be derived from the endopodites of trilobites.

The oldest diplopods are found in the Silurian (Ludlow) and Devonian (Lower Old Red) of Scotland, and three species belonging to two genera are known. The oldest is _Archidesmus loganensis_ Peach (1889, p. 123, pl. 4, fig. 4), and the Devonian species are _Archidesmus macnicoli_ Peach and _Kampecaris forfarensis_ Page (Peach 1882, p. 182, pl. 2, fig. 2, 2a, and p. 179, pl. 2, figs. 1-1g). All of these species show lateral expansions like the recent Polydesmidae, and these of course suggest the pleural lobes of trilobites. All three of the species are simpler than any modern diplopod, for there is only a single pair of appendages on each segment. No _foramina repugnatoria_ were observed, and the eyes of _Kampecaris forfarensis_ as described are singularly like those of a phacopid.

Peach says: "The eye itself is made up of numerous facets which are arranged in oblique rows, the posterior end of each row being inclined downwards and outwards, the facets being so numerous and so close together that the eye simulates a compound one." There is also a protecting ridge which somewhat resembles a palpebral lobe (1882, pl.

7, fig. la). Peach comments on the strength of the test, and from his description it appears that it must have been preserved in the same manner as the test of trilobites. It was punctate, and granules and spines were also present. The presence of the lateral outgrowths in these ancient specimens would seem to indicate that they are primitive features, and may have been inherited. While possibly not h.o.m.ologous with the pleural extensions of trilobites, they may be vestiges of these structures.

The limbs are made up of seven segments which are circular in section and expand at the distal end. The distal one bears one or two minute spines. They are most readily compared with the endopodites of _Isotelus_. The resemblance is, in fact, rather close. The sternal plates are wider and the limbs of opposite sides further apart than in modern diplopods. Except for one pair of antennae, no cephalic appendages are preserved.

While these specimens do not serve to connect the Diplopoda with the Trilobita, they do show that most of the specializations of the former originated since Lower Devonian times, and lead one to suspect that the derivation from marine ancestors took place very early, perhaps in the Cambrian. If no very close connection with the trilobites is indicated, there is also nothing to show that the diplopods could not have been derived from that group.

Primitive Characteristics of Trilobites.

TRILOBITES THE MOST PRIMITIVE ARTHROPODS.

The Arthropoda, to make the simplest possible definition, are invertebrate animals with segmented body and appendages. The most primitive arthropod would appear to be one composed of exactly similar segments bearing exactly similar appendages, the segments of the appendages themselves all similar to one another. It is highly improbable that this most primitive arthropod imaginable will ever be found, but after a survey of the whole phylum, it appears that the simpler trilobites approximate it most closely.

That the trilobites are primitive is evidenced by the facts that they have been placed at the bottom of the Crustacea by all authors and claimed as the ancestors of that group by some; that Lankester derived the Arachnida from them; and that Handlirsch has considered them the progenitors of the whole arthropodan phylum.

Specializations among the Arthropoda, even among the free-living forms, are so numerous that it would be difficult to make a complete list of them. In discussing the princ.i.p.al groups, I have tried to show that the essential structures can be explained as inherited from the Trilobita, changed in form by explainable modifications, and that new structures, not' present in the Trilobita, are of such a nature that they might be acquired independently in even unrelated groups.

The chief objections to the derivation of the remainder of the Crustacea from the trilobites have been: first, that the trilobites had broad pleural extensions; second, that they had a large pygidium; and lastly, that they had only one pair of tactile antennae.

It has now been pointed out that many modern Crustacea have pleural extensions, but that they usually bend down at the sides of the body, and also that in the trilobites and more especially in _Marrella_, there was a tendency toward the degeneration of the pleural lobes. A glance at the Mesonacidae or Paradoxidae should be convincing proof that in some trilobites the pygidium is reduced to a very small plate.

In regard to the second antennae standard text-books contain statements which are actually surprising. A compilation shows that the antennae are entirely uniramous in but a very few suborders, chiefly among the Malacostraca; that they are biramous with both exopodite and endopodite well developed in most Copepoda, Ostracoda, and Branchiopoda; and that the exopodite, although reduced in size, still has a function in some suborders of the Malacostraca. The Crustacea could not possibly be derived from an ancestor with two pairs of uniramous antennae.

Although I have defended the trilobites, perhaps with some warmth, from the imputation that they were Arachnida, my argument does not apply in the opposite direction, and I believe Lankester was right in deriving the Arachnida from them. If the number of appendages in front of the mouth is fundamental, then the trilobites were generalized, primitive, and capable of giving rise to both' Crustacea and Arachnida. As shown on a previous page (p. 119), the "connecting links" so far found tend to disprove rather than to prove the thesis, but the present finds should be looked upon as only the harbingers of the greater ones which are sure to come.

LIMBS OF TRILOBITES PRIMITIVE.

The general presence, in an adult or larva, of some sort of biramous limbs throughout the whole cla.s.s Crustacea has led most zoologists to expect such a limb in the most primitive crustaceans, and apparently the appendage of the trilobite satisfies the expectation. It is well, perhaps, as a test, to consider whether by modification this limb could produce the various types of limbs seen in other members of the cla.s.s. In the first place, it is necessary to have clearly in mind the peculiarities of the appendage to be discussed.

It should first of all be remembered that the limb is articulated with the dorsal skeleton in a manner which is very peculiar for a crustacean. The c.o.xopodite swings on a sort of ball-and-socket joint, and at the outer end both the exopodite and the basipodite articulate with it. Since the exopodite articulates with the basipodite as well as with the c.o.xopodite, the two branches are closely connected with one another and there is little individual freedom of movement. This is, of course, a necessary consequence of their articulation with a segment which is itself too freely movable to provide a solid base for attachment of muscles. The relation of the appendifer, c.o.xopodite, and two rami is here shown diagrammatically (fig. 33), the exopodite branching off from the proximal end of the basipodite at the junction with the c.o.xopodite.

In all trilobites the endopodite consists of six segments, and the c.o.xopodite of a single segment the inner end of which is prolonged as an endobase. There does not seem to be any variation from this plan in the subcla.s.s, although individual segments are variously modified. The exopodites are more variable, but all consist of a flattened shaft with setae on one margin. No other organs such as accessory gills, swimming plates, or brood pouches have yet been found attached to the appendages, the evidence for the existence of the various epipodites and exites described by Walcott being unsatisfactory (see p. 23).

[Ill.u.s.tration: Fig. 33.--Diagrammatic representation of an appendage of the anterior end of the thorax of _Triarthrus becki_ Green, to show relation of exopodite and endopodite to each other and to the c.o.xopodite. Much enlarged.]

In the Ostracoda the appendages are highly variable, but it is easily seen that they are modifications of a limb which is fundamentally biramous. In most species, both exopodite and endopodite suffer reduction. The exopodite springs from the basipodite and that segment is closely joined to the c.o.xopodite, producing a protopodite. In some cases the original segments of the endopodites fuse to form a stiff rod. While highly diversified, these appendages are very trilobite-like, and some Ostracoda even have biramous antennae.

The non-parasitic Copepoda have limbs exceedingly like those of trilobites. Many of them are biramous, the endopodites sometimes retaining the primitive six segments. c.o.xopodite and basipodite are generally united, and endopodite and exopodite variously modified.

Like some of the Ostracoda, the more primitive Copepoda have biramous antennae.

As would be expected, the appendages of the Cirripedia are much modified, although those of the nauplius are typical. The thoracic appendages of many are biramous, but both branches are multisegmented.

In the modern Malacostraca the ground plan of the appendages is biramous, but in most orders they are much modified. In many, however, the appendages of some part of the body are biramous, and in many the endopodites show the typical six segments. From the c.o.xopodites arise epipodites, some of which a.s.sist in swimming, and some in respiration.

Because of the many instances in which such extra growths arise, and because of the form of the appendages of the Branchiopoda, it has been suggested that the primitive crustacean leg must have been more complex than that of the trilobite. In looking over the Malacostraca, however, one is struck by the fact that epipodites generally arise where the exopodites have become aborted or are poorly developed, and seem largely to replace them. The c.o.xopodite and basipodite are usually fused to form a protopodite, and a third segment is sometimes present in the proximal part of the appendage.

In the Branchiopoda are found the most complex crustacean limbs, and the ones most difficult to h.o.m.ologize with those of trilobites. In recent years, Lankester's h.o.m.ologies of the parts of the limbs of _Apus_ with those of the Malacostraca have been quite generally accepted, and the appendages of the former considered primitive.

Now that it is known that the Branchiopoda of the Middle Cambrian (_Burgessia_ _et at._) had simple trilobite-like appendages, it becomes necessary to exactly reverse the opinion in this matter. The same h.o.m.ologies stand, but the thoracic limbs of _Apus_ must be looked upon as highly specialized instead of primitive.

[Ill.u.s.tration: Fig. 34.--One of the appendages of the anterior part of the trunk of _Apus_, showing the endites (beneath) and exites (above).

The proximal endite forms a gnathobase which is not h.o.m.ologous with the gnathobase (or endobase) of the trilobite. Copied from Lankester.

Much enlarged.]

Lankester (Jour. Micros. Sci., vol. 21, 1881) pointed out that the axial part of the thoracic limb of _Apus_ (fig. 34) is h.o.m.ologous with the protopodite in the higher Crustacea, that the two terminal endites corresponded to the exopodite and endopodite, and that the other endites and exites were outgrowths from the protopodite a.n.a.logous to the epipodites of Malacostraca. There seems to be no objection to retaining this interpretation, but with the meaning that both endopodite and exopodite are much reduced, and their functions transferred to numerous outgrowths of the protopodite. One of the endites grows inward to form an endobase, the whole limb showing an attempt to return to the ancestral condition of the trilobite. The limbs of some other branchiopods are not so easy to understand, but students of the Crustacea seem to have worked out a fairly satisfactory comparison between them and _Apus_.

The discovery that the ancestral Branchiopoda had simple biramous appendages instead of the rather complex phyllopodan type is another case in which the theory of "recapitulation" has proved to hold. It had already been observed that in ontogeny the biramous limb preceded the phyllopodan, but so strong has been the belief in the primitive character of the Apodidae that the obvious suggestion has been ignored.

Even in such highly specialized Malacostraca as the hermit crabs the development of certain of the limbs ill.u.s.trates the change from the schizopodal to the phyllopodan type, and Thompson (Proc. Boston Soc.

Nat. Hist., vol. 31, 1903, pl. 5, fig. 12) has published an especially good series of drawings showing the first maxilliped. In the first to fourth zoeae the limb is biramous but in the glaucothoe a pair of broad processes grow out from the protopodite, while the exopodite and particularly the endopodite become greatly reduced. In the adult the endopodite is a mere vestige, while the flat outgrowths from the protopodite have become very large and bear setae.

_Summary._

The limbs of most Crustacea are readily explained as modifications of 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 c.o.xopodite, basipodite, or both. A protopodite is generally formed by the fusion of c.o.xopodite 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 (prec.o.xal), 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 h.o.m.ology, 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 Palaeozoic. 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 Mesonacidae (_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 _Paedeumias_ (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. _Paedeumias_ 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 Mesonacidae, except one, _Olenelloides_, have far more thoracic and fewer pygidial segments than the average, while the reverse is true of the Eodiscidae, _Vanuxemella_, _Corynexochus_, _Bathyuriscus_, and Poliella.