Isopoda have a definite number of segments. There is less variation in the number of segments among the later than the earlier trilobites.

Isopoda have no facial suture. In at least three genera of trilobites the cheeks become fused to the cranidium and the sutures obliterated.

Isopoda have one or two segments of the thorax annexed to the head.

While this is not known to occur in trilobites, it is possible that it did.

Most Isopoda have a fairly stiff ventral test. The ventral membrane of trilobites would probably have become stiffened by impregnation of lime if the habit of enrollment had been given up.

In Isopoda the antennae are practically uniramous sensory organs. The second cephalic appendages of trilobites are capable of such development through reduction of the exopodite.

In the Isopoda the c.o.xopodites are usually fused with the body, remaining as free, movably articulated segments only in a part of the thoracic legs of one suborder, the Asellota. Endobases are entirely absent. This is of course entirely unlike the condition in Trilobita, but a probable modification.

In Isopoda there is a distinct grouping of the appendages, with specialization of function. The trilobites show a beginning of tagmata, and such development would be expected if evolution were progressive.

In both groups, development from the embryo is direct. Rudiments of exopodites of thoracic legs have been seen in the young of one genus.

The oldest known isopod is _Oxyuropoda ligioides_ Carpenter and Swain (Proc. Royal Irish Acad., vol. 27, sect. B, 1908, p. 63, fig. 1), found in the Upper Devonian of County Kilkenny, Ireland. The appendages are not known, but the test is in some ways like that of a trilobite. The thorax, abdomen, and pygidium are especially like those of certain trilobites, and there is no greater differentiation between thorax and abdomen than there is between the regions before and behind the fifteenth segment of a _Paedeumias_ or _Mesonacis_. The a.n.a.l segment is directly comparable to the pygidium of a _Ceraurus_, the stiff unsegmented uropods being like the great lateral spines of that genus.

The interpretation of the head offered by Carpenter and Swain is very difficult to understand, as their description and figure do not seem to agree. What they consider the first thoracic segment (fused with the head) seems to me to be the posterior part of the cephalon and it shows at the back a narrow transverse area which is at least a.n.a.logous to the nuchal segment of the trilobite. If this interpretation can be sustained, _Oxyuropoda_ would be a very primitive isopod in which the first thoracic segment (second of Carpenter and Swain) is still free.

According to the interpretation of the original authors, the species is more specialized than recent Isopoda, as they claim that two thoracic segments are fused in the head. The second interpretation was perhaps made on the basis of the number of segments (nineteen) in a recent isopod.

=Marrella splendens= WALCOTT.

Ill.u.s.trated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 192, pls. 25, 26.

Among the most wonderful of the specimens described by Doctor Walcott is the "lace crab." While the systematic position was not satisfactorily determined by the describer, it has been aptly compared to a trilobite. The great nuchal and genal spines and the large marginal sessile eyes, coupled with the almost total lack of thoracic and abdominal test, give it a bizarre appearance which may obscure its real relationships.

The cephalon appears to bear five pairs of appendages, antennules, and antennae, both tactile organs with numerous short segments, mandibles, and first and second maxillae. The last three pairs are elongate, very spinose limbs, of peculiar appearance. They seem to have seven segments, but are not well preserved. These organs are attached near the posterior end of the labrum.

There are twenty-four pairs of biramous thoracic appendages, which lack endobases. The endopodites are long and slender, with numerous spines; the exopodites have narrow, thin shafts, with long, forward pointed setae. The a.n.a.l segment consists of a single plate.

Further information about this fossil will be eagerly awaited. None of the ill.u.s.trations so far published shows biramous appendages on the cephalon. This, coupled with the presence of tactile antennae, makes its reference to the Trilobita impossible, but the present interpretation indicates that it was closely allied to them.

[Ill.u.s.tration: Fig. 32. _Marrella splendens_ Walcott. Restoration of the ventral surface, based upon the photographs and descriptions published by Walcott. Although all the limbs of the trunk appear to be biramous, only endopodites are placed on one side and exopodites on the other, for the sake of greater clearness in the ill.u.s.tration.

Drawn by Doctor Elvira Wood, under the supervision of the writer.

about 6.]

_Restoration of Marrella._

(Text fig. 32.)

The accompanying restoration of the ventral surface of _Marrella_ is a tentative one, based on Doctor Walcott's description and figures. The outline is taken from his plate 26, figure 1; the appendages of the head from plate 26, figures 1-3, 5, and plate 25, figures 2, 3; the endopodites, shown on the left side only, from figures 3 and 6, plate 25. I have not studied actual specimens, and the original description is very incomplete. The restoration is therefore subject to revision as the species becomes better known.

Arachnida.

No attempt will be made to pa.s.s in review all of the subcla.s.ses of the arachnids. Some of the Merostomata are so obviously trilobite-like that it would seem that their relationship could easily be proved. The task has not yet been satisfactorily accomplished, however, and new information seems only to add to the difficulties.

So far as I know, the Araneae have not previously been compared directly with trilobites, although such treatment consists merely in calling attention to their crustacean affinities, as has often been done.

Carpenter's excellent summary (1903, p. 347) of the relationship of the Arachnida to the trilobites may well be quoted at this point:

The discussion in a former section of this essay on the relationship between the various orders of Arachnida led to the conclusion that the primitive arachnids were aquatic animals, breathing by means of appendicular gills. Naturally, therefore, we compare the arachnids with the Crustacea rather than with the Insecta. The immediate progenitors of the Arachnida appear to have possessed a head with four pairs of limbs, a thorax with three segments, and an abdomen with thirteen segments and' a telson, only six of which can be clearly shown by comparative morphology to have carried appendicular gills. But embryological evidence enables us to postulate with confidence still more remote ancestors in which the head carried well developed compound eyes and five pairs of appendages, while it may be supposed that all the abdominal segments, except the a.n.a.l, bore limbs. In these very ancient arthropods, all the limbs, except the feelers, had ambulatory and branchial branches; and one important feature in the evolution of the Arachnida must have been the division of labour between the anterior and posterior limbs, the former becoming specialized for locomotion, the latter for breathing. Another was the loss of feelers and the degeneration of the compound eyes. Thus we are led to trace the Arachnida (including the Merostomata and Xiphosura) back to ancestors which can not be regarded as arachnids, but which were identical with the primitive trilobites, and near the ancestral stock of the whole crustacean cla.s.s.

TRILOBITES NOT ARACHNIDA.

While no one having any real knowledge of the Trilobita has adopted Lankester's scheme of the inclusion of the group as the primitive grade in the Arachnida, reference to it may not be amiss. This theory is best set forth in the Encyclopaedia Britannica, Eleventh Edition, under the article on Arachnida. It is there pointed out that the primitive arachnid, like the primitive crustacean, should be an animal without a fixed number of somites, and without definitely grouped tagmata. As Lankester words it, they should be anomomeristic and anomotagmatic. The trilobites are such animals, and he considers them Arachnida and not Crustacea for the following reasons:

Firstly and chiefly, because they have only one pair (apart from the eyes) of pre-oral appendages. "This fact renders their a.s.sociation with the Crustacea impossible, if cla.s.sification is to be the expression of genetic affinity inferred from structural coincidence."

Secondly, the lateral eyes resemble no known eyes so closely as the lateral eyes of _Limulus_.

Thirdly, the trilobation of the head and body, due to the expansion and flattening of the sides or pleura, is like that of _Limulus_, but "no crustacean exhibits this trilobite form."

Fourthly, there is a tendency to form a pygidial or telsonic shield, "a fusion of the posterior somites of the body, which is precisely identical in character with the metasomatic carapace of _Limulus_." No crustacean shows metasomatic fusion of segments.

Fifthly, a large post-a.n.a.l spine is developed "in some trilobites" (he refers to a figure of _Dalmanites_).

Sixthly, there are frequently lateral spines on the pleura as in _Limulus_. No crustacean has lateral pleural spines.

These points may be taken up in order.

1. If trilobites have one appendage-bearing segment in front of the mouth, they are Arachnida; if two, Crustacea. This is based on the idea that in the course of evolution of the Arthropoda, the mouth has shifted backward from a terminal position, and that as a pair of appendages is pa.s.sed, they lose their function as mouth-parts and eventually become simple tactile organs. Thus arise the chelicerae of most arachnids, and the two pairs of tactile antennae of most Crustacea. This theory is excellent, and the rule holds well for modern forms, but as shown by the varying length of the hypostoma in different trilobites, the position of the mouth had not become fixed in that group. In some trilobites, like _Triarthrus_, the gnathobases of the second pair of appendages still function, but in all, so far as known, the mouth was back of the points of attachment of at least two pairs of appendages, and in some at least, back of the points of attachment of four pairs. As pointed out in the case of _Calymene_ and _Ceraurus_, the trilobites show a tendency toward the degeneration of the first and second pairs of biramous appendages, particularly of the gnathobases. They are in just that stage of the backward movement of the mouth when the function of the antennae as mandibles has not yet been lost. If the presence of functional gnathobases back of the mouth, rather than the points of attachment in front of the mouth, is to be the guide, then Triarthrus might be cla.s.sed as an arachnid and _Calymene_ and _Isotelus_ as crustaceans. In other words, the rule breaks down in this primitive group.

2. Superficially, the eyes of some trilobites do look like those of _Limulus_, but how close the similarity really was it is impossible to say. The schizochroal eyes were certainly very different, and Watase and Exner both found the structure of the eye of the trilobite unlike that of _Limulus_.

3. The importance of the trilobate form of the trilobite is very much overestimated. It and the pygidium are due solely to functional requirements. The axial lobe contained practically all the vital organs and the side lobes were mechanical in origin and secondarily protective. That the crustacean is not trilobate is frequently a.s.serted by zoologists, yet every text-book contains a picture of a segment of a lobster with its axial and pleural lobes. It is a fundamental structure among the Crustacea, obscured because most of them are compressed rather than depressed.

4. The pygidium of trilobites is compared with the metasomatic shield of _Limulus_. No h.o.m.ology, if h.o.m.ology is intended, could be more erroneous. The metasomatic shield of _Limulus_ is, as shown by ontogeny and phylogeny, formed by the fusion of segments formerly free, and includes the segments between the cephalic and a.n.a.l shields, or what would be known as the thorax of a trilobite. No trilobite has a metasomatic shield. The pygidium of a trilobite, as shown by ontogeny, is built up by growth in front of the a.n.a.l region, and since the segments were never free, it can not strictly be said to be composed of fused segments. Some Crustacea do form a pygidial shield, as in certain orders of the Isopoda.

5. The post-a.n.a.l spine of Dalmanites and some other trilobites is similar to that of _Limulus_, but this seems a point of no especial significance. That a similar spine has not been developed in the Crustacea is probably due to the fact that they do not have the broad depressed shape which makes it so difficult for a _Limulus_ to right itself when once turned on its back. Relatively few trilobites have it, and it is probably correlated with some special adaptation.

6. There is nothing among the trilobites comparable to the movable lateral spines of the metasoma of _Limulus_.

While, as cla.s.sifications are made up, the Trilobita must be placed in the Crustacea rather than the Arachnida, there is no reason why both the modern Crustacea and the Arachnida should not be derived from the trilobites.

MEROSTOMATA.

It has been a custom of long standing to compare the trilobite with _Limulus_. Packard (1872) gave great vitality to the theory of the close affinity of the two when he described the so-called trilobite-stage in the development of _Limulus polyphemus_. His influence on Walcott's ideas (1881) is obvious. Lankester has gone still further, and a.s.sociated the Trilobita with the Merostomata in the Arachnida.