Trilobita

“ The cephalic appendages are specialized, and the antennules double as in most of the Malacostraca. External epipodites are very numerous on the anterior limbs.
This group extends back as far as the Pennsylvanian and had then probably already become adapted to fresh-water life. It may be significant that the Palæozoic syncarids appear to have lacked epipodites. While differing very considerably from the Trilobita, the Syncarida could have been derived from them.
3. Isopoda. ”

“ 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. ”

“ Tri"lo*bite (tr"l*bt) , n. [Cf. F. trilobite. See Trilobate.] (Paleon.) Any one of numerous species of extinct arthropods belonging to the order Trilobita. Trilobites were very common in the Silurian and Devonian periods, but became extinct at the close of the Paleozoic. So named from the three lobes usually seen on each segment.
Tri`lo*bit"ic (?) , a. Of, pertaining to or containing, trilobites; as, trilobitic rocks.
Tri*loc"u*lar (?) , a. [Pref. tri- + locular: cf. F. triloculaire.] Having three cells or cavities; as, a trilocular capsule; a trilocular heart. ”

“ The genera Belinurus, Aglaspis, Prestwichia, Hemiaspis and Bunodes consist of small forms which occur in Palaeozoic rocks. In none of them are the appendages known, but in the form of the two carapaces and the presence of free somites they are distinctly intermediate between Limulus and the Trilobitae. The young form of Limulus itself (fig. 40) is also similar to a Trilobite so far as its segmentation and trilobation are concerned. The lateral eyes of Limulus appear to be identical in structure and position with those of certain Trilobitae. ”

“ Students of trilobites have always wanted specimens to be delivered to them weathered out of the rock by nature and revealing the ventral anatomy without further work than the collecting, but the wish has never been fulfilled. In the Utica black shales, near Rome, New York, there was finally discovered in 1892 a layer less than ten millimeters thick, bearing hundreds of Triarthrus becki with most of the ventral anatomy intact. ”

“ 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. ”

“ Different genera and species of the trilobite are now found in almost every part of the globe, and are frequently exceedingly abundant in the rocks which contain them. That they must have swarmed in particular places, is abundantly evident from a number of localities in our own country,—millions, for example, must have lived and died not far from Trenton falls, in the State of New York. ”

“ 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-anal spine is developed "in some trilobites" (he refers to a figure of Dalmanites) . ”

“ The Trilobite called Bumastus by Sir R. Murchison (from its grape-like form) presents a very curious modification of the normal type. Both the head and caudal extremity are rounded, with no distinct longitudinal furrows; and the whole surface of the carapace is covered by extremely thin, apparently imbricated, lamellæ, the edges of which are undulated, and the intermediate spaces studded with minute dots. The eyes are smooth, and not granulose, as in Calymene. This genus is known in England by the name of the Barr Trilobite, from its occurrence in the limestone near Barr, in Staffordshire ”

“ The pygidium of trilobites is compared with the metasomatic shield of Limulus. No homology, if homology 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 anal shields, or what would be known as the thorax of a trilobite. ”

“ A high-speed, erratic motion which could be produced by the sudden flap of a pygidium would be of more service in escape than any amount of steady swiftness produced by the oar-like appendages of an animal of the shape of a trilobite. ”

“ He was twenty inches in length and carried himself with conscious dignity, albeit mud knocks the dignity out of almost anything but a trilobite. But this trilobite, for such he was, surveyed that Cambrian noon pensively, curled his whiskers with thought, and wriggled his shining joints in the sun. Presently a hen trilobite appeared and squatted beside him placidly.
“When I survey this spectacle,” said the trilobite, “when I reflect that the world is empty but for us, I am often tempted to wonder.” ”

“ The same inference follows, but with cumulative force, when we see nearly four hundred microscopic lenses set side by side, in the compound eye of a fossil Trilobite; and the weight of the argument is multiplied a thousand fold, when we look to the infinite variety of adaptations by which similar instruments have been modified, through endless genera and species, from the long-lost Trilobites, of the Transition strata, through the extinct Crustaceans of the Secondary and Tertiary formations, and thence onward throughout existing Crustaceans, and the countless hosts of living Insects. ”

“ The complete structure and zoological relationship of the trilobites has always been open to some doubt. As regards the former, within recent years exceptionally well-preserved specimens from the Utica Slates and the Cincinnati Limestone of Ohio, rocks of Ordovician age, have been discovered and dissected, whereby our knowledge of the organisation of this group is greatly advanced. These remarkable fossil remains show that the Trilobites bore on their under surface a number of appendages, one pair to each segment, except that of the anal. The front pair is whip-like and served as antennae ”

“ That the family to which her little fossil trilobite belonged the particular family for they were generally related, he said, to the lobster and crab, were found in the very oldest and deepest down rocks in which any sort of remains of living things have been found; therefore, it is likely they were among the earliest of earth's inhabitants. There were a great many of them, the doctor said, and many different species; for great numbers of them are found to this day in those particular rocks. The rocks must have been made at the time when the trilobites lived, and have somehow shut them in. ”

“ About this same time the Copepoda may have split off [142] 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. ”

“ The head is composed of five segments (if the first thoracic segment is really the fused first and second) , is often flattened, and lacks compound eyes. Pleural lobes are well developed, but instead of being flattened as in the trilobite, they are turned down at the sides or even incurved. A labrum is present.
The antennules, antennæ, and mandibles are quite like those of trilobites. The antennules are very long and made up of numerous segments. The antennæ are biramous, the junction between the coxopodite and basipodite is well marked, and the endopodite consists of only two segments. ”

“ I call this so because we see in the development of Sao, as represented by Barrande, most of the different' phases in evolution, which the ridge bearing trilobites of this second group have experienced during the Cambrian times.
I.
The body consists only of the head, neither pygidium, nor thorax yet developed.
1. Psilocephali (ψιλος, bald) , primeval, precambrian, adult stage not found, supposed to be like the head shield of Agnostus glandiformis and A. biplicatus quite round and bare without any glabella nor sutures. The corresponding stage in development also wanted. ”

“ An additional point to be brought forward to show that the trilobites probably pass the greater part of their life on their back, and died in that attitude, is that the young Limulus generally feed while turned on their back; moving at an angle with the bottom, the hind extremity raised, they throw out their feet beyond the anterior edge of the carapace, browsing, as it were, upon what they find in their road, and washing away what they do not want by means of a powerful current produced by their abdominal appendages. ”

“ This might be a very plausible explanation in the case of the bivalve-like Agnostidæ, and it is one I had suggested tentatively for that family before I read Staff and Reck's paper. In the case of the large trilobites with more segments, however, it would be more natural to think of a mode of progression in which there was an undulatory movement of the body and the pygidium, up-and-down strokes being produced by alternately contracting the dorsal and ventral muscles. Bending the pygidium down would tend to pull the animal backward, while bringing it back into position would push it forward. ”

“ When the free cheek is dissevered it shows no trace of the ridge, there is only a large scallop on the spot where it embraced the ridge. In the Paradoxidæ the rim of the indenture and the ridge are in so close contact that there is not the least place for an eye between them, as can be seen in the few specimens with a complete head. In all oculate trilobites again without any exception the facial suture separates that part of the eye which is the real visual organ with corneal facets, from the interior often elevated portion, opposite it, the so called palpebral lobe. ”

“ 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 [139] the base of his classification, and there is now less chance than ever that they can be called degenerate animals. ”

“ Well, you’re a good husband and father; what better could any trilobite be? And as to beauty, I shouldn’t have married you if you had not been about the handsomest gentleman trilobite that ever sat and curled his whiskers. Nature never made anything better than a trilobite. Why? Because she can’t. Can you picture anything different? Can you imagine any creature with more convenient limbs, more exquisite joints, more perfect claws, better eyesight, better senses, better manners, or more self-respecting? ”

“ Trilobites form an important part of all the faunas of the Cambrian system; they attain their greatest development in the Ordovician period, after which they become less numerous; their decline is very marked in the Devonian, in which nearly all the genera are but survivals from the Silurian period; in the Carboniferous, evidence of approaching extinction is seen in the small number of genera represented, all of which belong to one family—the Proëtidae, in the relatively few species in each genus and in the small size of the individuals of those species. ”

“ The eyes are marginal and pedunculate. The antennules are imperfectly known, but apparently short, while the antennas are long and slender, with relatively few, long, segments. The mandibles appear to be like endopodites of trilobites and show at least six segments. As so often happens in these specimens from British Columbia, [109] the preservation of the other appendages is unsatisfactory. As illustrated (Walcott, 1912 A, pl. 27, fig. 5) , both endopodites and exopodites appear to be present, and the shaft of the exopodite seems to be segmented as in Triarthrus. ”

“ There are no living trilobites, but the creatures now nearest them are the isopods. The trilobite was more like an isopod in its larva state than like a full-grown one. The trilobite had a hard plate or shell over his head and the front part of his body. This piece of armor was shaped nearly like the large front portion of the shell of a horseshoe [Pg 119] or king-crab. The king-crab uses this shell as a mud-plough to plough its way through the slime, where it finds its food and deposits its eggs. ”

“ This fine, large caudal termination of an Asaph is in the cabinet of the Albany Institute—and it is a subject of great regret, that all that has yet been discovered relating to this highly interesting trilobite, is to be seen in this fragment.
There are eighteen articulations of the tail and abdomen, which cannot be distinguished from each other. The middle lobe is composed of a series of straight, distinct, parallel articulations, very convex about the middle, so as to form a kind of longitudinal ridge down the back. ”