Trilobite

“ 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. ”

“ 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) . ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ In the meantime the creature had reached the center of the room and stopped. A metamorphosis was now taking place before my eyes. To my surprise, I beheld, in place of a magnified amoeba, a gigantic trilobite, larger, it is true, than any specimen which has ever been found, but, nevertheless, true to form in every detail.
The trilobite, in turn, changed to a brilliantly-hued star-fish with active wriggling tentacles. The star-fish became a crab, and the crab, a porpoise swimming about in the air as if it had been water. ”

“ 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. ”

“ For we see that one of the striking characteristics of such forms as Bunodes, Hemiaspis, etc., is the presence of segmented pleural flaps on each side of the main part of the body; and if we pass further back to the great group of trilobites, we find in the most manifold form, and in various degrees of extent, the most markedly segmented pleural folds. In fact, the hypothetical figure (Fig. 160, A) which I have deduced from the embryological evidence, might very well represent a cross-section of a trilobite, provided only that each appendage of the trilobite possessed an excretory coxal gland. ”

“ 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 Pædeumias or Mesonacis. The anal segment is directly comparable to the pygidium of a Ceraurus, the stiff unsegmented uropods being like the great lateral spines of that genus. ”

“ Walcott (1918, p. 170) has compared the limbs of Neolenus with those of the syncarid genera Anaspides and Koonunga. These are primitive Malacostraca without a carapace, but as they have a compressed test and Anaspides has stalked eyes, their gross anatomy does not suggest the trilobite. The thoracic appendages are very trilobite-like, since the endopodite has six segments (in Anaspides) and a multisegmented setiferous exopodite. The coxopodites, except of the first thoracic segment, do not, however, show endobases, and those which are present are peculiar articulated ones. ”

“ 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 ”

“ It needs only a glance at the very young to convince one that the embryos had swimming habits, so that in this form one sees the adaptation of the individual during its history to all modes of life open to a trilobite. The habits of the Harpedidæ may have been similar to those of the Trinucleidæ, but the members of this family are supplied with broad flat genal spines. ”

“ 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. ”

“ 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. ”

“ Modern Apus is certainly much less like a trilobite than has been supposed, and very far from being primitive. The Branchiopoda of the Middle Cambrian could have been derived from the trilobites by the loss of the pleural lobes, the development of the posterior margin of the cephalon to form a carapace, and the loss of the appendages from the abdominal segments. Modern branchiopods can be derived from those of the Middle Cambrian by the modification of the appendages through the reduction of the endopodite and exopodite and the growth of the endites and exites from the proximal segments. ”

“ 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 ”

“ In Burgessia, which seems to be a notostracan branchiopod, the trunk sinuses are very wide, and the appearance [83] is on the whole unlike that of any known trilobite. In Naraoia, however, the markings are much finer and directly comparable with those of Elyx. If my contention that Naraoia is a trilobite should be sustained, it might almost settle the question of the "nervures." In Burgessia these lateral trunks enter the main canal behind the fifth pair of appendages. In the trilobites they debouch much further forward. ”

“ Trilobite. “We’re the best and last, so there’s an end of it. The world was made for us.” She had the final word, very properly, and the trilobite shrugged his shoulders and waddled off to his family. Still he doubted.
[224]
II
Some millions of millions of years having passed by, we find ourselves, upon a bright afternoon of Mesozoic times, in the company of that genial and gigantic Deinosaur, Brontosaurus Excelsus. The monster, despite pleasant climatic conditions, was ill at ease. ”

“ Clarke cites the case of a trilobite with enormous compound eyes living among many Cambric forms wholly devoid of lenses, and other examples of a contradictory character, so that the size of the eye cannot be taken as proof of either deep or shallow water, but rather implies that the complex, highly-specialized eyes of certain forms enables these individuals better to adapt themselves to either deep or shallow water conditions. ”

“ The fossil remains of the trilobite family, are supposed by most naturalists to belong to a race of beings now extinct; but from the strong analogy which exists between them and certain species of crustaceous animals now living, it is highly probable that they will yet be found alive. ”

“ The enormous hypostoma of this giant resembles in a high degree that of the Asaphi, but is at the same time the most evident verification of the experience that the hypostoma per se cannot be regarded as the sole criterion for determinating the generic affinities of different species. The whole structure of this trilobite in other respects gives it a quite independent position, distinct, from Asaphus. ”

“ Perhaps the most characteristic and common trilobite of the Yeringian series in Victoria is Phacops sweeti (Fig. 110 D) , formerly identified with Barrande’s P. fecundus, from which it differs in the longer and larger eye with more numerous lenses. It is found in Victoria in the Upper Yarra district near the junction of the Woori Yallock and the Yarra Rivers; north-west of Lilydale; near Seville; at Loyola near Mansfield; and at Fraser’s Creek near Springfield, Kilmore.
Fig. 110—NEWER SILURIAN TRILOBITES.
A—Lichas australis, McCoy. ”

“ 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. ”

“ 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. ”