534
t
ARACHNIDA
of the Vertebrata, no sutures exist in the chitinous cuticle of Arthropoda. That any partial fusion of originally distinct chitinorts plates takes place in the cephalic shield of Trilobites, comparable to the partial fusion of bony pieces by suture in Vertebrata, is a suggestion contrary to fact. The Trilobites are known only as fossils, mostly Silurian and
appearance to the free somites. The genus Agnostus, which belongs to the last category, occurs abundantly in Cambrian strata and is one of the earliest forms known. This would lead to the supposition that the great development of metasomatic carapace is a primitive and not a late character, were it not for the fact that Paradoxides and Atops, with an inconspicuous telsonic carapace
Fig. 36.—Triarthrus Becki, Green. Dorsal view of second thoracic leg with and without setae. en, Inner ramus; ex, outer ramus. (After Beecher.)
Fig. 34.—Restoration of Triarthrus Becki, Green, as determined by Mr. Beecher from specimens obtained from the Utica Slates (Ordovician), New York. A, dorsal; B, ventral surface. In the latter the single pair of antennae springing up from each side of the camerostome or hypostome or upper lip-lobe are seen. Four pairs of appendages besides these are seen to belong to the cephalic tergum._ All the appendages are pediform and bi-ramose; all have a prominent gnathobase, and in all the exopodite carries a comb-like series of secondary processes. (After Beecher, from Zittel.)
Fig. 37.—Deiphon Forbesii, Barr. One of the Cheiruridae. Silurian Bohemia. (From Zittel’s Palceontology.)
prse-Silurian ; a few are found in Carboniferous and Permian strata. and numerous free somites, are also Cambrian in age, the latter As many as two thousand species are known. Genera with small indeed anterior in horizon to Agnostus. On the other hand, it may well be doubted whether the metasomatic carapace, consisting of three to six fused segments distinctly marked though not separated by soft membrane, are pygidial or posterior carapace is primarily due to a fusion of the Harpes, Paradoxides, and Triarthrus (Fig. 34). In Calymene, Homalonotus and Phacops (Fig. 38) from six to sixteen segments are clearly marked by ridges and grooves in the metasomatic tagma, whilst in Ilsenus the shield so formed is large but no somites are marked out on its surface. In this genus ten free somites (mesosoma) occur between the prosomatic and metasomatic caraa
Fig.* 35.—Triarthrus Becki, Green, a, Restored thoracic limbs in transverse section of the animal; h, section across a posterior somite ; c, section across one of the sub-terminal somites. (After Beecher.) paces. Asaphus and Megalaspis (Fig. 39) are similarly constituted. In Agnostus (F ig. 40) the anterior and posterior carapaces constitute almost the entire body, the two carapaces being connected by a mid-region of only two free somites. It has been held that the forms with a small number of somites marked in the posterior carapace and numerous free somites between the anterior and posterior carapace, must be considered as anterior to those in which a great number of posterior somites are traceable in the metasomatic carapace, and that those in which the traces of distinct somites in the posterior or metasomatic carapace are most completely absent must be regarded as derived from those in which somites are well marked in the posterior carapace and similar in
Fig. 39.—Megalaspis extenuatus. Fig. 38.—Dalmanites limuOne of the Asaphidse allied to lurus, Green. One of the Ihenus, from the Ordovician Phacopidse, from the Siluof East Gothland, Sweden. rian, New York. (From (From Zittel.) Zittel.) tergites of somites which were previously movable and well developed. The posterior carapace of the Trilobites and of Limulus is probably enough in origin a telsonic carapace—that is to say is the tergum of the last segment of the body which carries the anus. From the front of this region new segments are produced in the first instance, and are added during growth to the existing
