A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . edge ofthe carapace is turned over on the under side and supportsa thin membrane, in which are the two openings of thedigestive tract: the mouth, beneath the glabella, and theanus, beneath the pygidial axis. In front of the mouth isthe hypostoma or fore-lip, behind it is the metastoma or hind- G 2 84 GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. Gallery lip. Specimens of other species showing these structuresabiecase exhibited. Every body-segment, except that in which
A guide to the fossil invertebrate animals in the Department of geology and palaeontology in the British museum (Natural history) . edge ofthe carapace is turned over on the under side and supportsa thin membrane, in which are the two openings of thedigestive tract: the mouth, beneath the glabella, and theanus, beneath the pygidial axis. In front of the mouth isthe hypostoma or fore-lip, behind it is the metastoma or hind- G 2 84 GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. Gallery lip. Specimens of other species showing these structuresabiecase exhibited. Every body-segment, except that in which25. SiJius opens, bears a pair of appendages, attached to transverse thickenings of the ventral membrane. The frontpair form whip-like antennae. The remaining pairs arebranched, one branch being a crawling leg, the other branchbearing a fringe of bristles or of lamellae. The basal seg-ments of the four pairs on the head served to bite food andto pass it into the mouth. The lamellate branches of theremaining limbs may have served partly for swimming,partly for breathing. Trilobites lived only in the sea, some on reefs, some on. Fig. 39.—Reconstruction of a Trilobite, T7iarthrus Becki, from theOrdovician, Utica Slate of New York; natural size. (After 25.) muddy or sandy bottoms; some, it is inferred either fromthe absence or the extraordinary size of the eyes, in deepwater. In the growth of an individual trilobite of simplestructure, the free cheeks and the eyes borne by them areat first not seen on the upper surface of the head-shield. Asthe animal grows they appear at the edge, and graduallycome to occupy more and more of the upper surface. SomeTable-case early trilobites, however, such as Agnostus (Fig. 40 a),25. Harfes, and Trinucleus, never reach this stage, and may beseparated as a Grade Hypoparia (under-cheeks) from thosein which the free cheeks are visible on the upper these latter the free cheeks may be confined to the fore- AETHROPODA—T
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Keywords: ., bookauthorbritishmuseumnaturalh, bookcentury1900, bookdecade1900
