. Outlines of zoology. forms known as Hemi-aspidse, Hemiaspis, Belinurus. Order 2. Eurypterina (?=Mero-stomata or Gigantostraca), Large extinct forms found fromCambrian to Carboniferous body is divided into head, thorax,and abdomen. The head is smalland unsegmented. The thorax iscomposed of six distinct segmen,ts,the abdomen of six with a terminaltelson. On the head are borne sixpairs of appendages of varying shape,two lateral compound eyes, and twomedian ocelli. On the ventral surfaceof the thorax there are five pairs ofgills covered by flat plates, of whichthe
. Outlines of zoology. forms known as Hemi-aspidse, Hemiaspis, Belinurus. Order 2. Eurypterina (?=Mero-stomata or Gigantostraca), Large extinct forms found fromCambrian to Carboniferous body is divided into head, thorax,and abdomen. The head is smalland unsegmented. The thorax iscomposed of six distinct segmen,ts,the abdomen of six with a terminaltelson. On the head are borne sixpairs of appendages of varying shape,two lateral compound eyes, and twomedian ocelli. On the ventral surfaceof the thorax there are five pairs ofgills covered by flat plates, of whichthe most anterior pair are very large, and form the so-called operculum(cf. Limulus), The surface of the body was covered with of the Eurypterids reached a length of 6 ft. The oldestMerostomes are referred by Walcott to a sub-order Limulava somewhatdivergent from other Eurypterids. This order is sometimes placed near the Crustacea, but the generalopinion is that they are linked through Limulus to Fig. zoo.—Young Limulus.—After Walcott. Order 3. Trilobita. Trilobites, Calymene, Phacops,Asaphtis Extinct forms chiefly found in Cambrian and Ordovician strata, butextending up to the Carboniferous. The body as found is divisibleinto three parts—the unsegmented head shield, often prolonged back-wards at the angles; the flexible thorax of a varying number ofsegments; the unsegmented abdomen or pygidium. A medianlongitudinal ridge, or rachis, divides the body into three longitudinalportions. 378 PHYLUM ARTHROPODA. Traces of limbs are only rarely preserved. In the head region there are four pairs, apparently have been recently foundin this region. The thorax andabdomen are furnished with biram-ose appendages, with long-jointedendopodite, short exopodite, and agill (or epipodite ?) of varying the abdominal region the gillswere perhaps rudimentary. Trilobites are often found rolledup in a way that reminds one ofsome wood-lice. So abu
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
