. Outlines of zoology. Zoology. ARANEWM. 327 anterior pair are much used as feelers. In the embryo there are four pairs of abdominal appendages which abort. The nervous system is of the usual Arthropod type, but shows much centralisation. Thus the ventral ganglia are fused into one large centre in the cephalothorax (see Fig. 141), a condition comparable to that in crabs. There are two or three rows of simple eyes on the cephalothorax, whose focal distance is very short, spiders trusting most to their exquisite sense of touch, by which they discriminate the various vibrations on a web line. The
. Outlines of zoology. Zoology. ARANEWM. 327 anterior pair are much used as feelers. In the embryo there are four pairs of abdominal appendages which abort. The nervous system is of the usual Arthropod type, but shows much centralisation. Thus the ventral ganglia are fused into one large centre in the cephalothorax (see Fig. 141), a condition comparable to that in crabs. There are two or three rows of simple eyes on the cephalothorax, whose focal distance is very short, spiders trusting most to their exquisite sense of touch, by which they discriminate the various vibrations on a web line. The senses of smell, hearing, and taste are also pre- sent, but little is known in re- gard to the organs. Body cavity, endosternite, and coxal glands generally re- semble those of scorpions. The spider usually sucks the blood and juices of its prey, and behind the gullet lies a powerfully suctorial region, strengthened by plates, and worked by muscles. From the small mid-gut arise five pairs of long caeca, a pair running forwards and a pair passing into the bases of each pair of legs, and then back again. These csca sometimes anastomose. Further back the mid-gut gives off numerous digestive outgrowths, which fill a large part of the abdomen. Their secretion digests proteids. Terminally there is a large cloaca, and where the intestine joins this, four much-branched excretory Malpighian tubes are given off, and are again said to be endodermal in rhirinniK; FlG- 141-—Dissection of Mygale Ul from the ventral surface. —After Cuvier. :, Cheliceras ; 2, pedipalps cut short; 3-6, walking legs ; g1. large thoracic ganglion ; gK ganglion at base of abdomen; , chambered tracheae or lung-books—at the left side the anterior is cut open to show the lamellae (/.) ; m., muscle of abdomen; st1. and stigmata of lung-books ; ov., ovary; sfi., Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabili
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
