. Outlines of zoology. Zoology. 328 ARACHNOIDEA AND PALMOSTRACA. A three-chambered heart, containing colourless blood, lies within a pericardium near the dorsal surface of the abdomen. It gives off an anterior and a posterior aorta and lateral vessels; and the circulation corresponds in general to that of the scorpion. In a few forms (Tetrapneumones) respiration is effected by four "lung-books," in the large bird-catching Mygale (Fig. 141). In the vast majority (Dipneumones) there are two lung-books, and tubular tracheae in addition. The stigmata of the lung-books lie on the ant
. Outlines of zoology. Zoology. 328 ARACHNOIDEA AND PALMOSTRACA. A three-chambered heart, containing colourless blood, lies within a pericardium near the dorsal surface of the abdomen. It gives off an anterior and a posterior aorta and lateral vessels; and the circulation corresponds in general to that of the scorpion. In a few forms (Tetrapneumones) respiration is effected by four "lung-books," in the large bird-catching Mygale (Fig. 141). In the vast majority (Dipneumones) there are two lung-books, and tubular tracheae in addition. The stigmata of the lung-books lie on the anterior ventral sur- face of the abdomen; the tracheae open posteriorly near the spinnerets, or just behind the opening of the lung-books, or at both places. The spinnerets (4-6) lie posteriorly a little in front of the anus. They are movable organs, perforated by numer- ous (often many hundred) fine tubes or "spinning ; The tubes are connected with numerous compressible glands secreting liquid silk. There are various kinds of *-'3^VZ?^!7t&& glands, and both the amount chamber; x., posterior wall; st., and the nature of the secre- stigma; ch., one of the interlamellar .- * ,, . , chambers. tion are under the spinner s control. The spinnerets arise from modifications of abdominal appendages, and the glands are ectodermic invaginations. The males are usually smaller and often more brightly coloured than their mates. From the paired testes, in the anterior part of the abdomen, two vasa deferentia pass to a common aperture beside the openings of the lung-books. From the paired ovary two oviducts likewise arise and open into a uterus, whose external aperture is surrounded in the mature female by a complex genital armature or epigynium. Here also in most females are the openings of two receptacula seminis, in which the sperms received from a male are stored, and from which they pass by a pair of internal ducts to the oviducts, there to fertilise the. Please not
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
