. The biology of spiders. Spiders; Insects. INTERNAL STRUCTURE 67 knot or ganglion from which paired nerves arise. The " brain " is represented by two supra-pharyngeal ganglia above the pharynx, joined by nerves to two sub-pharyngeal ganglia below. These connecting nerves are called circumpharyngeal commissures and form a " nerve- collar " through which the pharynx passes. The form of the nervous system found in the adult spider (Fig. 43) has been considerably modified from this primitive arrangement, and, as in other examples of the same process, the modi- fication consist
. The biology of spiders. Spiders; Insects. INTERNAL STRUCTURE 67 knot or ganglion from which paired nerves arise. The " brain " is represented by two supra-pharyngeal ganglia above the pharynx, joined by nerves to two sub-pharyngeal ganglia below. These connecting nerves are called circumpharyngeal commissures and form a " nerve- collar " through which the pharynx passes. The form of the nervous system found in the adult spider (Fig. 43) has been considerably modified from this primitive arrangement, and, as in other examples of the same process, the modi- fication consists in an ap- parent reduction in the num- ber of ganglia, owing to their fusion with one another. Thus the " brain" of the spider is a composite syn- cerebrum, composed of three lobes. One of these is the so-called prostomial ganglion and two were the ganglia of the prosthomeres, or seg- ments which in development have passed in front of the mouth. In the same way the ganglia of the other seg- ments of the cephalothorax have fused instead of remain- ing separate, and the nerve- collar through which the fore-gut passes is thick and con- spicuous. The ganglia of the abdomen are evanescent, like the segments themselves. In very young spiders there is a stage in which as many as six ganglia are present along the floor of the abdomen, but these disappear in the course of development, and in the adult there is no trace of a ventral nerve-cord in the Fig. 43.—The Nervous System. From Dahl after Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Savory, Theodore Horace, 1896-. London : Sidgwick & Jackson
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Keywords: ., bookcentury1900, bookdecade1920, booksubjecti, booksubjectspiders
