. Elementary text-book of zoology. Zoology. 60 MOLLUSCA. on the stalk of the optic ganglion and they cause a rapid inter- change of blue, red, yellow and dark colours. In addition to the chromatophores, there is a deeper layer of small shining spangles which produce interference colours, and thus give rise to the peculiar iridescence and lustre of the skin. The Cephalopoda possess an internal cartilaginous skeleton, which serves for the protection of the nerve centres and sense organs and attachment of mus- cles. In the Dibranchiata this skeleton constitutes a cartilaginous. -Gst which enclose
. Elementary text-book of zoology. Zoology. 60 MOLLUSCA. on the stalk of the optic ganglion and they cause a rapid inter- change of blue, red, yellow and dark colours. In addition to the chromatophores, there is a deeper layer of small shining spangles which produce interference colours, and thus give rise to the peculiar iridescence and lustre of the skin. The Cephalopoda possess an internal cartilaginous skeleton, which serves for the protection of the nerve centres and sense organs and attachment of mus- cles. In the Dibranchiata this skeleton constitutes a cartilaginous. -Gst which encloses the cere- bral ganglia, resophageal ring, and the auditory organ, while its lateral portions are hollowed out and represent the orbits. There are also (Decapods) optic cartilages, a so-called brachial cartilage and dorsal cartilage, various small cartilages for the closure of the mantle cavity, and fin cartilages for the support of the fins. Alimentary canal. - The mouth, which is placed within the circle of arms, is surrounded by a cir- cular fold forming a kind of lip (fig. 532). It is armed with two powerful jaws, an upper and a lower, which resemble in form a reversed parrot's beak. The radula, which recalls that of the Heteropoda, bears in each row a tooth-like median plate, and on each side three long hooks, adapted for drawing in the food ; in addition there may also be some flat non-toothed plates. The oesophagus usually receives two pairs of salivary glands, and either has the form of a simple narrow tube, or presents before its junction with the stomach a crop-like dilatation (Octopods, fig. 535 Jn). Fro. 533.—Nervous system of Sepia officinalis (after Cheron). Cg, cerebral ganglion; Vg, visceral gang- lion ; Eg, buccal ganglion ; Spy, suprapharyugeal ganglion ; Tg, ganglia of the tentacles ; Gst, stellate ganglion ; Ot, auditory Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - c
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
