Réaumur observed, “that although the workmanship of the land and sea animals when completed is alike, the manner of its production is very different. Spiders, caterpillars, &c., form threads of any required length, by making the viscous liquor of which the filament is formed pass through fine perforations in the organ appointed for spinning. But the way in which muscles form their thread is widely opposite; as the former resembles the work of the wire-drawer[184], so does the latter that of the founder who casts metals in a mould.” The canal of the organ destined for the muscle’s spinning is the mould in which its thread is cast, and gives to it its determinate length. [184] This remark of M. Réaumur confirms the observations of M. H. Straus, quoted in Chapter VII. that the thread of the silk-worm is not produced by a simple emission of liquid matter through the orifices of the spinner, or that it acquires solidity at once from the drying influence of the air. Indeed, silk cannot be produced in this manner, but is _secreted in the form of silk in silk vessels_, and the spinning apparatus, so called, only unwinds it. Mr. Straus’s observations on this head admit of no argument. The discovery reduces all that has been heretofore written upon the subject to the character of old lumber. Réaumur learned the manner of the muscle performing the operation of swimming by actually placing some of these fish under his constant inspection. He kept them in his apartment in a vessel filled with sea water, and distinctly saw them open their shells and put forth their tongues. They extended and contracted this organ several times, obtruding it in every direction, as if seeking the fittest place whereon to fix their threads. After repeated trials of this kind, the tongue of one was observed to remain for some time on the spot chosen, and being then drawn back with great quickness, a thread was very easily discerned, fastened to the place: this operation was again resumed, until all the threads were in sufficient number: one fibre being produced at each movement of the tongue.