An example of the use of simile in scientific descriptions of the natural world (here a group of arrow worms):
"The head when not in action is slightly flattened and of a truncate-conical shape; when in action its basal part assumes a semilunar or horse-shoe form, in the concavity of which lies the longitudinally-folded mouth. On each arm of the fleshy horse-shoe, a comb, formed of eight strong, curved, slightly hooked claws or teeth, is attached. The animal when lively is constantly clasping these bristle-like teeth together, over its mouth; when clasped together, and the head in a state of inaction, they appear to be situated much nearer to the mouth than when their fleshy bases are expanded in action.
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The animal moves quickly by starts, bending its body. The two pair of lateral fins and that on the tail lie in the same horizontal plane: viewed with a lens of small power they appear formed of a delicate membrane, but under a lens of 1/20th of an inch focal distance they appear to consist of excessively fine transparent rays, touching each other, like the barbs of a feather, but not, as it appeared to me, actually united by a membrane.
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When the tail is filled with vigorously circulating matter two large cul-de-sacs or gut-shaped ovaries are invariably present, extending, as represented (o o) in the diagram, from the base of the tail along each side of the intestinal tube…"
Charles Darwin (1844) Observations on the structure and propagation of the genus Sagitta. Annals and Magazine of Natural History
Biological descriptions often include comparisons to more familiar structures (e.g., 'like the barbs of a feather'). The reference to "the fleshy horse-shoe" would seem to be a metaphor (that is, the structure is here described as, not like, a horseshoe), except that Darwin had already made the comparison to a half-moon or horse-shoe explicit.
I have included the reference to the cul-de-sac which may seem to be another metaphor – but the term culd-de-sac was used in anatomy as a technical term and alternative to diverticulum to label a sac or pouch connected to a hollow organ. To refer to the head part as having an arm could be considered metaphorical, though this is a common usage and so would be likely read as literal.
Read about examples of science similes
