Science Art: Sagitta atlantica and Sagitta equatoria, 1922.

Scientific illustration of bristle-jawed arrow worms, fast-moving marine predators.
Scientific illustration of bristle-jawed arrow worms, fast-moving marine predators.

These are illustrations from “Notes on Species of Sagitta Collected on a Voyage from England to Australia” by B.B. Gray, as published in The Proceedings of the Royal Society of Queensland, which I found here, in the Biodiversity Heritage Library.

Sagitta are arrow worms, predatory marine worms in a phylum called “bristle-jaws,” or Chaetognatha. The biggest of them is around 120 mm long. That’s less than 5 inches.

Like all the bristle-jaws, they’re hermaphrodites, and when they mate, each individual fertilizes the other. They also look more or less the same throughout their lives, unlike other invertebrates who spend a while as a nymph and then metamorphosize into some entirely different shape. The little ones are just little, hopping and darting around and swallowing their little prey whole.

The one on the left, Sagitta atlantica, is described in The Proceedings… as “a transparent but fairly firm form,” which I suppose is flattering enough. It was found between Madeira and the Canary Islands. Sagitta equatoria is less flatteringly “a firm opaque form with a very large head.” It was captured off the coast of West Africa at 15 degrees of latitude.