One day after swimming front crawl, which works the triceps nicely, it all became clear. The triceps muscle has a curious shape at the top on account of its three 'heads' — or connecting origins. And the name derives from tri-ceps from Latin meaning three-heads. And so it goes. The biceps is a two-headed muscle attached with a long and short connection at the top. And the more complicated quadriceps has four heads that attach at the top, though they're a little harder to see.

Beyond this concept finally clicking for me, it was fun to emulate some of the anatomical graphical style of Leonardo Da Vinci — perhaps the ultimate at sketchplanations? — which I have long admired. For inspiration, this is a cracking collection: Leonardo Da Vinci the Graphic Works.

Biceps, triceps, quadriceps

For a closer look next time you’re at the beach. Pay close attention to whether you find one with a dextral or sinistral aperture.

Gastropods are the largest class of molluscs which includes limpets, snails and sea slugs. For those who weren’t sure.

Info from the 1971 book, Discovering Sea Shells by Barry Charles

Typical gastropod anatomy

A centipede has 1 pair of legs on each body ring, and a millipede has two on most body segments. Hence millipedes will generally have plenty more legs to go round. As far as I know this is fairly foolproof. Though generally speaking there are more obvious ways to tell, such as millipedes looking more like long armoured nice round worms and usually moving slowly, and centipedes zipping around while generally seeming more long-legged and insecty, with longer antennae at the front and longer angled back legs, sometimes with hooks for defence.

If you were offered to pick one of them up, you’d probably pick the millipede.

Also, in case you were tempted to think they are insects, they’re not because insects have six legs. They are arthropods (loosely meaning jointed feet) along with insects, spiders, scorpions and crustaceans, but in their own subphylum of, aptly named, myriapoda (loosely meaning many legs).

Check out core insect anatomy for a sketchplanation insect refresher.

How to tell a centipede from a millipede

Head, thorax - with 6 legs or wings, abdomen and an exoskeleton and you have an insect. One of over a million species. As impressive as snowflakes.

Core insect anatomy

The anatomy of a wave showing the key features of a wave: the crest, amplitude, trough, wavelength and when and how it starts to break on a beach.

Common in experience. Less commonly understood.

Anatomy sketches