Nectocaris had a flattened, kite-shaped body with a fleshy fin running along the length of each side. The small head had two stalked eyes, a single pair of tentacles, and a flexible funnel opening out to the underside of the body. The funnel often gets wider away from the head. Internally, a chamber runs along the body axis, containing a pair of gills; the gills comprise blades emerging from a zig-zag axis. Muscle blocks surrounded the axial cavity, and are now preserved as dark blocks in the lateral body. The fins also show dark blocks, overprinted by fine striations that sometimes stand proud of the rock surface.
Although Nectocaris is known from Canada, China and Australia, in rocks spanning some 20 million years, there does not seem to be much diversity; size excepted, all specimens are anatomically very similar. Historically, three genera have been erected for nectocaridid taxa from different localities, but these ‘species’ – Petalilium latus and Vetustovermis planus – likely belong to the same genus or even the same species as N. pteryx. Within N. pteryx, there seem to be two discrete morphs, one large (~10 cm in length), one small (~3 cm long). These perhaps represent separate male and female forms.
The unusual shape of the nectocaridid funnel has led to its interpretation as an eversible proboscis, but this is difficult to reconcile with the fossil evidence. Instead, its fluid dynamics seem optimal for a role in jet propulsion, where it would have maintained an efficient slow flow of water over the large internal gills. The eyes of Nectocaris would have had a similar visual acuity to modern Nautilus (if they lacked a lens) or squid (if they did not).