<p>Understanding different manifestations of complexity is central to comparative anatomy. The evolution of metazoans to bilaterians, eucoelomates, amniotes, to the higher-level nervous system of brains is reasonably thought of in terms of a rise in complexity. However, it tends towards a bias of complexity being present in vertebrates. Recent work in comparative anatomy resulted in a detailed atlas of anatomical sections for 6 vertebrates and 5 vertebrates at the adult stage: the roundworm, earthworm, swan mussel, Roman snail, giant cockroach, spiny-cheek crayfish, carp, edible frog, red-eared slider, domestic chicken, and rat. Similarly, recent research in complexity analysis has uncovered several metrics to analyze complexity mathematically. This work investigated the complexity of anatomical sections from the Atlas using three different metrics. Spatial information represents the proportion of an anatomical section with sharp edges; approximate Kolmogorov complexity represents the proportion of an anatomical section’s information that can be compressed algorithmically; and semantic complexity that counts the number of anatomical terms identified by an expert. This study found that expert-based measures determined the vertebrates to be 1.4–1.75 times more anatomically complex than lower invertebrates in the atlas, but no significant difference was found for the algorithmic metrics, possibly due to the small pixel scale on which they operate or confounding factors. Important caveats in this analysis include differences in image resolutions (pixels per area), as well as different numbers of sections per organism. This study finds semantic complexity the most consistent with evaluating complexity in comparative anatomy.</p>