The fin whale is brownish to dark or light gray dorsally and white ventrally. They are dark gray dorsally and clean white ventrally. This species exhibits countershading, with brilliant metallic blue coloration dorsally and white ventrally. Both sexes are generally a reddish-brown dorsally and colored a dirty cream ventrally. Its body is a deep blue dorsally and shades of silvery white ventrally. The head capsule is sclerotized anteriorly and deeply incised ventrally and often dorsolaterally. It is a rose red color dorsally and orange ventrally. These spinules are in short transverse rows both ventrally and posteriorly. Simultaneously, the stomach begins to expand in width dorsally and ventrally in an asymmetric manner. It is ventrally wider at the rostral end. Motor axons leaving the abducens nucleus run ventrally and caudally through the pons. In the case of hammerheads the rostrum (hammer) extends both ventrally and laterally (sideways). Behind the blades is the mouth, located ventrally at the anterior end of the body. The female body tends to be thickened and curved ventrally. The male worm is smaller, long and wide, and features a ventrally curved tail.