Johannes Müller classed the lancelet with the fishes, although he pointed out that the differences between this simple vertebrate and the lowest fishes are much greater than between the fishes and the amphibia. But this was far from expressing the real significance of the animal. We may confidently lay down the following principle: The Amphioxus differs more from the fishes than the fishes do from man and the other vertebrates. As a matter of fact, it is so different from all the other vertebrates in its whole organisation that the laws of logical classification compel us to distinguish two divisions of this stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, the Craniota (man and the other vertebrates). The first and lower division comprises the vertebrates that have no vertebræ or skull (_cranium_). Of these the only living representatives are the Amphioxus and Paramphioxus, though there must have been a number of different species at an early period of the earth’s history. Fig.210. The lancelet (Amphioxus lanceolatus), left view. Fig. 211. Transverse section of the head of the Amphioxus. Fig. 210—The lancelet (_Amphioxus lanceolatus_), left view. The long axis is vertical; the mouth-end is above, the tail-end below; _a_ mouth, surrounded by threads of beard; _b_ anus, _c_ gill-opening (_porus branchialis_), _d_ gill-crate, _ e_ stomach, _f_ liver, _g_ small intestine, _h_ branchial cavity, _i_ chorda (axial rod), underneath it the aorta; _k_ aortic arches, _l_ trunk of the branchial artery, _m_ swellings on its branches, _n_ vena cava, _ o_ visceral vein. Fig. 211—Transverse section of the head of the Amphioxus. (From _Boveri._) Above the branchial gut (_kd_) is the chorda, above this the neural tube (in which we can distinguish the inner grey and the outer white matter); above again is the dorsal fin (_fh_). To the right and left above (in the episoma) are the thick muscular plates (_m_); below (in the hyposoma) the gonads (_g_). _ ao_ aorta (here double), _c_ corium, _ec_ endostyl, _f_ fascie, _gl_ glomerulus of the kidneys, _k_ branchial vessel, _ld_ partition between the cœloma (_sc_) and atrium (_p_), _mt_ transverse ventral muscle, _n_ renal canals, of upper and _uf_ lower canals in the mantle-folds, _p_ peribranchial cavity, (atrium), _ sc_ cœloma (subchordal body-cavity), _si_ principal (or subintestinal) vein, _sk_ perichorda (skeletal layer).