The outer form of our hypothetical primitive vertebrate was at all events very simple, and probably more or less similar to that of the lancelet. The bilateral or bilateral-symmetrical body is stretched out lengthways and compressed at the sides (Figs. 98–100), oval in section (Figs. 101, 102). There are no external articulation and no external appendages, in the shape of limbs, legs, or fins. On the other hand, the division of the body into two sections, head and trunk, was probably clearer in _Prospondylus_ than it is in its little-changed ancestor, the amphioxus. In both animals the fore or head-half of the body contains different organs from the trunk, and different on the dorsal from on the ventral side. As this important division is found even in the sea-squirt, the remarkable invertebrate stem-relative of the vertebrates, we may assume that it was also found in the prochordonia, the common ancestors of both stems. It is also very pronounced in the young larvæ of the cyclostoma; this fact is particularly interesting, as this palingenetic larva-form is in other respects also an important connecting-link between the higher vertebrates and the acrania.
The head of the acrania, or the anterior half of the body (both of the real amphioxus and the ideal prospondylus), contains the branchial (gill) gut and heart in the ventral section and the brain and sense-organs in the dorsal section. The trunk, or posterior half of the body, contains the hepatic (liver) gut and sexual-glands
in the ventral part, and the spinal marrow and most of the muscles in the dorsal part.
Figs. 98-102. The ideal primitive vertebrate (prospondylus). Diagram. Figs. 98–102.—The ideal primitive vertebrate (prospondylus). Diagram. Fig. 98 side-view (from the left). Fig. 99 back-view. Fig. 100 front view. Fig. 101 transverse section through the head (to the left through the gill-pouches, to the right through the gill-clefts). Fig. 102 transverse section of the trunk (to the right a pro-renal canal is affected). _a_ aorta, _af_ anus, _au_ eye, _b_ lateral furrow (primitive renal process), _c_ cœloma (body-cavity), _d_ small intestine, _e_ parietal eye (epiphysis), _f_ fin border of the skin, _g_ auditory vesicle, _gh_ brain, _h_ heart, _i_ muscular cavity (dorsal cœlom-pouch), _k_ gill-gut, _ka_ gill-artery, _kg_ gill-arch, _ks_ gill-folds, _l_ liver, _ma_ stomach, _md_ mouth, _ms_ muscles, _na_ nose (smell pit), _n_ renal canals, _u_ apertures of same, _o_ outer skin, _p_ gullet, _r_ spinal marrow, a sexual glands (gonads), _t_ corium, _u_ kidney-openings (pores of the lateral furrow), _v_ visceral vein (chief vein). _x_ chorda, _y_ hypophysis (urinary appendage), _z_ gullet-groove or gill-groove (hypobranchial groove).