Probably we must trace to the same phylogenetic cause the defective nature of the sense organs of the amphioxus, which we will describe later (Chapter XVI). Prospondylus, on the other hand, probably had three pairs of sense-organs, though of a simple character, a pair of, or a single olfactory depression, right in front (Figs. 98, 99, _na_), a pair of eyes (_au_) in the lateral walls of the brain, and a pair of simple auscultory vesicles (_g_) behind. There was also, perhaps, a single parietal or “pineal” eye at the top of the skull (_epiphysis, e_).
In the vertical median plane (or middle plane, dividing the bilateral body into right and left halves) we have in the acrania, underneath the chorda, the mesentery and visceral tube, and above it the medullary tube; and above the latter a membranous partition of the two halves of the body. With this partition is connected the mass of connective tissue which acts as a sheath both for the medullary tube and the underlying chorda, and is, therefore, called the chord-sheath (_perichorda_); it originates from the dorsal and median part of the cœlom-pouches, which we shall call the skeleton plate or “sclerotom” in the craniote embryo. In the latter the chief part of the skeleton—the vertebral column and skull—develops from this chord-sheath; in the acrania it retains its simple form as a soft connective matter, from which are formed the membranous partitions between the various muscular plates or myotomes (Figs. 98, 99 _ms_).
To the right and left of the cord-sheath, at each side of the medullary tube and the underlying axial rod, we find in all the vertebrates the large masses of muscle that constitute the musculature of the trunk and effect its movements. Although these are very elaborately differentiated and connected in the developed vertebrate (corresponding to the various parts of the bony skeleton), in our ideal primitive vertebrate we can distinguish only two pairs of these principal muscles, which run the whole length of the body parallel to the chorda. These are the upper (dorsal) and lower (ventral) lateral muscles of the trunk. The upper (dorsal) muscles, or the original dorsal muscles (Fig. 102 _ms_), form the thick mass of flesh on the back. The lower (ventral) muscles, or the original muscles of the belly, form the fleshy wall of the abdomen. Both sets are segmented, and consist of a double row of muscular plates (Figs. 98, 99 _ms_); the number of these myotomes determines the number of joints in the trunk, or metamera. The myotomes are also developed from the thick wall of the cœlom-pouches (Fig. 102 _i_).