[Illustration: FIG. 47.--_Amphioxus lanceolatus_. (After Häckel.) _a_, anus; _au_, eye; _b_, ventral muscles; _c_, body-cavity; _ch_, notochord; _d_, intestine; _do_ and _du_, dorsal and ventral walls of intestine; _f_, fin-seam; _h_, skin; _k_, gills; _ka_, gill-artery; _lb_, liver; _lv_, liver-vein; _m 1_, brain-bladder; _m 2_, spinal marrow; _mg_, stomach; _o_, mouth; _p_, ventral pore; _r_, dorsal muscle; _s_, tail-fin; _t_, aorta; _v_, intestinal vein; _x_, boundary between gill-intestine and stomach-intestine; _y_, hypobranchial groove.] Now I should not have presented this ideal representation of a primitive vertebrate--for I have very little faith in the "scientific use of the imagination" where it aspires to discharge the functions of a Creator in the manufacture of archetypal forms--I say I should not have presented this ideal representative of a primitive vertebrate, were it not that the ideal is actually realized in a still existing animal. For there still survives what must be an immensely archaic form of vertebrate, whose anatomy is almost identical with that of the imaginary type which has just been described. I allude, of course, to _Amphioxus_, which is by far the most primitive or generalized type of vertebrated animal hitherto discovered. Indeed, we may say that this remarkable creature is almost as nearly allied to a worm as it is to a fish. For it has no specialized head, and therefore no skull, brain, or jaws: it is destitute alike of limbs, of a centralized heart, of developed liver, kidneys, and, in short, of most of the organs which belong to the other Vertebrata. It presents, however, a rudimentary backbone, in the form of what is called a notochord. Now a primitive dorsal axis of this kind occurs at a very early period of embryonic life in all vertebrated animals; but, with the exception of _Amphioxus_, in all other existing Vertebrata this structure is not itself destined to become the permanent or bony vertebral column. On the contrary, it gives way to, or is replaced by, this permanent bony structure at a later stage of development. Consequently, it is very suggestive that so distinctively embryonic a structure as this temporary cartilaginous axis of all the other known Vertebrata should be found actually persisting to the present day as the permanent axis of _Amphioxus_. In many other respects, likewise, the early embryonic history of other Vertebrata refers us to the permanent condition of _Amphioxus_. In particular, we must notice that the wall of the neck is always perforated by what in _Amphioxus_ are the gill-openings, and that the blood-vessels as they proceed from the heart are always distributed in the form of what are called gill-arches, adapted to convey the blood round or through the gills for the purpose of aeration. In all existing fish and other gill-breathing Vertebrata, this arrangement is permanent. It is