fied cartilage. This ideal animal, therefore,
is to all appearance as much like a worm as a fish, and swims by means
of a lateral undulation of its whole body, assisted, perhaps, by a
dorsal fin formed out of skin.

[Illustration: FIG. 45.--Ideal primitive vertebrate, seen from the
left side. (After Haeckel.) _na_, nose; _au_, eye; _g_, ear; _md_,
mouth; _ks_, gill-openings; _x_, notochord; _mr_, spinal tube; _kg_,
gill-vessels; _k_, gill-intestine; _hz_, heart; _ms_, muscles; _ma_,
stomach; _v_, intestinal vein; _c_, body-cavity; _a_, aorta; _l_,
liver; _d_, small intestine; _e_, ovary; _h_, testes; _n_, kidney
canal; _af_, anus; _lh_, true or leather-skin; _oh_, outer-skin
(epidermis); _f_, skin-fold, acting as a fin.]

[Illustration: FIG. 46.--The same in transverse section through the
ovaries; lettering as in the preceding Fig.]

[Illustration: FIG. 47.--_Amphioxus lanceolatus_. (After Haeckel.)
_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,
kid