a rudimentary eye which has neither pigment nor cornea and is coloured like the general surface of the body.

Eryonicus (fig. 63) belongs to the same group, and comes from the region of Saint-Thomas in the Antilles, where it lives at a depth of about 825 yards. This animal has a reduced optic stalk, but at the extremity of this, where in littoral

FIG. 63. Eryonicus cœcus. Bate? (After W. Faxon,
The Stalk-eyed Crustacea, Mem. of Mus. of Comp.,
Zool. Harvard College, vol. xviii., 1895.)

Atlantic at a depth

pletely devoid of

forms the eye is borne, there is only a depression as if the eye had been carefully scooped out.

Willemæsia

(fig. 64), a relative of the marine crayfish and an in

habitant of the

of about 3500 yards, is comeyes in the adult condition,

although it possesses them in the larval stage. Scolophthalmus (fig. 65), which lives down to 4000 yards, is quite devoid of eyes, but possesses eye-stalks which terminate in spines.

It seems, then, that different species of deep-sea Crustacea may present different degrees of degeneration of the eye. One species in itself exhibits all

grades of degradation according to the depth at which it lives. This creature—Cymonomuswhich, when near the surface, has fully formed eyes upon movable stalks, at a depth of a few hundred yards exhibits movable stalks without eyes; and at 1500 yards the stalks are fixed and end in spines.

Isopod Crustacea, which live in the deep sea, present similarly degenerate eyes. Many are blind

FIG. 65.-Scolophthalmus lucifugus, FAX.

a, optic peduncle transformed to a spine. (After W. Faxon, The Stalk-eyed Crustacea, Mem. of Mus. of Comp. Zool. Harvard College, vol. xviii., 1895.)

and display all kinds of optic degeneration. Noesa, for instance, simply has eyes devoid of pigment. Thus, in abysmal Crustacea, the degeneration of the eyes is in no sense a retracing of developmental stages.

Another instance chosen from examples of the atrophy of organs in individuals, shows that the supposed law of retracing cannot be made universal.

4. Atrophy of the branchial vessels in man.-Examination of a human embryo of about three

weeks old shows the presence of a series of slits on the sides of the neck, the slits not being parallel, but converging towards the ventral surface. Between these slits are swellings, or pads, which pass up towards the dorsal surface and appear like the beginnings of hoops or ribs enclosing the visceral cavity; the elevations are the branchial arches, the slits are the gill-slits.

In the human embryo (fig. 66, A), as in fish, these slits appear from above downwards, and

as they are p formed, the corresponding blood

vessels arise.

These vessels,

m'

m2.

ap

C

C

a

or aortic arches,

arise from a ven

B

-AD

A

tral aorta (a.) FIG. 66.-Diagram of branchial arches in mammals.

which gives off six lateral

A. Embryonic stage. a, aorta; c, aortic arches ;
AD, dorsal aorta. B. Adult stage. The parts
represented by dotted lines have degenerated.
A, aorta; v, carotid; ap, pulmonary artery.

branches (c.) at each side. These lateral branches run up between the gill-slits and form two main trunks on the dorsal side which converge to form the descending aorta (ad.).

In man the branchial arches are transformed, parts of them entering into the structure of the face, and during the transformation parts of the aortic arches atrophy (fig. 66, B). But the order

of this atrophy does not correspond in any way to the order of the formation of the vessels.

The median parts of the anterior two lateral branches (m1 and m2) disappear, and the vertical parts remain as the internal and external carotid. vessels. The vertical piece which joined the posterior parts of the third and fourth arches disappears the internal and external carotids thus acquire a stem of their own. The parts of the fourth arch remain; the fifth arch disappears at each side, and the sixth arch forms the pulmonary artery (ap.).

Thus the degeneration of these vessels represents in no way whatever a retracing of their developmental history. All that occurs is that the useless parts disappear and the useful parts persist. A comparative study of this example would only enforce our conclusion.

In ontogeny the neurapophyses are more ancientthan the vertebral centres. None the less, as we have already seen, the examination of any vertebral column from head toward tail shows a gradual disappearance of all parts except the centra, although the centra are the last to be formed.

SECTION II.

The path of degeneration in plants.

1. Rarity of cases of recapitulation in the organogeny of leaves.—We have already said that