Evolution of eyes in Mollusca.

n, Layer of nerve tissue; o, Optic nerve; 1, Lens.

A.-Eye of Solen, consisting of sensitive pigmented cells at the bottom of pit-like depressions in the skin.

B.-Eye of Patella. The pit is more developed, and the sensory cells less exposed.

C.-Eye of Haliotis. The mouth of the pit is narrow, and the sensitive cells still more protected. A well-developed optic nerve is present, and a layer of nerve tissue outside the sensitive cells of the retina.

D.-Eye of Fissurella. The mouth of the pit is closed, and a lens is developed.

eye of a cuttle-fish, we find that the eye, although a complicated one, yet passes in its own development through all the above series of stages, from the slight depression of skin, through the stages of a pit

FIG. 18.

JOO

PATELLA

HALIOTIS

FISSURELLA

Diagram of eyes of Patella, Haliotis, and Fissurella.

In Patella no image is formed. In Haliotis an indistinct image is formed by narrowing of the mouth of the pit. In Fissurella a distinct image is formed by the lens.

with large and small mouth; lens and finally eyelids being developed. (Fig. 19.)

The important point here is that we are able to show that the series fulfils our conditions, that all stages are possible ones, and that each is a distinct step onwards and an improvement on its predecessor; and furthermore, that each stage is retained as the actual permanent condition in some actually living mollusc.

It is not always possible to point out so clearly as

Loligo, one of the higher Cephalopoda, showing development of eye. o, Optic nerve and ganglion; n, Nerve layer of retina; 1, Lens; i, Iris; c, Cornea; p, Pigment layer of retina; s, Layer of sensory cells.

A.-First stage; a simple pit in the skin. This corresponds to the adult condition in Patella.

B.-Narrowing of mouth of pit, corresponding to Haliotis.

C.-Closing of pit, and formation of lens, corresponding to Fissurella. D.-Formation of iris.

E.-Adult condition.

F.-Figure of adult Loligo.

S n

in the above instance the particular advantage gained at each step, even when a complete developmental series is known to us; but in such cases our difficulties may be largely ascribed to ignorance of the particular conditions that confer advantage in the struggle for existence.

EMBRYONIC STAGES VIEWED AS ANCESTORS.

Early larval stages are of much interest, as possibly indicating the forms of the earliest ancestors. The most important and fundamental point is the fact that all the higher animals arise from eggs, and that the bodies of the higher animals are built up of cells or units, as a wall is built of bricks.

The lowest animals, or Protozoa, are single units or cells, and the egg of the higher animals is also a single cell. Therefore each of the higher animals begins its life as a single cell-i.e., in a Protozoon stage. Does not this indicate the descent of Metazoa or multicellular animals from Protozoa or unicellular ones? If there is a blood-relationship between the highest and lowest animals, and if the higher are descended from the lower, is it not reasonable to look for the origin of Metazoa in the Protozoa ? May not this be the explanation of the origin of all Metazoa in their actual development from single cells, and may not the egg represent the Protozoon stage in the ancestry?

Further, if animals really recapitulate, and if the reason why all Metazoa begin life as single cells

which is the most remarkable fact in the whole of embryology - is that they are descended from Protozoa, may not we hope to find from the study of early stages of development some hint as to the mode of origin of Metazoa in the first instance?

Let us consider, for example, the actual development of Amphioxus. The egg divides into a number of cells, which, instead of separating, remain together and continue to divide again and again, giving rise to the morula stage. The next stage is the tubular condition, where the cells are arranged regularly round a central cavity with an aperture at each end. This is followed by the blastula stage, which consists of a hollow ball, the outer cells of which are furnished with cilia enabling the embryo to swim freely. During later stages foldings take place, caused by outgrowths in some places and depressions in others, whereby the shape is gradually altered. (Fig. 20.)

Our present point is to ascertain whether these earliest stages are possible ones; whether there are organisms which remain permanently in one of these conditions-viz., (1) a single cell; (2) a heap of similar cells; (3) a hollow tube; (4) a ciliated hollow ball.

As examples of the first condition, or that of a single cell, the Monads may be taken. These are among the most minute and the simplest of living organisms, having an oval body, a nucleus, and a flagellum. They are found in infusions of animal and vegetable matter. An example of the second