attention. It is generally referred to as the x-shaped eye-spot which occurs throughout the Entomostraca, but disappears during larval life in the Malacostraca. It is, as a rule, so small, that investigation of its finer structure is difficult. In Apus, however, the unpaired eye is so large that its finer details are made out with comparative ease. This fact is especially important from our point of view, for if Apus is really the (or a) primitive Crustacean, then all the unpaired eyes throughout the whole class are in all probability only modifications of that of Apus. Hence it is necessary, for a comparative study of these organs, to have some accurate knowledge of their original form. We feel justified in assuming that this organ in Apus is in its original form, not only because Apus has retained so many primitive (ie. Annelidan) characteristics, but because, as will be described below, this form gives us a clue as to the origin of the organ out of an anterior pair of Annelidan eye-spots. In these pages we must of course confine our attention exclusively to the unpaired eye of Apus, describing in order (1) its general structure; (2) its probable origin; and (3) its present functions.

Structure.-Two groups of sensory cells, each forming what is in this connection generally called a retina, yield the two side walls of a cavity which is flat at the top and rounded below. The top consists of the chitinous fold, already described as forming the water-sac, while the lower part hangs free in the body cavity. Anteriorly, the cavity runs to a point along the watercanal (see Fig. 25); posteriorly, it ends in a blunt

point very nearly abutting on the brain. The posterior wall of the cavity and the posterior half of its floor are also composed of somewhat similar retina. The

FIG. 25.-Longitudinal median section through the unpaired "eye," diagrammatic. vr, ventral; dr, dorsal retina; p, tangle of pigment cells; s, water-space over the eyes; c, canal of the same, giving off a branch (1) into the interior of the organ; b, brain.

sensory ends of all these retinal cells point inwards; the nerves from the outer ends of the cells unite together to run towards the brain, forming from the four retinæ four nerve strands on which the un

paired "eye" seems to stand on the brain, as on four stalks, between the stalks of the optic ganglia. The stalk of the ventral retina is distinguished by the presence of several enormous ganglion cells, apparently the largest in the whole nervous system of Apus.

FIG. 26.-Lateral view of the unpaired "eye," diagrammatic. vr, ventral; dr, dorsal; Ir, lateral retina; b, brain; S, water-space; c, canal of the same; p, tangle cf pigment cells. Only the outer ends of the lateral retinal cells can be seen.

A cross section of the posterior end of the organ looks to the naked eye like an X. The retina, bulging in somewhat towards the interior of the cavity, give its lumen this form. (See Fig. 27, 3.) The pigment cells which fill the cavity form a tangle of

pscudopodia without any apparent arrangement. They contain the same minute olive green pigment granules as the other pigment cells, which spread out in great numbers over the subhypodermal connective tissue layer throughout the body. This, however, is not always the case; some specimens had brown pigment in large granules like those of the paired eyes. Some of the pseudopodia penetrate a long way between the retinal cells, and, at the surface of the organ where there are no retinal cells, the pigment cells with their long processes form together the external surface, the whole structure having apparently no enclosing membrane. The pigment cells, in fact, are nothing more than a plexus of the ordinary pigment cells which spread out irregularly throughout the whole body, among the subhypodermal connective tissue.

In a well-preserved specimen, in which the pigment did not happen to be very dense, the cells were seen to send down processes towards the inner sensory ends of the retinal cells. As these processes were regularly arranged, and free from pigment, it was difficult at first to decide whether they belonged to the retinal or to the pigment cells. We mistook them at first for large cilia belonging to the retinal cells.

It will be seen from the drawings that the retinal cells are not all of the same size. The lateral retinæ are composed of two distinct groups, an anterior group of long narrow cells, and a posterior group of short thicker cells. The ventral retina is composed solely of large thick cells, and the dorsal of two groups of