Baur, and others, look upon it as being a layer of epithelial cells. It is admitted, says M. Baur, that the chitinous inner membrane of the intestine is secreted by the surrounding layer of epithelial cells, and passes without any line of separation into the ordinary outer layer of chitine. But, on the other hand, M. Leydig urges that the chitinous, spirally-thickened lining of the tracheæ, which also is continuous with the outer skin, is clearly formed by the connective tissue.
Under these circumstances, we naturally ask for a definition of the terms "epithelium" and "connective tissue." If with Baur we define the former to be a layer of cells, or nuclei, clothing a free surface, it is evident that the chitinogenous layer of the skin would be an epithelial structure. But it may be asked, what is a free surface? Before the spiracles are formed, and the tracheæ thus provided with an external opening, the cells which secrete the chitinous inner membrane are certainly connective tissue; when, however, the trachea is complete, they clothe an outer surface just as much as the chitinogenous cells of tendons, which may indeed be regarded as tracheæ, in which the chitine fills up the cavity, instead of lining it. In this case, however, the cells themselves have undergone no change whatever, nor, even of position. It seems to me, therefore, that we are not able to draw any satisfactory line of separation between epithelium and connective tissue; nor can we look upon them as essentially distinct, although it may be convenient to retain the names as expressing different forms of the same tissue.
The accessory eye-spot does not appear to be so rudimentary an organ as has been generally supposed; and Prof. Leydig was able to detect in it several distinct though small bodies, of high refractive power. He thinks that it is a mistake to suppose that this spot belongs to the period of embryonal life, since, in all species in which he found it present in the embryo, it also occurred in mature specimens. The sense of hearing resides, he believes, as in other Crustacea, in the anterior antennæ; these organs are provided with a large nerve, which, in the middle of the antennæ, forms a small ganglion. From this ganglion, again, nerve-filaments, more or less separate, pass to the group of auditory hairs, generally from five to ten in number, which form a tuft at the end of the antenna.
No organ of smell has as yet been satisfactorily proved to exist in Daphniidæ. The so-called shell-canals of the Entomostraca are regarded by Prof. Leydig as homologous with the equally enigmatical "green glands" of the higher Crustacea, and these again with the projecting green organ in the embryo of Asellus. This curious structure was first discovered by Rathke, who supposed it to act as an embryonal gill. Prof. Leydig's reasons for considering it as homologous with the above-mentioned organs are, that he can suggest no other homologue for it, and that it occupies a similar position; and that, whereas the "green glands" are found in all other aquatic Crustacea, no other homologue for them has been detected in Asellus. In none of these cases has the organ any external orifice.
The blood in Daphnia consists of a fluid which is either colourless,
