30 REVIEWS. 



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 in- 

 testine 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 tracheae, 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 tracheae thus provided with an external 

 opening, the cells which secrete the chitinous inner membrane are cer- 

 tainly 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 trachea), in which the chitine fills up 

 the cavity, instead of lining it. In this case, however, the cells them- 

 selves 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 re- 

 tain 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 re- 

 sides, he believes, as in other Crustacea, in the anterior antennae ; these 

 organs are provided with a large nerve, which, in the middle of the an- 

 tennae, forms a small ganglion. From this ganglion, again, nerve-fila- 

 ments, more or less separate, pass to the group of auditory hairs, gene- 

 rally from five to ten in number, which form a tuft at the end of the an- 

 tenna. 



No organ of smell has as yet been satisfactorily proved to exist in 

 Daphniidae. The so-called shell-canals of the Entomostraca are re- 

 garded by Prof. Leydig as homologous with the equally enigma- 

 tical " green glands" of the higher Crustacea, and these again with the 

 projecting green organ in the embryo of Asellus. This curious struc- 

 ture was first discovered by Rathke, who supposed it to act as an 

 embryonal gill. Prof. Leydig' s reasons for considering it as homo- 

 logous 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 Crus- 

 tacea, 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, 



