172 CRUSTACEA. 



for the enlargement of the optic ganglion. If so, it would correspond 

 to the proximal budding point (for the enlargement of the ganglion, 

 Fig. 85, h') in the eye-stalk of Branehipus. This last view has 

 recently been accepted by Kingsley also, who originally thought 

 that the layer of the crystalline cones and the retinular layer arose 

 from the outer wall of the optic invagination. 



The above view receives its chief support, as Carriere pointed out, from 

 the position of a pigmented layer of mesoderm - cells, which, according to 

 Reichenbach, develops between the outer wall of the optic fold and its 

 crystalline cone-layer, and Avhich is, nevertheless, evidently identical with 

 the layer of pigment-cells below the basal membrane of the eye, described 

 above (p. 169) for My sis. 



With regard to the significance of the separate parts of the ommatidium, 

 as to which Grenacher and Patten have recently taken opposite views, 

 attention should be drawn to Parker's observations, which revealed a con- 

 nection of the retinular cells with fine nerve -fibres, while the crystalline 

 cone-cells, which doubtless reach to the basal membrane, end at that point. 

 This is in agreement with the view of Grenacher, who saw in the retinular 

 cells the percipient elements, while Patten considered that the crystalline 

 cone-cells known as retinophorae were the elements connected with the nerves. 



In the development of the compound eye of the Cladocera, which was care- 

 fully described by Grobben (No. 11), special interest is awakened by the 

 formation of an integumental fold which grows over the eye, cutting off a 

 hemispherical precorneal space. The movement of the sunken eye is thus 

 assisted. Similar conditions are found in Apus, Esthcria, Limnaclia, and 

 Limnetis. The compound eyes, in these forms and perhaps also in the 

 Ostracoda, may be regarded as movable stalked eyes with degenerated stalks 

 which have sunk below the surface. Where, as in the Cladocera, an unpaired 

 compound eye is found, this must be considered to have arisen by the fusion 

 of paired rudiments. 



An auditory organ was observed by Reichenbach (No. 65) in 

 Astacus, as a dorsal invagination in the basal joint of the antennule. 

 Even in early stages, the ectodermal sensory epithelium, which 

 probably yields the auditory ridges, is distinguished by the multi- 

 laminar and regular arrangement of its cells. Nusbaum (No. 39) 

 similarly was able to observe the origin of the auditory sac in 

 Mysis in the endopodite of the last pair of pleopoda out of an 

 ectodermal invagination. 



E. Gills. 

 The branchiae first appear as simple outgrowths of the superficial 

 body- epithelium (ectoderm), within which lacunar blood-spaces 

 traversed by connective tissue-strands develop (Reichenbach). We 

 may with some probability regard all the branchial sacs or tubes 

 which belong to the outer side of the basal joints of the limbs, 



