328 PHYLUM ARTHROPODA. 



The brain is thus in all cases a composite structure containing 

 the neuromeres of the preoral segments fused together. The 

 mode of its development in Scolopendra would indicate that there 

 is anteriorly, in addition, a median unpaired element (the 

 archicerebrum of Heymons) together with lateral elements 

 (forming, with the archicerebrum, the syncerebrum of Heymons) 

 with which the neuromeres have united, though how this con- 

 dition is related to that presented by Peripatus remains for the 

 present obscure. 



The number of neuromeres entering into the brain appears 

 to be two in Peripatus, most Arachnids, and in Daphnia 

 (Fig. 250) and Limnadia (Fig. 241 D) among the Crustacea, 

 three in the Malacostraca, Myriapoda and Insecta. 



For the further discussion of the segmentation of the arthropod head 

 and other cognate questions the reader is i-eferred to the literature on 

 this subject quoted at the beginning of the chapter. 



The conclusions here arrived at are to a large extent in accordance 

 with those of Heymons, Goodrich and Lankester, but the designation of 

 the segments by a numerical nomenclature, adopted by Lankester, has 

 been as far as possible avoided, because of the uncertainty of deciding 

 in the several groups, which is the first segment. 



The Eyes of Arthropods. The eyes of Peripatus are vesicular 

 structures which arise by invagination of a portion of the brain 

 rudiment (while it is still part of the skin) with which they 

 retain their connexion by the optic nerve. In this they appear 

 to resemble the simple eyes of Arachnida. 



Little has been ascertained throwing light on the origin of the 

 unpaired " nauplius eyes" of the lower Crustacea. 



Apart from Peripatus the simplest form in which the paired 

 eyes are met with in Arthropods is that presented by the stemma 

 or ocellus of Insects (Fig. 231). The chitinous cuticle is thick- 

 ened to form a lens, and beneath this the cells of the hypo- 

 dermis are disposed in a cup-shaped manner, the adjacent 

 cells being highly pigmented. The cells in the floor of the cup 

 form the retina, and are in connexion with the nerve ; while the 

 transparent ends of the surrounding cells, bending over the 

 retina from the sides, form a clear medium, which has been called 

 the vitreous body, intervening between the retina and the lens. 

 It appears evident from the arrangement of the parts of such 

 a simple eye that the light entering through the lens is focussed 

 on the layer of retinal cells, and hence that some representation 



