620 L. H. GOtJGlt. 



is pierced by the alimentary canal in the region where the 

 two parts are joined to each other. 



Five pairs of nerves issue from the ventral or post-oral 

 part of the brain; these belong to the pedipalpi and the legs. 

 The nerves and ganglia belonging to the cheliceras are 

 pr^oral. 



No other nerves as yet leave the dorsal portion of the 

 brain, though two swellings on each side of it, just behind 

 the ganglion of the chelicerse, denote the optic ganglia. 

 They are not yet connected with the eyes by nerves, these 

 being very late to appear. The dorsal portion of the central 

 nervous system contains the accessory brain, the four 

 cerebral ganglia, and part of the ventral ganglion-cord, con- 

 taining the ganglia of the cheliceras. 



The ventral ganglion-cord consists of eighteen ganglia, 

 six belonging to the extremities. The other twelve are very 

 small; although lying in the cephalothornx they really 

 belong to the abdomen. 



The elements forming the brain and ventral ganglion- cord 

 are histologically mostly the same as those described in the 

 fourth stage. We now remark, however, small masses of 

 darker stained fibres in the commissural parts of the central 

 nervous system, the origin and structui'e of which it is diffi- 

 cult to understand. They are not to be found in the adult 

 brain. 



6. The Eyes. — The median eyes have become much 

 further developed since the fourth stage. This can best 

 be described in connection with the draAving (fig. 6). The 

 median eyes are still situated on a fold in front of the 

 mouth. The mouth opens into the cavity (c.) formed between 

 the fold carrying the eyes and the cephalothorax. We can 

 distinguish between three distinct layers of the fold — the 

 corneal, the retinal, and the subretinal layer. This last is 

 separated from the retinal by a fissure. 



The stratum corneum {co.) consists of a layer of nuclei, two 

 or three deep. In front it has begun to deposit chitin, the 

 future lens {I.). 



