1917] The Nervous System of Thysanura 309 



which is nearly a cross-section through the brain of an adult. 

 In this a much more complicated structure of fibers and fibrils 

 is presented than in any of the others so far compared. The 

 stalks and roots of the mushroom-bodies are shown in Fig. 32, 

 also the so-called central body. The arrangement of the 

 fibrillar material of the mushroom seems to have a different 

 arrangement and development than in Lepisma saccharina 

 studied by Bottger, 1910. In his descriptions and figures 

 an anterior and posterior division of the cap of the fibrous 

 material or "Traube," is given but in the species studied at 

 this time the "Traube" has a different position and is not 

 clearly made up of more than one main lobe on each side. 

 Each of these main lobes as shown in the figure has five secondary 

 lobes instead of four shown by Bottger. In the region of these 

 masses of fibrous material of the mushroom-bodies there are a 

 number of irregular fibrous bundles which may represent other 

 parts of this structure. On the whole, there is a fairly close 

 resemblance to the brains studied by Bottger and differences 

 may be due to the fact that this is not the species which he 

 studied, but our most common local species whose exact identity 

 has not yet been reported to me by special students of this 

 group. 



One of the first differences between the brains of Machilis 

 and Lepisma is shown because of the different position of the 

 eyes. The eyes of Machilis are connected with the caudal 

 end and this dorso-caudal region forces the parts of the brain 

 usually found here, farther forward. This is not an unusual 

 condition among the brains of invertebrate animals, in some 

 amphipod Crustaca as compared with nearly related isopods 

 there is a similar shifting. In general, the posterior lobe region 

 of the brain of segmented animals seems to represent its highest 

 center, but shiftings such as just mentioned may often change 

 the usual arrangements. 



The second marked difference between Machilis and Lepisma 

 at least in all specimens studied, both small and large, was in the 



Fig. 31. Longitudinal section through the edge of the brain of Machilis, the 



section is through the eye above and to the right. The cephalic end 



is towards the left. X 150. 

 Fig. 32. Cross section through the brain of Lepisma, only the right half is 



shown. X 150. 

 Figs. 33 and 34. Cross sections through two levels of the first thoracic ganglion 



of Lepisma. The dorsal side is up. X 150. 



