192 ONYCHOPHORA. 



a thin layer of fibres had already appeared on the dorsal side of the 

 former. As development proceeds this gradually thickens (Figs. 101, 

 p. 202, and 102, p. 205). The position of this fibrous layer on 

 the mass of the ganglion-cells is practically retained in the adult, 

 for even there the fibrous mass lies dorsally to the ganglionic cells 

 (Balfour, No. 1), and only a very few of the latter attain a 

 position dorsally to the fibrous mass. This feature must be regarded 

 as a primitive one. In more highly differentiated forms, e.g., the 

 Crustacea and the Arachnida, the fibrous mass is indeed peripheral 

 when it first appears, but is soon covered by ganglionic cells, and 

 comes to lie within the mass of the ganglion. It has already been 

 pointed out, in connection with the Crustacea (Vol. ii., p. 162), 

 that the appearance of the fibrous substance on the inner periphery 

 of (i.e., dorsally to) the ventral strands might represent a primitive 

 condition. 



The transverse commissures which are, in Peripatus, found in large numhers 

 connecting the longitudinal nerve-trunks, grow out from the latter like the 

 peripheral nerves, which are said to be formed by the outgrowth of nerve- 

 fibres (v. Kennel). 



The brain arises in a manner agreeing with the origin of the rest 

 of the nervous system, but certain complications are caused by the 

 fact that it is formed by the fusion of the ganglia of two distinct 

 segments. The separation of the ganglionic rudiment of the 

 cephalic segment from the epidermal thickening (ventral organ) 

 takes place somewhat as in the trunk-segments, but the fibrous tissue 

 here lies much deeper in the mass of ganglionic cells, and is partly 

 covered dorsally by the latter (Fig. 96 B). From this dorsal cell- 

 mass a strand of cells is continued into the antennal rudiment, and 

 forms the rudiment of the antennal nerve (v. Kennel, No. 4, 

 Sedgwick, Xo. 10, Pts. iii. and iv.). The latter therefore appears 

 as a direct continuation of the cerebral ganglion, and is in this way 

 distinguished from all the other peripheral nerves, which are merely 

 outgrowths of nerve-fibres (without participation of ganglionic 



cells). 



The nerve-mass yielded by the cephalic segment soon grows to 

 such a size as to occupy the greater part of the head; the two 

 masses of ganglion cells, from which the antennal nerves proceed, 

 shift towards the middle dorsal line, where they form a pair of large 

 egg-shaped swellings (Fig. 97, g). The pair of ganglia composing 

 the brain are at first separated by a deep slit. This becomes bridged 

 over later, the fibrous mass of the two halves of the brain uniting 



