THE MOLLUSC A 17 



exclusively composed of prolongations of sensory or centripetal 

 nerve fibres. The prolongations of the superficial ganglion cells are 

 continued into motor or centrifugal nerve fibres. Isolated ganglion 

 cells are to be found in the muscles of the heart and in those 'of the 

 buccal bulb (Pulmonata). 



The nervous system, being in close relation to all the other 

 rgans, is. of great importance to the morphologist, and the more so 

 oecause it is the last to be influenced by the modifications under- 

 gone by the organism. At the same time, every modification of 

 an organ is faithfully reflected by the nervous system. In such 

 forms as are still slightly differentiated, the large nerve cords are 

 uniformly covered with ganglion cells : in less primitive forms the 

 special development of certain parts of the body has produced a 

 preponderant development of certain nerves corresponding to them, 

 and nerve cells accumulate and give rise to ganglia at the bases of 

 these more highly developed nerves. Accessory ganglia may also 

 be formed at different points of the nervous system, either at the 

 bases of sensory organs (Figs. 94, br.g ; 214, os, etc.) or at the 

 origin of important nerve trunks (Fig. 159, i). The ganglion 

 centres may be shifted along the cords on which they are situated 

 in consequence of changes in the parts that they innervate. 

 Similarly, a nerve may be shifted along the cord from which it 

 issues until it seems to have changed its place of origin, but its 

 fibres always maintain their connection with their primitive nerve- 

 centre. 



The approximation or the union of two parts of the body in- 

 volves the approximation or fusion of the corresponding ganglia ; 

 or if one of the two parts is atrophied, its nerve-centre is reduced 

 and may be fused with the adjacent nerve-centre. The various 

 ganglia exhibit a general tendency to centralisation, all the 

 principal sensory organs being aggregated at the anterior part 

 of the body. At first all the sensory and motor nerve-centres, 

 and finally all the others (Cephalopoda, Nudibranchia, many 

 Pulmonata), become localised in the same region and are grouped 

 together. 



Organs of Sense. The function of general sensibility is spread 

 over the whole of the free surface of the envelope of the 

 body and the surfaces in continuity with it : included among the 

 latter are the internal surface of the mantle, and especially its 

 glandular tracts, and all invaginations of the ectoderm, such as the 

 pedal glands, the terminal portion of the rectum, the kidneys, etc. 

 On these various surfaces sensory elements are found among the 

 epithelial cells in the form of neuro-epithelial or end-cells, which 

 sometimes traverse a thick calcified cuticle (aesthetes of Chiton y 

 Fig. 24). These elements are particularly numerous in the most 

 exposed parts, such as the cephalic tentacles of Gastropods, the 



