THE MOLLUSCA 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 
organs, is of great importance to the morphologist, and the more so 
because 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, ete.) or at the 
origin of important nerve trunks (Fig. 159, 7). 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, ete. 
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, 
Fig. 24). These elements are particularly numerous in the most 
exposed parts, such as the cephalic tentacles of Gastropods, the 
2 
