NERVOUS SYSTEM. 
209 
The Internal Organization. 
The internal organization of the mollusca is characterized by a 
certain degree of uniformity due to the common origin of the phylum, 
but this general ancestral likeness has been partially obliterated by 
the development of adaptive characters, more especially correlative 
with the special habits of the animals. 
NERVOUS SYSTEM. 
The nervous system, upon which sensation and muscular motion 
depend, resembles in some respects that of Vermes ; it is of para- 
mount importance in the animal organization, while being the last to 
be influenced by the morphological modifications undergone hy the 
animal, its peculiarities acquire very great phylogenetic significance, 
and have been used by Scbimkewitsch 
as the basis for the classification of all 
Bilateralia, the mollusca being termed 
Tetraneura, and characterized by the 
possession of cephalic ganglia, with paired 
ventral and lateral nervous cords. It is 
also more generally used in a restricted 
way to separate two great groups of 
Gastropods, according as the pleuro- 
abdoniinal commissures are straight or 
still twisted by the torsion which the 
body of the animal has undergone. 
The nervous system consists essentially 
of nucleated ganglion cells and nerve 
fibres, forming Ganglia, Connectives, 
Commissures, and Nerve Fibrils, which 
are invested by the Neurilemma, a some- 
what fibrous and more or less deeply 
pigmented sheath of connective tissue. 
Although primitively characterized by 
the absence of any local concentration 
of its various parts, which were probably 
constituted by ganglionic nerve cords, such as still represent the 
pedal 
Fig. 412. — Diagram of the nervous 
system of a mollusk, showing the 
bilateral arrangement of its consti- 
tuent parts and their relation to the 
median line of the body. 
g. ganglia, or paired aggregations 
of nervous tissue ; con. connectives, 
the longitudinal nerve cords joining 
dissimilar ganglia and never crossing 
the median line of the body ; co7n, 
commissures or transverse nerve 
cords, connecting the moities of the 
same ganglionic centre and therefore 
always crossing the median line of 
the body ; n.f. nerve fibrils. 
ganglia of Vivipara, yet usually the nervous tissue has now 
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