4 
HISTORY AND CLASSIFICATION. 
several pairs of irregularly di.sposed ganglia, which arrangement led 
Professor Owen to apply the term Heterogangliata to them, in contra- 
distinction to Homogangliata, by which term he designated the 
iVrticulata on account of their ganglia being arranged in a paired 
longitudinal series. Prom the Vertebrata, they are distinguished by 
the absence of an internal bony skeleton. 
History. 
The extensive and important sub-kingdom Mollusca {mnllh, 
soft), as now understood, embraces four Classes only. Cephalopoda, 
(ta.stropoda, Scaphopoda, and Pelecypoda. 
The PohjZDH, Bntchiopudd, and other groups, which were formerly 
included, have been successively removed from the molliLscan sub- 
kingdom and placed in other divisions. 
I.innti originally separated the sub-kingdom Mollusca as now under- 
stood, into two great divisions, placing the shell-bearing species 
associated with other organisms in a groti]) he designated as Vermes 
Tesfdced, and the nakeil, or internally-shelled s})ecie,s, he grouped 
with many other very dilferent forms of life, and designated them 
lVr;«p.s‘ Zoo'phiitd, a term which lie afterwards changed to Vermes 
Mo! I used. 
Paron Cuvier was the tirst to unite the Mollusca into one great 
sub-kingdom, and though he e.xcluded many groujis of organisms 
which were united with them by Linnb, he still retained several 
wliich have been since excluded by more modern authors. 
Our Briti.sh land and fre.shwater mollusca belong exclusively to tbe 
two classes. Gastropoda and Pelecypoda, the remaining groups being 
exclusively marine in babit. 
Classification. 
(Tissitication has for its object, not only the systematic arrange- 
ment of the objects of study, but the combination in suitable groups 
of those sjiecies having most affinity with each other, and jios.sessing 
in common some recognizable determinate characters. When a number 
of species possess this suitable similarity of organization they are 
united in a group termed a genus; such of these groups or genera as 
are distinguished by some common character are united in larger 
grou])S, called families ; the.se larger groups or families are gathered 
into still more numerous as.semblages, termed orders ; and finally 
