2 MOLLUSCA—CLASS III. GASTEROPODA. 
with the earlier system of Linneus. We find two genera, for example, 
Chiton and Patella, associated together in the same family, the one having 
a multivalve shell, the other a univalve, yet intimately allied by their 
natural organization and habits; and, again, we perceive an affinity 
between two genera, Haliotis and Siliquaria, whose shells are of the most 
opposite construction ; this apparent anomaly of arrangement, however, 
will be found to be most simple, and in perfect accordance with the true 
characters of the animal. Linnzus divided the Mollusca into classes 
according to the number of pieces of which their shell is composed, wni- 
valve, bivalve or multivalve ; but, in devising a new and more legitimate 
arrangement, the great author of the ‘ Régne Animal’ selected those cha- 
racters which seemed to him to represent the most important features in 
their natural organization. He first notices the progress of the nervous sy- 
stem, dividing the cephalous from the acephalous, according to the absence 
or development of a brain ; he then selects the foot, or organ of locomotion, 
for the purpose of establishing his classes, and the branchie, or organs of 
respiration, for dividing these into orders. Now in determining the subdi- 
vision of the Gasteropoda after this manner, we are naturally led to sup- 
pose that the most simple arrangement would have been to have separated 
the hydrobranchiate, or water-breathing kinds, from the pneumobranchiate, 
or air-breathing ; but so large a proportion would fall into the first of these 
divisions, that it becomes necessary to examine the system of respiration 
more minutely: the structure or position of the breathing organs is there- 
fore considered. Upon this plan the Gasteropoda are divided into seven 
orders: the first have the branchiz or organs of respiration cirrous or 
hair-like, whence they are called Cirrhobranchiata ; the second are remark- 
able in having them placed in a circle round the body under the edge of 
the mantle, Cyclobranchiata ; the third have them situated in a particular 
cavity in the back of the neck, Cervicobranchiata ; the fourth are distin- 
guished by having them on the right side only, Pleurobranchiata ; the fifth 
have them contained in a lump or nucleus on the back, Nucleobranchiata ; 
the sixth have them in the form of a pulmoniferous net-bag, Pulmobran- 
chiata ; and the seventh have them pectinated, or of the shape of a comb, 
Pectinibranchiata. 
