16 CLASS III. GASTEROPODA. ORDER III. CERVICOBRANCHIATA. 
Order III. GASTEROPODA CERVICOBRANCHIATA. 
Branchiz pectinate, propé ad collum plerumque in cavitate speciali im- 
posite ; pallio supra cavitatem interdum fisso. 
The Cervicobranchiate Gasteropoda are so called, because the respira- 
tory organs are situated at the back of the neck, in an oblique line either 
upon the surface or in a particular cavity. Some authors have instituted 
a still further subdivision of these organs; Deshayes, for example, di- 
stinguishes the Scutibranchiate, the Tubulibranchiate and the Aristero- 
branchiate. Gray, on the other hand, refuses to accept the arrangement 
or position of the branchiz as typical; in consequence of their pectinate 
structure, he includes the whole of these mollusks with the great mass 
of Pectinibranchiata, changing the title to Cteno- (Latiné Pectini-) bran- 
chiata. 
The breathing apparatus of the Cervicobranchiata undoubtedly passes 
through a considerable variety of modification, and their shells are of 
very anomalous construction. Those of the Siliquarie and Haliotides, 
for instance, differ materially in their growth, notwithstanding the affinity 
of their animal inhabitants ; there is an analogy between these two genera 
which could not have been anticipated until the nature and position of 
the breathing organs became known. The respiration of each requires a 
continual current of water; and not only is the mantle slit or perforated 
in that part which covers the cervical cavity, but the shell also; so that 
the important consideration of the branchiz has detected an affinity 
between two mollusks which had been previously assigned to different 
parts of the system. 
We divide this order into four families, as follows : 
FISSURACEA. MacrostomatTa. 
CAPULACEA. TUBISPIRACEA. 
