269 
The respiratory function is performed by means of gills, 
which lie in the branchial cavity on the ventral aspect of 
the body; these giUs are two in number in the higher 
forms, and, along their whole length, are joined to the walls 
of the mantle cavity, which, as it expands and contracts 
alternately, allows the ingress and egi'ess of water to aerate 
the blood. In the lower group, the giUs are four in number, 
and are only attached to the walls of the mantle cavity by 
their posterior ends. The gills form a cylinder in Octopus 
and Sepia, whilst in Loligo and other genera they form only 
half a cylinder. 
The nervous system consists of three large pau*s of 
nervous ganglia, which are arranged around the upper part 
of the oesophagus, and are protected by a cartilaginous 
covering ; this concentration of the nervous system, corre- 
lated as it is with a high development of the sensory organs, 
justifies the position in which systematists have placed the 
Cephalopoda at the head of the Molluscan sub-kingdom. 
The three pau's of nervous ganglia (pedal, cerebral, and 
parieto-splanchnic), characteristic of all true Mollusca, are 
supplemented in the Cephalopods by subsidiary and acces- 
sory ganglia and their connecting commissures. 
According to Owen, the nervous system of the Nautilus 
is inferior to that of the Dibranchiata ; " the pai-t which cor- 
responds to the brain of the Cuttle Fish is neither enlarged 
nor lobulated, nor contained in a cartilaginous receptacle ; 
but is a simple, rounded chord, or commissure, placed trans- 
versely upon the oesophagus, and connected at its ex- 
tremities to the great nervous ganglions."* 
• For description of Nervous System of ('cphnlopodn, see " Kcchercbcs pour 
scrvir a TListoire du systemo uervcux dcs Cupbalopodes Dihraachiaux." Jules 
Charon, Paris, 1866. " Memoria sulla Structura del Si«tonia Xervoso doi Ccfalo- 
podi," S. Trinclicso, Fircnze, 18G8. " Norvous System of the Cephalopoda," A. 
Hancock, Ann. and Mag. Nat. lliet., Bor. ii., vol. i., London, lt<52. 
