CHAPTER VI 
THE CEPHALOPODA 
CLASS V.—THE CEPHALOPODA, Cuvier. 
Order 1. Tetrabranchia. 
Sub-Order 1. Nautiloidea. 
+ 2. Ammonitoidea. 
Order 2. Dibranchia. 
Sub-Order 1. Decapoda. 
Tribe 1. Oigopsida. 
», 2. Myopsida. 
Sub-Order 2. Octopoda. 
Tribe 1. Leioglossa. 
,, 2. Trachyglossa. 
Definition. —The Cephalopoda are perfectly symmetrical Mollusca, 
in which the edges of the foot are transformed into circumoral 
appendages completely surrounding the head, and the epipodium 
is modified to form an exhalant muscular tube or funnel consisting 
of two free or united lobes, situated behind the head at the opening 
of the pallial cavity, and serving as a conduit for the water from 
this cavity. In the nervous system all the typical ganglion-pairs 
are concentrated in the head, and are applied to or contained in 
the interior of a cartilaginous skeletal piece. The renal organs are 
constituted by the glandular covering of the afferent branchial 
vessels. The coelom communicates with the exterior either directly 
or by the intermediary of the paired kidneys, and by a second pair 
of ducts serving as gonaducts. ‘The gonad is situated in the coelom 
and is not continuous with the gonaducts. A portion of the 
circumoral pedal crown is “ hectocotylised,” that is to say, modified 
to form a copulatory organ in the male. The development is 
characterised by the incomplete segmentation of the ovum. 
IJ. GENERAL DESCRIPTION AND EXTERNAL CHARACTERS. 
In comparing the Cephalopoda with other Mollusca, one finds 
that the ventral surface is much abbreviated and the length of the 
285 
