THE CEPHALOPODA 331 
tentacular arms are taken into account, they may be from twelve to 
eighteen metres long. Hence these Mollusca have given rise to 
various fabulous tales, and they have been known by man from 
the remotest periods of antiquity, as is evidenced by their representa- 
tions on some of the most ancient monuments from Mycenae, Egypt, 
and Greece. 
In the present day some four hundred species of Cephalopoda 
are distributed throughout all the seas of the world. Some species, 
especially those with a short and rounded pallial sac, such as the 
Octopodidae and Sepiola, are strictly littoral—indeed Sepiola and also 
Rossia are fossorial in habit. Other species are inhabitants of the 
open sea, and among these various forms of Oigopsida dwell in 
great depths: Spirula is found down to 1000 fathoms; Cranchia 
and Bathyteuthis down to 1700 fathoms; Histiopsis at a depth of 
nearly 2000 fathoms; Calliteuthis at 2200; and Cheiroteuthis 
down to 2600 fathoms. Many of these deep-sea Oigopsida are 
luminous. 
The history of the Cephalopoda extends back to the remotest 
geological times. Orthoceras and other forms allied to Nautilus, but 
as yet uncoiled, are abundant in the most primitive Palaeozoic 
formations. The subdivision of the Ammonitoidea, related to the 
Tetrabranchia, is distributed from the Devonian to the end of the 
Secondary period. The Dibranchia do not appear till the end of 
the Secondary epoch, during which they were characteristically 
represented by the Belemnitidae, a group which, like the Ammoni- 
toidea, became nearly completely extinct at the end of this period. 
V. REVIEW OF THE ORDERS, SUB-ORDERS, AND FAMILIES 
OF THE CEPHALOPODA. 
The class Cephalopoda comprises two orders, the Tetrabranchia 
and the Dibranchia. Palaeontology, as well as morphology, shows 
that the Tetrabranchia (Nautilus, etc.), that is to say, the Cephalopods 
with multiple branchiae, auricles, and kidneys, and with an external 
chambered shell, are the most archaic. The Dibranchia are more 
specialised, inasmuch as they have lost the anterior branchiae, 
auricles, and kidneys, and their shell has become rudimentary. 
The earliest Dibranchia were descended from rectilinear forms with 
a multilocular external shell devoid of a rostrum, and they gave 
rise in turn to Spirula, the Belemnitidae, and the allied Oigopsida. 
From the last named were derived, as the result of a yet more 
profound specialisation, on the one hand the Myopsida, on the other 
hand the Octopoda, by the loss of the tentacular arms (already so 
much reduced as to be almost lost in some Oigopsida), and by the 
more and more complete atrophy of the shell. 
