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 

 JKossia 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. 



