178 MOLLUSC A. CEPHALOPODA. 



of the modern cuttle-fishes attain an enormous size, for 

 instance Loligo sometimes has a length of forty feet or 

 more. 



The Dibranchiata, as the name indicates, possess one 

 pair of gills only ; there is also one pair of auricles and 

 one pair of kidneys. The number of arms is limited to 

 eight or ten ; and on the inner side that facing the 

 mouth they are provided with rows of sucking discs, 

 which sometimes possess horny hooks. The jaws are not 

 calcified. An ink-sac is always present and is occasionally 

 found preserved fossil; and even in this condition it is 

 capable of being used for artistic purposes. The funnel is 

 in the form of a complete tube. A skeleton is absent in 

 some forms ; when present it is internal, with the single 

 exception of the genus Argonauta ; it may be either horny 

 or calcareous. In some (Sepia etc.) it has the form of a 

 flattened spoon-like body, known as the pen, which is 

 composed mainly of laminated calcareous material ; occa- 

 sionally at the posterior end there is a small chambered 

 portion. The pen is placed on the dorsal surface of the 

 animal in a sac formed by the mantle. In Spirula the 

 shell is situated in the posterior part of the body, and 

 consists of a tube coiled in a plane spiral and divided up 

 into chambers by means of septa, which are traversed by 

 a siphuncle placed near the inner margin. In Belemnites 

 it consists of a solid portion (the "guard") and of a 

 chambered part with a siphuncle. The shell in Argonauta 

 is of quite a different nature to that found in other forms ; 

 it is spiral but not chambered, and is secreted, not by the 

 mantle, but by the two dorsal arms, and is found only in 

 the female, serving for the reception of the eggs. 



The Dibranchiata are divided into two sub-orders 

 (1) the Decapoda, (2) the Octopoda. 



