THE CEPHALOPODA 295 
feuthis among the Oigopsida, there are two infundibulo-pallial 
sutures, 
In the Dibranchia the mantle is a very muscular organ, which, 
by its contractions, serves two purposes. By alternately and 
rhythmically drawing in and forcing out the water that enters 
the pallial cavity between the funnel and the border of the mantle, 
it acts as an accessory respiratory organ, and by violently expelling 
water through the funnel it acts as an efficacious locomotory organ, 
causing the animal to execute sudden retrograde movements. 
In the majority of Cephalopods with internal shells (Decapoda) 
and in the Cirrhoteuthidae, the mantle is produced into lateral 
symmetrical expansions or fins of various form and position (Figs. 
253, 260, and 268, fi). ‘These organs always originate at the aboral 
Fic. 261. 
Two fossil Nautilids, left-side view. A, Ophidioceras simpler, Barrande; A’, mouth of the 
shell; B, Ptenoceras alatwm, Barrande. /f.s, foot (arms) sinus ; i.s, infundibular sinus. (After 
Barrande.) 
extremity of the mantle (Fig. 290, (4) @)—even in Octopus, in which 
genus they eventually disappear—as two triangular or rounded out- 
growths. ‘They remain localised at the aboral extremity in Spirula, 
in which genus they are situated close together at the point where 
the two halves of the mantle reunite behind the shell (Fig. 295). In 
most other Oigopsida they are still terminal and close together, but 
they tend to shift further forward on the anterior or dorsal surface, 
as in ZYaonius (Fig. 253) and Ommatostrephes (Fig. 297), and they 
may be duplicated, the two fins on each side lying close together, 
as in Grimalditeuthis (Fig. 258, fi’, fi’) and Vampyroteuthis. But in all 
other cases they diverge to take up positions opposite one another on 
the right and left sides of the body, and show an increasing tendency 
to occupy the whole length of the body, as in 7hysanoteuthis, where 
they are triangular (Fig. 298, B), and in Sepioteuthis, where they are 
rounded. In Sepia the fins extend the whole length of the mantle, 
