THE CEPHALOPODA 



295 



teuthis 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, Ji). These organs always originate at the aboral 



A 



FIG. 261. 



Two fossil Nautilitls, left-side view. A, OplihVioceras simplex, Barrande ; A', mouth of the 

 shell ; 11, J'tenocwas alatum, Barrande. f.s, foot (arms) sinus ; i.s, infundibular sinus. (After 

 Barrande.) 



extremity of the mantle (Fig. 290, (4) ft) 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 Taonius (Fig. 253) and OmmatostrepJies (Fig. 297), and they 

 may be duplicated, the two fins on each side lying close together, 

 as in Grimalditeuthis (Fig. 258,/,/') 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 Thysanoteuthis, 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, 



