204 SPIRULA. 



attach the posterior end of its body to an}' object, leaving the 

 arms free to exercise their prehensile power on passing objects of 

 food. This wonderful terminal sucking organ is not found in 

 any other cephalopods, but may have been possessed by the ani- 

 mal of Ammonites, supposing it to have been related to the 

 Spirula rather than the Nautilus. The anatomy of Spirula, 

 which is carefully worked out and illustrated in Prof. Owen's 

 memoir, shows it to belong to the dibranchiate decapod cuttle- 

 fishes, as already indicated by previous studies. Whilst Spirula 

 possesses natatory powers superior to the Nautilus, in the action 

 of its webbed arms, additional to that of the funnel, the former 

 are so small in proportion to the size of the animal, and the fins 

 are so rudimentary as to indicate sedentary habits. Prof. Owen 

 observes that in Spirula, as in Nautilus, " the shell serves as the 

 point d'appui of the retractors of the funnel and of the head 

 with its locomotive and prehensile organs. Moreover, the last 

 chamber of the shell in Spirula also receives part of the visceral 

 mass, viz., the hind termination of the liver, which, covered by 

 its capsule, and this again by the peritoneum or a delicate 

 aponeurosis continued from the attached shell-muscles, consti- 

 tutes the hemispheric mass that fills the chamber and forms or 

 sends off the beginning of the membranous siphon. 



In another memoir, Prof. Owen shows that the dorsal portion 

 of the animal of Spirula is placed towards the outer wall of the 

 shell, which is the reverse of the relative positions of animal and 

 shell in both Nautilus and Ammonites, showing that the spiral 

 growth of the shell cone took a contrary direction. He agrees 

 that the aptychi are developed on the spadix of Ammonify, and 

 are true opercular bodies ; consequently the Ammonite could not 

 have been like the Spirula, an internal shell, but must luive been 

 closely related to Nautilus.* 



According to some recent investigators, there is a marked re- 

 semblance between the recent Spirula and the fossil Ammonites, 

 particularly in the initial whorl, and ;; difference in the latter 

 character between Ammonites and Nautilus which indicates that 

 the Ammonites should be separated from the tetrabranchiate and 

 united with the dibranchiate cephalopods. If this should prove 



* Owen, on the Relative Positions to their Constructors of the Cham- 

 bered Shells of Cephalopods. Zool. Proc., 055, 1878. 



