SPIRULID^. 49 



Shell placed vertically in the posterior part of the bod}^, with 

 the involute spire towards the ventral side. 



Although thousands of shells of these mollusks are washed 

 ashore in all parts of the world, the animal is almost unknown, 

 but three perfect individuals, and several others, more or less 

 imperfect, having been collected. The U. S. Coast Survey Steamer 

 Blake, in 1878, dredged a Spirula with its mollusk, in the West 

 Indies, at the great depth of 950 fathoms. Prof. Owen's last 

 memoir on the Spirula adds materially to what was heretofore 

 known respecting it. (Ann. Mag. Nat. Hist.^ 5 ser.,iii, 1, 1879.) 

 He shows that the mantle terminates posteriorly in two lateral 

 flaps which cover the sides of the shell, leaving it partly exposed 

 dorsall}'' and ventrally. Posteriorly, Ibetween the lobes, is an 

 elliptical, convex body, with a central depression or disk, flanked 

 by a pair of oblong productions, perhaps homologous with fins, or 

 at any rate resembling the small lateral-terminal fins of Loligopsis. 

 The terminal disk is, perhaps (as long ago described by Rum- 

 phius), a true sucker, enabling the animal to attach the posterior 

 end of its body to any 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 animal 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, addi- 

 tional 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 cVappui 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 ter- 

 mination of the liver, which, covered by its capsule, and this again 

 by the peritoneum or a delicate aponeurosis continued from the 

 attached shell-muscles, constitutes 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 Ammonites, and 

 are true opercular bodies ; consequently the Ammonite could 



