12 
THE VOYAGE OF H.M.S. CHALLENGER. 
with the aid of this little organ. It is now necessary to abandon this idea, for there is 
no evidence that Spirula fixes itself by its aboral extremity. 
There is besides nothing in the fossa and its papilla which recalls the structure 
of an acetabulum or sucker, especially there are no retractile muscles at the bottom 
of the cavity. Nor is this fossa a mucous pore, as other authors have supposed ; 
there is no trace of special glandular apparatus. Its functions still remain very 
problematical. One might perhaps suppose that the papilla is a protective prominence ; 
we know that owing to the rapid swimming of Cephalopods, being retrograde, their aboral 
extremity is much exposed. And Spirula, having a well-developed funnel, is organised 
for swimming backwards, like the other Dibranchiates, and its terminal papilla has 
VIII VII 
Fig. J.—Left view of Spirula reticulata (the 
internal part of the shell and the visceral 
sac represented by broken lines) ; x 2. i, 
pallial cavity ; ii, ventral external part of 
the shell; iii, terminal disk ; iv, fin ; v, 
dorsal external part of the shell; vi, last 
septum ; vii, dorsal projection of the 
mantle edge; viii, ventro-lateral projection 
of the mantle edge. 
Fig. Iv.—P art of the external face of the 
shell of Spirula; x 90. 
precisely the situation of an organ of protection for the aboral extremity. In the 
majority of Decapods this situation is occupied by the point of the rostrum. Still, on 
the hypothesis that in the living animal this papilla would be covered by a small 
external shell, it does not appear that one could see in this the homologue of the rostrum 
of the Decapods, since the latter is formed in the shell sac. 
c. Shell .—The shell of Spirula, being very common, can be easily studied, and is 
well known to zoologists. Therefore it is not proposed to dwell on this subject, except 
in so far as concerns the relations of the shell to the animal. 
In the adult it is as we know multicellular, siphonated, and rolled like a spiral, in a 
