1*J 
THE VOYAGE OF II.M.S. CHALLENGER. 
with the aiil of this little organ. It is now necessary to abandon this idea, for there is 
no evidence that Spirilla fixes itself by its aboral extremity. 
'riiere 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 speciid glandular apparatus. Its functions still remain very 
prul)leraatic;il. 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 Sph'ula, having a well-developed funnel, is organised 
fitr swimming backwards, like the other Dibranchiates, and its terminal papilla has 
Kl'i. J. — Lea Wew of Spirula reticulata (tbe 
ioternn |«rt of the ihcll and the vinceral 
i«c n-tire*cnte<l hy liTokeii linen) ; x 2. i, 
f«llUI r*Tlty ; ii, ventral external part of 
the »hell ; ill, tcniiinal dink ; iv, fin ; v, 
dorial external rmrt of the shell ; vi, last 
sr|>tam ; vil, ilorval jirojoction of the 
r:~.iUee<lj{e; vlii, rentro-latcral projection 
of U.e mantle edge. 
Fio. K. — Part 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 hy|Kjthcsis that in the living animal this papilla would be covered by a small 
external shell, it docs not appear that one could see in this the homologue of the rostrum 
of the DecajKxls, since the latter is formed in the shell sac. 
c. SltcU . — '1 he shell of Spirula, being very common, can be easily studied, and is 
well known to zoologists. 'J’hcrcfore 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 a/lult it is os we know multicellular, siphonated, and rolled like a spiral, in a 
