30 
THE VOYAGE OF H.M.S. CHALLENGER. 
As to what is the mode of life of Spirilla, Agassiz has supposed, from the disposition 
of the chromatophores, that the posterior, part of the living individual remains plunged 
in the mud at the bottom. However, in the two specimens taken by the Challenger 
and the “ Blake,” it is noticed that it is precisely at the posterior part that the 
chromatophores are specially preserved (PI. II. figs. 1, 2 ; PI. YI. fig. 14). Further, the 
funnel of Spirula is as well developed as in the other Cephalopods. This animal is not 
then sedentary, but a good swimmer, which the existence of fins at the posterior part 
likewise confirms ; these fins would evidently not be found there if the part were 
plunged in the mud. 
If we now compare the following facts: on the one hand, that Spirula is a swimmer 
and that it keeps to the greater depths (since it never has been observed living at the 
surface or near it), and on the other hand that as soon as dead the animal is carried 
away by its shell towards the surface, we ought evidently to conclude that the living 
animal compresses a part of the gas contained in the “phragmocone,” apparently the gas 
enclosed in the siphuncle, and this by the action of the pallio-siphonal sinus, as has been 
explained above (see Circulatory System). 
The rarity of Spirula is thus explained by the abyssal nature of the animal. That 
the animal is extremely rare is proved by the fact that among the inhabitants of certain 
islands of the South Pacific—where the shells of Spirula are extremely abundant—the 
opinion prevails that the shell “ has no animal.” 
The circumstance that the individuals found floating, or thrown up on shore, are 
generally incomplete and mutilated, might be explained by the interpretation of Kobert, 
according to whom Siphonophores ( Physalia ) prey upon Spirula ; he says 1 that one of 
the specimens captured by the “ Ptecherche ” “ had been taken among the tentacles of a 
Physalia .” 
IX. Phylogeny. 
The systematic position of Spirula among the Dibranchiate Cephalopods is in no way 
fixed. In 1879, Brock expressed the opinion that Spirula must be ranked among the 
Myopsid Decapods. 2 But in a later work 3 he retracted that view, declaring that 
Spirula had nothing to do with the Myopsids, that it is doubtful if it be an (Egopsid, 
and that it probably represents a special group. In 1881, Steenstrup maintained, 
1 Comptes renclus, t. ii., 1836, p. 363. 
2 Brock, Studien iiber die Verwandtschaftsverhaltnisse der dibranchiaten Cephalopoden, Erlangen, 1879, 
p. 21. 
3 Brock, Versucb einer phylogenie der dibranchiaten Cephalopoden (Morph. Jahrh., Bd. vi., 1880, 
p. 84). 
