10 
THE VOYAGE OF H.M.S. CHALLENGER. 
formed later by the pressure of the shell upon the thinnest points of the pallial envelope ; 
it is solely on account of this pressure that the mantle has not completely closed and 
that the openings have therefore persisted. The development of the shell sac of other 
Cephalopods (Sc^u'a, LoUgo) takes place in an essentially similar manner, except that 
the edges join not only in one part but throughout their whole length. 
The terminal disk is remarkable for the organs which it carries : Fins and Aboral Fossa. 
a. Fins. — Tlie fins are relatively small, rounded, almost semicircular, longer in the 
" dorso-ventral ” direction than in the cephalo-caudal (PI. I. fig. 1 ; PL II. fig. 2 ; Fig. J 
in the text). Their muscular mass is not attached to the cartilaginous supports, as 
in the other Decapods, the fibres being inserted directly on the shell sac. The fins of 
Spinila are characterised by their direction and situation. 
* Direction. — The fins are parallel to the median sagittal plane, inserted to the 
right and left of the terminal disk, and not arranged in the same right and left horizontal 
plane as is the case with other Dibranchiate Cephalopods ; they are thus to the fins 
of other Dibranchiates much as the tails of fishes are to those of Cetaceans. 
•• Situation at the posterior part of the body. — In the Cephalopods which have been 
most generally studied [Sejna, Loligo, &c.), the fins extend in the direction just indicated 
along a rather considerable length of the sides of the mantle. Nevertheless the dis- 
position presented in Spirula cannot be considered as a state of reduction or of retro- 
grade development. In fact in Nautilus, where the shell is quite external, there are no 
fins. In the ®gopsids, such as Ommatostrephes, the most archaic of living Dibranchiate 
Cephalopoda, the two fins are at the posterior extremity, attached side by side on the 
median dorsal line (where they cannot be attached in Spirula, on account of the opening 
of the shell cavity), and do not extend so far forward as in the other Decapods, in which 
the line of attachment is by degrees removed from the median dorsal line (Loligo) 
towanls the sides where the fins extend over a greater and greater length {Sepioteuthis, 
Sipia). On the other hand, in the development of Decapods with long fins [Loligo, 
Sijna), as well as in the others, these organs are found originally situated at the aboral 
extremity, as in the adult Spirula, and are there separated by a space of greater or less 
extent (Figs. G, II). Spirula thus presents the most primitive state of the fins of 
Ccphalo|*ods. 
0. Aboral Fossa. — Between the two fins in the middle of the terminal disk is found 
a rounded oj»ening (Bl. I. fig. 4; Bl. II. fig. 3), which in Professor Giard’s specimen 
upj»ear8 rather elongated across (apparently by deformation, see Fig. I in the text). 
I his ojMjning leads into a cavity of greater diameter than that of the orifice (PI. II., 
Js.\ Bl. \ I. fig, 14 ). This cavity is occuj)icd by a conical papilla, the summit of which 
in certain individuals projects a little to the outside (PI. VI. fig. 14). This papilla 
pr= nts a |»cculiar structure, but the state of preservation of the two specimens studied 
unfortunately prevents a ver}’ detailed description. The two figures relative to this 
