EEPOUT ON SPIEULA. 
9 
deep (PL II. fig. 1 ; and Figs. C, I, N in the text). However, in the Challenger specimen, 
the terminal disk, although as thick (PI. III., P^), is less distinct from the rest of 
the mantle (PI. I. figs, 1-6) than in the two other forms, the furrow above mentioned 
being in it almost indistinguishable. 
This disk is laterally continuous with the rest of the mantle ; ventrally and dorsally 
it limits aborally the shell openings. However, in Professor Giard’s smaller specimen 
{Spirula reticulata) this lateral continuity with the mantle does not extend over so 
great a space as in the larger specimen ; the shell openings are there consequently 
proportionally longer. 
Thus in the adult the shell tends to become still more internal than in the young. 
On the other hand the three forms [Spirula australis, Spirula reticulata, and Spirula 
peronii) all show an increasing gradation in the encroachment by the mantle on the 
shell, the minimum dorso-ventral extension of the aboral disk being found in 
Spirula australis,^ corresponding to the maximum length of the shell openings and 
Figs. D, E, F. — Three schemes of supposed developmental stages of Spirula. D, in the embryonic shell ; E, with a bilocular shell ; 
F, ivith a quinquelocular shell. — i, infundibulum ; ii, pallial cavity ; ii', terminal disk ; iii, shell ; iv, mantle ; v, head. 
the maximum of extension of that disk ; in Spirula. peronii (PI. I. fig. 4) it is 
more completely fused with the rest of the mantle, and at the same time the shell 
openings there are smaller than in the two other forms, Spirula reticulata (Figs. C, 
J, N) being in this respect intermediate between Spirula australis and Spirula peronii. 
It is necessary then to suppose that, in the process of development, the free margins 
of the mantle are reflected over the shell, as in numerous Molluscs (and especially over 
the dorsal side of the shell in Nautilus), since the shell has gradually become more and 
more covered (Fig. E). The right and left sides of the mantle evidently must send, 
towards the aboral extremity, prolongations which have united over the median line, 
giving rise to the terminal disk, which was apparently of small extent at first (Fig. F). 
It must not be supposed that the shell might have been completely enclosed at a very 
early stage, and that the dorsal and ventral openings of the shell cavity might have been 
^ Owen, Ann. Mag. Nat. Hist., ser, 5, voL iii. pi. i. fig. 3. 
(ZOOL. CHALTj. exp. — PART LXXXIII. — 1894.) 
2 
