OBSERVATIONS ON SPIRULA. 
I. External Conformation and Teguments. 
The body of Spirula is elongated, cylindrical, truncated posteriorly. The posterior 
extremity is constituted by a “ terminal disk,” which is less distinct in Spirula peronii 
(PI. I. figs. 1-4) than in Spirula australis (PI. II. figs. 1-3) and in Spirula reticulata 
(Figs. C, J, and Is in the text); it carries two small fins, parallel to the median sagittal 
plane, between which is an orifice leading into a little fossa. In the posterior half 
there are, both dorsally and ventrally, two antero-posterior oval openings through which 
the last whorl of the shell very slightly projects. This “body” corresponds to the 
visceral sac, covered by the mantle, of Nautilus and of the Gastropods (“ episoma,” 
Huxley) ; the cephalo-pedal mass (“ prosoma” or cephalopodium, Huxley) is united to the 
body by the “ mesosoma ” (Huxley), that is to say, by the nuchal region and the funnel, 
the latter (epipodium) belonging morphologically to the foot. 
These different parts : head, foot (arms), funnel, mantle, &c., must be separately 
examined. 
1. Head. — In the Challenger specimen this part was retracted into the pallial 
cavity (PI. I. figs. 1, 2, 3) and a little distorted (PI. I. figs. 5, 6). The “ Blake” specimen 
(PI. II. fig. l) had also the head slightly twisted. This asymmetry is not normal, however, 
and has not been observed in other specimens ; 1 it is apparently a deformation due to con¬ 
traction. The head in its general conformation, like that of other Decapods, is characterised 
(in the Challenger and “ Blake ” specimens) by its rather considerable width, especially in 
the posterior part; it is more developed laterally than dorso-ventrally. The eyes are 
placed laterally, and behind them, but more ventrally, are found the olfactory organs 
(see further on for the conformation of these organs). 
2. Arms. —As in all the Decapods, there are eight sessile arms, which are in Spirula 
rather short, and two long pedunculated or tentacular arms. 
a. Sessile Arms. —The eight short arms are perceptibly equal in length (PI. I. figs. 
1-3); their aboral or external surface is rounded, except that there is a median longitudinal 
ridge. The internal face is flattened so as to form a surface for the acetabula, extending 
from the base to the summit of each arm; this surface is broad and bounded by a curved 
contour at the basal extremity and narrows gradually towards the summit. The bases 
1 For example, the individual studied by Owen in 1879. 
