ORDER II. MONOTHALAMIA. 307 
Nautilus. The two hindermost or dorsal pair of tentacles present, how- 
ever, a modification of structure in this octopod unlike any of the class. 
The Argonaut possesses the peculiar faculty of secreting a light boat- 
shaped shell, for the purpose of containing her eggs; and as this elastic 
envelope could not serve to protect the ovary, if it had been moulded as 
in the Nautilus, immediately on the body, it is formed from the outside 
by an expansion of each of the dorsal tentacles, which undertake the 
office of calcification like the expanded lobes of the Cypree*. The cal- 
cifying power is thus transferred from the mantle to this pair of arms or 
tentacles ; it is, however, much more feebly developed than in the Nau- 
tilus ; the shell remains in a perfect state of elasticity during the life of 
the animal, and it contracts and swells to suit the respiratory and loco- 
motive movements of its inhabitant. 
The velated or web-like arms, so celebrated in poetic fiction as supply- 
ing the service of sails}, are much larger in some species of Argonauta 
than in others; (for there are several distinct kinds of Argonauts, each 
presenting, with a constancy of correspondence in their shells, a joint 
assemblage of characters specifically distinct from each other ;) in the 
Argonauta argo, for example, the common species of the Mediterranean, 
* In the living Argonauts alluded to as having been experimented upon by Madame Power, 
she distinctly observed the gradual formation of the shell as above described; and upon pur- 
posely breaking it whilst the animal was in life and vigour in her marine vivarium, she was 
still further gratified to find that the fracture became duly repaired under the influence of the 
calcifying membranes. We have also the additional testimony of D’Orbigny on this head, 
who relates having frequently discovered Argonauts with the margin of the shell in a mem- 
branous and soft state, from having just received an additional secretion of calcareous matter. 
+ ‘These membranes,” says Owen, “have been described by naturalists and poets, from 
Aristotle and Callimachus down to Cuvier and Byron, as serving the office of sails; the ani- 
mal being supposed to have the power of rigidly extending the soft fleshy arms which sup- 
port the membranes, and maintaining the latter tensely outstretched to meet the breeze. It 
is scarcely necessary to observe, that the structure of the parts in question is incompatible 
with this hypothesis of the use of the ve/a in navigating the frail boat of the Argonaut.” «It 
has been ascertained, indeed,” continues the Professor, ‘‘ by direct observation, that these 
vela, or rather velamenta, have not only a relation of coexistence, but one of direct physiolo- 
gical import to the development of the shell, serving as the organs both of secreting and of 
retaining this part.” 
