EXTERNAL SHELL. 15 



placed upon the other. It is secreted by the palmate arms of the 

 female, which are constantly applied to its sides and envelop 

 them. In the male Argonaut, always much smaller, there are no 

 palmated arms, and consequently, no shell. The shell itself 

 appears to be useful only as a portable cradle for the development 

 of the eggs, and the animal which forms it does not appear to 

 differ greatly from the shell-less Octopus. 



The question of the parasitism of the animal of the Argonaut 

 in its shell, originally assumed by distinguished naturalists, has 

 been so long debated, that quite a literature upon the subject has 

 accumulated. The want of attachment of the animal by adductor 

 muscles, and the fact that the shell itself is not moulded on the 

 animal's body, nor does it correspond to the shape of the latter, 

 were so strong evidence of parasitism, that the animal itself was 

 described as Ocythoe, and the shell as Argonauta. The obser- 

 vations of Madame Jeanette Power first set this vexed question 

 at rest, by showing that the animal builds its shell by the exu- 

 dation of material from the expanded or velamentous arms ot 

 the female, instead of from the mantle, as in true molluscous 

 shells. 



The texture of the Argonaut shell, or egg-nest, is porcellanous, 

 composed of small plates or prisms ; its earlier portion is covered 

 with a chagrined cuticle, and its toothed periphery is stained 

 with brown. On either side the velamentous dorsal arms are 

 applied to its external surface, and not only do they add to the 

 margin when growth takes place, but they suffice also to renew 

 any broken portion of the already existing walls. 



In a specimen of Aryonati/a Aryo, which forms a part of the 

 collection of Amherst College, a portion has been broken out 

 near the middle of the left side, and not far from the sinus of the 

 aperture. A new deposit of testaceous substance, together with 

 a broken fragment, has closed the opening in the rude manner 

 common in the shells of the mollusca. But the most extraor- 

 dinary circumstance, is that a fragment which Avas broken out in 

 the accident which befel the animal, now constitutes two-thirds 

 of the repaired portion, and that the original inner surface is now 

 the outer surface, as is evident from its concavity, style of undu- 

 lation, and texture. It is also nearly at right angles to its orig- 

 inal position. These facts show that the piece was totally 



