The Pearly Nautilus. 



By W. T. Manger. Read April 22,rd, 1903. 



Class — Cephalopoda. 



/vz;/2//v— ARGON AUTID^. 

 Species — Argonauta argo. 



This is one of the most interesting of the Octopods, and occupies 

 a perfectly unique position among the Mollusca. It is not homo- 

 logous with the ordinary molluscan shell, not being attached to the 

 animal by a special muscle, but is embraced or held to the body 

 by two of the arms, which are dilated at the extremity and specially 

 adapted for this purpose ; the arms are furnished with minute 

 secretive glands endowed with a power of calcification similar to 

 that in the mantle of other Molluscs. The Argonauta has eight 

 arms, each having two rows of suckers. When floating on the 

 surface of the water, the two dilated arms mentioned are not expanded 

 aloft to catch the breeze as has been so often represented : they are 

 busy holding on to the shell from the outside, so that if the shell were 

 taken out of the water and reversed, the animal would very soon fall 

 completely out of the shell by its own weight. The Argonauta can 

 float on the water, using its arms as oars ; it can swim through the 

 water by the successive injection and ejection of water in and out of 

 the branchial cavity ; and it also crawls on the bed of the ocean in an 

 inversed position on its arms, feeding on Molluscs and Crustaceans. 

 The mouth, with its parrot-like beak, is in the middle of the bases 

 of the arms. One of the most curious attributes of this species is 

 that the shell is constructed by the female only. The shell is simply 

 involuted and not chambered, and forms a receptacle for the ova, a 

 cradle for the young, and a protection for the parent. 



The male is about an inch in length, and resembles an ordinary 

 octopus, having neither shell nor palmate arms. The arms are 

 tapering and alike, excepting the third on the left side, which is 

 specialised. Perhaps the most remarkable in the sexual relations of 

 all the Mollusca (says the " Cambridge Natural History ") is the so- 

 called Hectocotylus of the Cephalopoda. In the great majority of 

 the male Cephalopods one of the arms, which is modified for the 

 purpose in various ways, becomes charged with spermatophores, and 

 sometimes becomes detached, and remains attached to the female ; 

 the lost portion is gradually reproduced, and in due time resumes its 

 former appearance. It was a long time before naturalists could bring 



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