384 OCTOPODA 



Blainville maintained that the animal which inhabits the 

 Argonaut shell is a parasite, without any means of depositing 

 or forming a shell of its own, but which possesses itself of the 

 Argonaut shell, either by expelling or succeeding the original 

 inhabitant, a supposed nucleo-branchiate (Heteropod) mollusc 

 akin to Oarinaria. The final blow to this strange hypothesis — 

 which was urged by the most ingenious series of arguments — was 

 given by Professor Owen, who in 1839 brought before the Zoo- 

 logical Society of London the admirable observations of Madame 

 Jeannette Power, who made a continuous study of a number 

 of specimens of Argonauta in her vivarium at Messina. The 

 result of these observations tended to show that the young 

 Argonauta when first excluded from the Qgg is naked, but that 

 in ten or twelve days the shell begins to form , that the 

 principal agents in the deposition of shell are the two velated 

 or web-like arms ; and that portions of the shell, if broken 

 away, are repaired by a deposition of calcareous matter.^ 



Fam. 4. Philonexidae. — Mantle supported by two ridges 

 placed on the funnel ; large ' aquiferous ' pores (supposed to 

 introduce water into the tissues) near the head or funnel; 

 suckers in two rows, pedunculate. — Atlantic and Mediterranean. 



Genera : Ocythoe^ arms of unequal size, no intervening mem- 

 brane, third arm on the right hectocotylised (see Fig. 51, p. 138), 

 two aquiferous pores at the base of the siphon ; male very small ; 

 Tremoctopus^ two aquiferous pores between the eyes, two on the 

 ventral side of the head. 



Fam. 5. Alloposidae. — Mantle edge united to the head by 

 three commissures ; arms extensively webbed, acetabula sessile. 

 Hectocotylised arm developed in a cavity in front of the right 

 eye. — N. Atlantic. 



Fam. 6. Octopodidae. — Head very large, arms elongated, 

 similar, more or less webbed, acetabula usually in two rows, sessile; 

 mantle supported by fleshy bands, no cephalic aquiferous pores. 



In Octopus proper the web is usually confined to the lower 

 part of the arms ; Fischer separates off as Pteroctopus a form in 

 which it reaches almost to their extremity. The third right 

 arm (Fig. 52, p. 140) is hectocotylised, the modified extremity 

 being, according to Hoyle, sometimes minute, sometimes spoon- 

 shaped, with a tendency to transverse ridges, rarely slender and 

 1 Bep. Brit. Assoc. 1844, Transactions, p. 74 ; P. Z. S. 1830, p. 36. 



