426 PROF. OWEN ON THE ARGONAUT. 



of the shell, while no contradictory inference had heen, or could be, dedu- 

 ced from an examination of the specimens themselves. 



" With reference to the second suite of specimens, viz. the ova of the 

 argonaut in different stages of development, Mr. Owen entered into a de- 

 tailed account of the new and interesting facts which they revealed. In 

 the ova most advanced, the distinction of head and body was established ; 

 the pigment of the eyes, the ink in the ink-bladder, the pigmental spots on 

 the skin, were distinctly developed ; the siphon, the beak, — which was co- 

 lourless and almost transparent, — and the arms, were also discernible by a 

 low microscopic power; the arms were short and simple; the secreting 

 membranes of the shell were not developed, and of the shell itself there was 

 no trace. 



" In the second memoir of 1838, published by Madame Power, it is stat- 

 ed that the young argonaut is excluded from the egg, as such, but naked, 

 twenty-five days after oviposition, and that in ten or twelve days more she 

 discovered that they had formed their little shell. Mr Owen regretted 

 that there were no specimens in the present collection exhibiting the com- 

 mencing formation of the shell ; — these were still a desideratum : but he 

 proceeded to say, that the observations on the development of the ova of the 

 Mollusca in general, which science possessed, would be greatly overstated 

 if one per cent, of the known species of Mollusca were allowed to have been 

 subjected to such examination ; he could not therefore admit, or indeed 

 understand, the philosophy of regarding the period of the development of 

 a mere dermal production, like the shell, as being subject to so precise a 

 law, that its non-appearance in an embryo-mollusc, prior to its exclusion 

 from the egg-covering, was to be considered proof positive that such mol- 

 lusc should never thereafter have the power of secreting a shell. Now it 

 was evident, from the observation of Madame Power's specimens, indepen- 

 dently of any statements respecting them, that the expanded membranes of 

 the dorsal pair of arms are not formed until the development of the embryo 

 has far advanced : if, therefore, these membranous arms be, as Madame 

 Power states, the organs of the secretion of the shell, that shell may not be 

 formed until after the exclusion of the young argonaut. 



" The proof that the velated arms possess, like the expansions of the 

 mantle of the Cyprcea, a calcifying power, was afforded by the third series 

 of specimens on the table of the Society. These consisted of six shells of 

 the argonaut, from which Madame Power had removed pieces of shell while 

 the argonauts were in life and vigour, in her marine vivarium. One of the 

 shells had been removed from the animal ten minutes after- the fracture ; 

 another argonaut had lived in the cage two months after being subjected 

 to the experiments : the remaining specimens exhibited intervening periods 

 between the removal of a portion of the shell and its reparation. The frac- 

 tured shell first described had the breach repaired by a thin, transparent, 

 membranous film ; the piece removed was taken from the middle of the 

 keel. In a second specimen calcareous matter had been deposited at the 

 margins of the membrane, where it was attached to the old shell. In a 

 third specimen, in which a portion of the shell had been removed from the 

 keel, about two inches from the mouth of the shell, the whole breach had 

 been repaired by a calcareous layer, differing only in its greater opacity and 

 irregularity of form from the original shell. In the specimen longest re- 

 tained after the fracture, a portion had been removed from the margin of 

 the shell ; here the new material next the broken edge presented the opa- 

 city characteristic of the repairing substance, but the transition of this sub- 

 stance into the material of the shell, subsequently added in the ordinary 

 progress of growth, was so gradual, in the resumption in the repairing ma- 

 terial of the ordinary clearness and striated structure of the shell, that it 



