PROF. OWEN ON THE ARGONAUT. 



423 



" A second fact, of considerable weight in the debated point of the para- 

 sitism of the argonaut, was afforded by this collection, viz., that in ten of 

 the younger specimens there were no ova in the shell, but the body of the 

 cephalopod occupied the whole of the cavity of the shell, to which it accurately 

 corresponded inform. It was scarcely possible, Mr. Owen observed, to con- 

 template these specimens without deriving a conviction that the body had 

 served as the mould upon which the shelly matter had been deposited ; and 

 with reference to the expanded membranes of the dorsal arms, to which the 

 office of calcification was assigned by Madame Power and M. Rang, these, 

 it should be remembered, were, in fact, essentially productions of the man- 

 tle, and possessed the same structure. It was only in the smaller specimens, 

 however, that the body filled the shell ; when the ovarium begins to enlarge 

 the body is drawn from the apex of the shell, and the deserted place is oc- 

 cupied chiefly by the mucous secretion of the animal, until the ova are de- 

 posited therein. 



" Mr. Owen then reminded the members present, that in former discus- 

 sions on the nature of the argonaut, he had opposed to the parasitic theory 

 an observation made by himself on a series of young argonauts, of a differ- 

 ent species from the Argonauta Argo, all captured at the same time, and 

 exhibiting different sizes and degrees of growth, viz., the exact correspon- 

 dence between the size of the shells and that of their inhabitants, every trifling 

 difference in the bulk of the latter being accompanied with proportional dif- 

 ferences in the size of the shells which they occupied. 1 Madame Power's 

 collection of young argonauts afforded the means of pursuing this compa- 

 rison to a much further extent, and Mr. Owen had not only done so in re- 

 ference to their relative size, but had also weighed the shell and its inha- 

 bitant separately of each specimen, from the smallest up to that in which 

 the ov:i were fully developed in the ovarium. The following tabular view 

 was given of the weights and measurements of ten of the alternate speci- 

 mens in this series. 



1 Zool. Trans. Vol. ii. pt. ii. p. 115. 



2 In each case the cephalopod was removed from the shell, and both were 

 placed on blotting-paper, to absorb the superfluous liquor ; due care was 

 taken to weigh each specimen under conditions as precisely similar as pos- 

 sible. 



3 The disproportionate ratio in the increase of the shell B arises from the 

 additional portion of the shell being thicker and heavier in proportion to 

 the previously-formed part, than in the subsequent periods of growth, so 

 that the increase of weight is in a greater ratio than the increase of size. 



4 Clusters of ovisacs were conspicuous in D to the naked eye in the ova- 

 rium, which had already begun to expand under the sexual stimulus. 



5 The ovarium has now begun rapidly to enlarge. 



6 This admeasurement was taken in a straight line, traversing the longest 

 diameter of the shell ; it was found impracticable to give any constant ad- 

 measurement of the cephalopod, in consequence of the varying state of con- 

 traction and form of its soft and changeable body. 



