Respiration in Invertebrate Animals. 197 



authoi'^s examinations have extended^ he has succeeded in 

 establishing only two main varieties, — that of Octopus and 

 Sepia (PI. XV. tig.l), in which the secondary lobuli circumscribe 

 a hollow axis, by curving from the dorsal to the ventral aspect y 

 and that, 2ndly, oiLoliyo and Loligopsis (PI. XV. fig. 3, 3^), in 

 which the secondary parts of the organ pass only half-way 

 round, and float freely in the branchial chamber, instead of 

 being, as in Octopus, tied down to the longitudinal pallial 

 muscle upon which the gills rest. He has no doubt, however, 

 that other modifications of structure in the branchial organs of 

 this class exist ; but, in consequence of the want of specimens, 

 he is not at present in a position to speak of them. The Di- 

 branchiate order is thus obviously resolvable into two well- 

 marked suborders, — the one in which the gill forms a cylinder-, 

 the second, in which it forms a hemicylinder . This distinction 

 has never before been noticed. 



No example of the Tetrabranchiata has ever fallen under my 

 notice. Prof. Owen states that they stand in closer connexion 

 with the Gasteropod Mollusks than the Dibranchiala. In Nau- 

 tilus Pompilius this distinguished observer figures the branchiae 

 as having a completely formed cylindrical arrangement. 



It must therefore be concluded that the Tetrabranchiata are 

 lower than the Dibranchiata in the Molluscan scale, and that 

 Octopus should stand beneath the Calamary in zoological rank. 

 This interesting fact, as the ground of classification, is com- 

 mended to the attention of those who may enjoy opportunities 

 of studying the organization of this most interesting class of 

 animals. In accordance with this view, which regards the gill 

 of Octopus as ranking below that of Loligo, let us proceed first 

 to the detaileji description of the lower variety. 



The gills in the genus Octopus are strikingly distinguished in 

 apparent structure from those of the other families of the Di- 

 branchiate tiibe. Although only two in number, in all the 

 species of this order they exhibit, as stated before, very extra- 

 ordinary diversities of shape. In the size and disposition of the 

 branchial chamber there is no corresponding variation : it is 

 nearly the same in all species. In Octopus the branchia on 

 either side is attached in a peculiar manner to the ventral wall 

 of the breathing-chamber. The dorsal surface is free and un- 

 attached : it is so situated as to float on a thin supporting 

 ridge. The water thus rushes into and out of its interior with 

 equal facility. The branchial artery, or afferent venous trunk 

 (fig. 1 b), lies on and courses up along the ventral side of the 

 organ : it emanates from the branchial hearts*. The branchial 



* It will afterwards be shown that these cordiform dilatations of the blood- 

 channels in the vicinity of the branchiae may be viewed as mere safety 



