198 Dr. T. Williams on the Mechanism of Aquatic 



vein or efferent trunk (fig. 1 a) runs along the dorsal border of 

 the gill, and terminates in the aorta (these two trunks are 

 seen in section, fig. 2«, i). These two longitudinal trunks 

 constitute the framework by which the entire apj^aratus of the 

 gill is supported. To them are attached respectively the two ends 

 of the secondary lobuli in Octopus : in Luligo, as will be again 

 explained, a slight variation from this type occurs. The plan 

 of this structure is readily understood on a transverse section of 

 the gill (fig. 2). In the gill of Octopus viilgaiis there are 

 twelve pinnae or secondary lobuli on either side (fig. 1 d,d). Along 

 the external and internal margins respectively (fig. 2?,z, & e,e), 

 efferent and afferent vessels are observed to travel. These second- 

 ary branches, like the primary trunks from which they proceed, 

 serve to support, in their turn, the tripinnaj [g), or the ultimate 

 leaflets in which are distributed the final capillaries of the 

 branch i 86. As observed by Professor Owen, the gills of this 

 Cephalopod are tripinnate. This general term, however, serves 

 but very rudely to express the extreme refinement of structure 

 and arrangement which these organs exhibit. The secondary 

 and tertiary divisions are so much longer than the straight space 

 between the points to which they are fixed, that in the ordinary 

 state of the animal, after death, they present remarkably com- 

 plex foldings and convolutions ; but when the organ is in the 

 condition of full and complete distension (with blood in the 

 vessels and water in the hollow axis), the secondary lobuli and 

 their appended system of leaflets are straight or smooth laminae 

 of exquisite slenderness, delicacy, and translucency. Although 

 the secondary divisions (fig. 1 d, d) amount to no more than 

 twelve in number on either side, the tertiary laminae [c, c) which 

 each of them supports are as many as from twenty-five to 

 thirty on either side. The multiplication thus insured is ex- 

 traordinary. The secondary lobuli are separated from each 

 other by free, open water-passages (fig. 2 h). It is through 

 these spaces that the fresh inspiratory streams enter the axial 

 interior of the gill (fig. 2). No exact parallel to these spaces 

 is found in any other MoUusk. The water, indeed, rushes be- 

 tween the secondary divisions of the gills in all the Pectini- 

 branchiata. In these latter, however, the organ does not cir- 

 cumscribe a hollow interior. The tertiary branchial foliage is 

 disposed at right angles on the secondary, just as the latter rest 

 at right angles on the main primary trunks. According to this 

 arrangement, no obstacle is offered to the rapid and free passage 



receptacles for the I'etrogressing column of blood (which is either stopped 

 or thrown back in its course during the extreme changes of size which the 

 gills are constantly undergoing), with quite as much propriety as they are 

 now considered to be propulsive hearts. 



