•J4 THE VOYAGE OF 1 1. M.S. CHALLENGER. 



decrease from the base to the apex ; but for about one-half the distance there appears to 

 be a. stronger and more important fibrous arrangement than there is within the remainder ; 



and, coinciding with this variation in internal structure, there is an alteration in the 

 direction of the small stiff cilia, that run spirally round the surface externally — those 

 towards the base being directed backwards, those towards the extremity being' directed 

 forwards and spirally reversed. This appendage, from its extent and position in the 

 branchial chamber, evidently plays an important function in the branchial system, 

 situated as it is at the anterior opening, and lias probably the control over the circulation 

 of the water within the branchial chamber. 



The third pair of siagnopoda has not even the rudiment of any mastigobranchial plate. 

 In this it differs from most of the higher Crustacea. Its position is so forced in and com- 

 pressed that there appears to he scarcely room for the requirements of such an addition, 

 and is absent accordingly. 



The mastigobranchia of the firsl pair of gnathopoda is short, bifid, almost rudimentary . 

 furnished with long rigid hairs at the extremity, and carrying at its base a short podo- 

 branchiaJ plume. This plume consists of long trichobranchiate filaments, until they 

 approach the extremity, when I hey assume a pbyllobranchiate character, the petals of 

 which are arranged in a. position longitudinal to the axis of the plume. Above, on the mem- 

 branous articulation, are two arthrobranchiae. The anterior is trichobranchiate, with the 

 exception of two other phyllobranchiate petals that terminate the plume. This plume is 

 also short, being scarcely as long as the podobranchia. The posterior arthrobrancbia is 

 very long and entirely phyllobranchiate, with the exception of a few filaments of an in- 

 termediate character at the base. These latter, as they approximate the phyllobranchiate 

 petals, increase their diameter at the base, and become flattened, and so gradually pass 

 from one form into the other. The petals are arranged longitudinally, with the longer axis 

 of the plume 1 on each side of the median line — the longer on the anterior and the shorter 

 on the posterior margin, the longest being the terminal petals, each of which is traversed 

 by a set of channels, without definite walls, that assume an arborescent appearance. 



The mastigobrancbia attached to the second pair of gnathopoda is short, although a, 

 little longer than that of the first pair. It is slightly curved, rigid, furred with numerous 

 short hairs along the lower margin, and subapically tipped with long, straight hairs (which 

 have been accidentally omitted by the lithographer). It supports at its base a long ami 

 well-developed podobranchial plume, consisting of a series of phyllobranchiate petals 

 traversing the whole length of the under margin of the longitudinal axis of the plume : 

 while a, series of trichobranchiate filaments are attached to the base. On the coxa, near 

 the branchial articulation, are two fasciculi, or bundles of straight hairs. There are two 

 arthrobranchiae attached to the membranous articulation; the anterior is rather shorter 

 than the posterior. Both have a mass of trichobranchiate filaments attached to the base 

 of a long plume of phyllobranchiate petals of considerable number and size. In the 



