174 THE VOYAGE OF H.M.S. CHALLENGED. 



towards the distal extremity, where it is prolonged into a flat and narrow pollex that 

 articulates with a similarly formed dactylos. The entire appendage is spinous, the spines 

 being of a similar character to those on the body of the animal. They are larger on the 

 upper and lower margins of the meros, the largest existing at the upper distal or carpal 

 angle. A large tooth or spine exists on each side at the base of the dactylos. The 

 dactylos is spinous on the inner and outer margins, having very regularly three short 

 to one long tooth on the outer margin, and four or five short to one long one on the inner 

 side ; this is the case with the inner side of the pollex also, the outer side of which is 

 smooth. 



The second pair of pereiopoda is very slender, smooth, chelate, and cylindrical ; the 

 meros and carpos are subequally long, and the propodos is short. 



The third pair is slightly spinous, much more so than either of the others, and more 

 robust than the second ; the carpos is shorter than the meros, but larger than the propodos. 



The fourth pair is about the same length as the third ; the five first joints are spinous ; 

 the propodos is slender, longer than the carpos, and smooth ; the dactylos is slender and 

 styliform ; the meros is armed with a strong tooth at the upper distal angle. 



The fifth pair resembles the fourth in most points, except that the propodos is still 

 longer than the carpos, and has the inferior distal angle (o) produced to a point that 

 resembles a short, obtuse pollex ; the dactylos is fringed with hairs on the outer side. 



Each of these pairs of appendages, except the posterior, carries a mastigobranchia and 

 its corresponding podobranchial plume, and two arthrobranchise. The four posterior 

 pairs carry, moreover, a corresponding pleurobranchial plume. 



The branchiae may be arranged according to the following table : — 



Pleurobranehi®, . . 1 1 1 1 



Arthrobranchise, 



Podobranchiaj, 



Mastigobranchise, 



The podobranchise and pleurobranchise are the largest, and the arthrobranchise are the 

 least important ; the four plumes correspond to each other according to the position shown 

 in the diagram on PL XXII. The podobranchiae {pel. b.) overlie the others, and the pleuro- 

 branchise (pi. br.) are below them ; lying between them on each side, anteriorly (ar. b.) and 

 posteriorly (arth), are the arthrobranchise, having but a single row of digital processes ou 

 the inner side, and these lie between the base of the podobranchia and the upper surface 

 of the pleurobranchia; the outer surface of each is furnished with a number of closely- 

 packed digital processes, the various parts being kept separate by numerous long and 

 slender hairs attached to the base or stalk of the podobranchial plume, and also to the 

 surface of the anterior and posterior mastigobranchise (m. b.) which divide and separate 



