162 



The four pairs of le<^R now described form an .interior series, 

 differing considerably in form from the remaining three which 

 are similai- to one another and form a posterior series, the members 

 of wliich increase in size posteriorly. 



They?/5'/i tJtoracic leg (third per eiopod) (See Plate xxv., fig. 3) 

 is slightly longer than the fourth. The basos is produced poster- 

 iorly into a thin flat expansion with convex margin fringed with 

 long setae — much in the same way as in many Amphipoda. The 

 ischios is large, about two thirds as long as the basos and is sub- 

 rectangular, though expanding somewhat distally; it bears tufts 

 of stout seta^ on both margins. The meros, carpus, and propodos 

 are all similar and subrectangular, but each is longer and narrower 

 than the preceding, they all bear numerous tufts of stout spiniform 

 seta^ on both margins. The meros ha<? the postero-distal angle 

 slightly produced as in many Amphipoda. 



The si:rfh and sei-enth thoracic legs ( fourth n\\(\ fifth pereiopoda) 

 are similar to the fifth but are much largei', the various joints are 

 all similar to the corresponding joints of the fifth leg, but they 

 bear a greater number of setfe, and the setfe themselves are stouter, 

 the basos has the posterior margin more produced and more con- 

 vex and the dactylos is longer and more slender. 



Figure 3 of Plate xxv., shows the last (seventh) leg, but will 

 do almost equally well to represent the general appearance of the 

 fifth and sixth. 



It is worthy of note that although these thoracic legs are very 

 An)phipodan-like in general appearance, they all have the ischios 

 large and well developed instead of being small and very short as 

 in almost all the Amphipoda, and that although the first pair of 

 legs has a well developed subchelate hand, the second pair is quite 

 simple, while as a general rule both pairs are more or less sub- 

 chelate in the Amphipoda. The resemblance of these legs to 

 those of the Amphijwda is therefore more superficial than real, 

 and a comparison of my figures with those given by Sars of Asellus 

 aquatiocs will show that there is a close general resemblance 

 lietween the two. 



I have called all these legs "thoracic legs" instead of speaking 

 of the first two pairs as " gnathopoda " and tiie remainder as 

 " pereiopoda ' as is usually done in the Amphipoda. This would 

 have made an arbitrary and misleading distinction between the 

 second and third pairs which are precisely similar to each other, 

 while it would probably have led to confusion if T had spoken of 

 them all as "pereiopoda," as tho.se of the last pair would then be 

 the seventh, pereiopoda, while if the plan adopted with the Amphi- 

 poda were adhered to they would be the fifth pereiopoda. It 

 seems to me a pity however that the term "pereiopoda" has not 

 been applied to all the appendages of the pereion both in the 

 Amphipoda and Isopoda, leaving the term "gnathopoda" a special 



