290 ME. GEOEGE M. THOMSON ON A FEESHWATER 



Second maxillce (PL XXIV. figs. 7 and 8).— These are thick, fleshy, and 4-branched, 

 each branch being 1-jointed. The broad basal portion which bears these branches 

 is rounded on its outer edge, which bears a few small spines ; at its upper and outer 

 extremity it has a small crown of setse at the articulation of the outer branch. This 

 basal portion is longitudinally a little bent on itself, so that while the two outer branches 

 on each side work against each other, the two inner lie more in the same plane and move 

 obliquely one against another. The first (outer) branch is short and rounded, and, as is 

 the case with the second branch, ends in a great number of setse, which are somewhat 

 curved inwards and are toothed along their inner margins. The third branch resembles 

 the narrow second one in form, but the setae are not toothed. The fourth (innermost) 

 branch is setose along its inner margin, and is expanded on its outer into a thin rounded 

 lamella which partly covers the third branch. These four setose plates are evidently 

 for the purpose of preventing particles of food from escaping from the oral aperture. 



3IaxilU])eds (PL XXIV. figs. 13, 14, and 15).— These organs are distinctly pediform, 

 differing from the succeeding pairs of legs only in the degree of their development. They 

 are 7-jointed. The coxal joint bears on its outer side two lamellar bi-anchite, each of 

 which is obliquely crossed near its base by a tine line of articulation ; the upper of these 

 is more than twice as broad as the lower, which is long and very narrow. On its inner 

 side the coxa bears two small lameUar organs fringed all round with setae, the inner and 

 narrower of which stands alongside, but at right angles to, the outer. These setose 

 plates serve to close the oral aperture behind. The upper portion of this joint bears at its 

 outer extremity a slender appendage which is subequal in length to the rest of the 

 joint. This exopodite appears at first sight to be unjointed, but is seen to bear a minute 

 articulation at its apex. 



The basos is Ions, and is rounded on the anterior or outer side, while the keeled inner 

 side bears a double row of plumose setae. The ischium, which is similarly shaped in 

 section, is about half as long, and the meros is as long, as the basos. The two succeeding 

 joints form a distinct — and in spirit-specimens a sharp — geniculation with the meros, 

 being bent in towards the middle line of the body ; they are both somewhat shorter 

 than the meros, and are rather densely setose. The propodos is more flattened than the 

 carpos, and is fringed with numerous curved hairs on both margins, while the dactylos 

 ends in three powerful hooked claws, which are almost hidden among setae. 



These organs I have termed maxillipeds, as in position they correspond to those 

 appendages in other Crustacea; but they might more appropriately be called the 

 first pair of loalk'mg legs. 



The next seven pairs of legs resemble the preceding pair in general structure, but differ 

 considerably in detail ; they all want the setose lamellae on the inside of the coxal joint, 

 which indeed appear to act partly as oral organs, but several of them have a small pointed 

 lobe which may be a homologous development. The exopodites and branchiae are 

 differently developed in all the limbs, while the basal joint is shorter than in the first 

 pair. In the second to the sixth pairs, the long plumose exopodites apparently serve as 

 natatory organs and facilitate progression though the water. 



In the second pair of legs (PL XXIV. fig. 16) the joints are similar in number, shape, 

 and relative length, as well as in distribution of setae, to those of the first pair. The 



