REPORT ON THE CRUSTACEA MACRURA. xlvii 



The Third Pleopoda. — The third pair of pleopoda likewise exhibits some of the 

 characteristics of those preceding, but in many genera, and more especially in those in 

 which the anterior pairs depart less from the common type, it differs but little from those 

 that follow. This and the following two are the chief egg-carriers in the Trichobranchiata 

 and Phyllobranchiata, although in some genera they may be found on the second, and in 

 Stenopus a few ova are attached to the first pair. 



The Fourth and Fifth Pleopoda. — These pairs are generally only broad and leaf-like 

 appendages. 



The Sixth Pleopoda. — The sixth pair of pleopoda is the only feature that is in- 

 variably constant, existing in a more or less perfect form throughout the whole of the 

 Macrura. 



Its articulation with the somite differs from the preceding by the reduction of the 

 large coxal plate to a rudimentary condition, and in its being placed at the postero- 

 lateral angles. The basisal joint, instead of being long as in those anterior to it, is short, 

 and the terminal branches are generally broad and leaf-like, but in some instances slender 

 and style-like, as may be seen in Tludassina (PI. III.), in Clieiroplatea (PI. I. fig. v.v), 

 and in other aberrant forms through the Anomura, where it may be seen in a rudimentary 

 condition in the cancriform genera. 



In the course of development it is the first appendage present of those that belong to 

 the pleon, and it appears almost simultaneously with the antennae, while the pereiopoda 

 are yet in an incipient condition, and the other pleopoda are visible only as immature buds. 



The basis, or peduncular joint, is short, and the two branches generally bear a strong 

 resemblance to each other, although each of them possesses features peculiar to itself. 

 The inner branch is generally the more flexible, and has the two margins similar and 

 fringed with hairs, whereas, the outer is generally slightly longer than the inner; the 

 outer margin is smooth and strengthened by a strong rib, and is ciliated with hairs on 

 the inner side and distal extremity only ; it is divided by a diaeresis, or transverse line 

 of articulation. This feature is one that is very general in the Macrura, being present, 

 so far as my experience teaches me, in each division, and absent only in the genera that 

 belong to the families Palinuridae, Scyllaridse, and Eryonidae. 



In the Dendrobranchiata the diaeresis is, I believe, universally present ; but in some 

 genera, as Benthesicymus and Gennadas, it is imperfect in character, although, even in 

 these, a small bundle of muscular tissue occupies the position where the line of diaeresis 

 should be (PI. LV. fig. lr), thus demonstrating by its presence that previously there 

 existed an articulation between the two parts ; the muscles, ceasing to be of use, have 

 gradually diminished in size and value. This pair of appendages bears a considerable 

 analogy to the second pair of antennae, the outer branch representing the scaphocerite ; 

 and to add to this comparison we find in the Atyidae that the scaphocerite is furnished 

 with a diaeresis of similar character. 



