xliv THE VOYAGE OF H.M.S. CHALLENGER. 



attached to the pleopoda. But large squamose plates exist also in Penseus and its 

 congeners, which also so far as known never carry ova. 



If we turn to Apseudes, that anomalous little genus, which appears in its general 

 character and condition to be rather a Macruran than an Isopod, we find this lateral plate 

 distinctly recognisable as a joint articulating with its somite, and with the basisal joint 

 which supports its two branches attached at its extremity. 



It appears to me that what is true of a small Crustacean is also true of a large one, 

 and this interpretation is most consistent with the homology of these parts. 



If the ecphysis or exopodite be a branch of the second joint, as it is acknowledged to be, 

 then it is clear that the part generally denominated the peduncle of the pleopod must be 

 the second joint, that is the basis, and not the coxa, or the branches arising from it cannot 

 be the homologues of the basecphysis ; or, finally, the branches springing from the first 

 joint or coxa of the pereiopoda must also be homologically the same as those which spring 

 from the second joint or basis ; which is absurd, since it would make the exopodite and 

 the epipodite, or as I have named them, the basecphysis and mastigobranchia, homologous 

 with each other. 



The pleopoda undergo various modifications of form, the three anterior more or less 

 after one type, the three posterior after another ; the first pair in the female varies but 

 little from the succeeding which carry the ova, and what change there may be is in the 

 direction of depreciation. t 



In the Trichobranchiata in the female it is very much reduced^in character, and some- 

 times, as in the Astacidse, it is almost rudimentary ; whereas in the male of the same 

 species it is a large and powerful organ adapted for its special purpose. 



In Astacus it is developed into an almost cylindrical tube, but in the genera belonging 

 to the family of the Eryonidse it forms rather a broad spoon ; and judging from wdiat 

 we have observed of the habits of the higher forms, as exhibited in Carcinis msenas, 

 in which the extremity of the vas deferens is projected into folds of the first pair 

 of pleopoda and inserted into the female, so in these, although in a less perfect manner, 

 the pleopod may be utilised to similarly direct same organ to the entrance of the 

 oviduct. 



In Ibaccus, and I believe in all the Synaxidea, the first pair of pleopoda is wanting 

 in both male and female, but in the Stenopidse it is well developed, but only single- 

 branched, and utilised in the female as an ege-carrier. 



In the Dendrobranchiata even a greater change takes place in the male, and one 

 that is well worthy of close consideration. 



The inner branch is transformed into a very thin membranous plate, which I have 

 called the petasma, and which is capable of very large extension ; it generally lies folded 

 longitudinally in a narrow compass, and is frequently studded at the proximate margins 

 with hooks and teeth which vary in form ; its minor differences are so numerous and 



