1916-17.] Experiments and Observations on Crustacea. 2 75 
read : “ Each consists of a large plate formed by the expanded protopodite 
with the small endopodite at its tip while the exopodite is vestigial or 
absent,” to which succeed the following sentences as a footnote, “ What is 
here called the exopodite is usually regarded as the endopodite, and vice 
versa. The interpretation given above depends on the assumption that the 
uropods have reached their present position by a movement of rotation , 
not of simple translation.” As to the constitution of the uropodal sym- 
podite I take the following specific statement from Pfefier (1887) : “The 
first segment cannot be made out, having disappeared completely in the 
tail-shield. The second segments are large plates which completely cover 
the plates of the pleopods.” 
In failing to specify the exact nature of the rotation assumed, Caiman’s 
footnote is not clear. Pfefier ’s statement as to the fate of the original 
uropodal sympodite is apparently at variance with that implied in Caiman’s 
phrase “ formed by the expanded protopodite.” I will here attempt to 
show (1) that the large plate of the uropod is a compound piece, and (2) 
that what is usually called the exopodite of the uropod is a real exopodite. 
Noting the oblique position of the long axis of the sternal foramina 
corresponding to the last two pleopods, one concludes that the anterior 
border of the large uropodal plate is its original medial border. The 
anterior part of this plate is prolonged in front of its articulation with the 
sixth pleonic segment ; so too is each pleopodal sympodite (or rather the 
most distal segment thereof) prolonged medially beyond the articulation 
with the corresponding sternite : this probably means that the greater part 
of the uropodal plate corresponds to the distal segment of the pleopodal 
sympodites. 
More conclusive evidence as to the orientation of the uropods is, however,, 
obtained from the form of the articular foramen in the sixth pleonic segment, 
which shows an articular spur projecting backwards and laterally from the 
anterior border of the foramen — see fig. 11. It is evident that this articular 
spur serially corresponds with those in the other sternites ; its direction 
shows that the sixth sternite as a whole has undergone a rotation more com- 
plete than that of the fourth and fifth sternites, but with the same angular 
sign. This means that the lateral surface of the opened uropod corresponds 
to the anterior surface of the pleopods, the medial or gill-directed surface 
of the uropod corresponding to the posterior surface of the pleopods. The 
mode of articulation of the uropodal plate with the sixth sternite by means 
of an articular spur is evidence that the plate is compound. 
Having determined by what kind of rotation the uropodal sympodite 
has acquired its present position, it is easy to settle the relationships of 
