196 ON THE ANATOMY AND AFFINITIES OF 
articulation is a broad joint produced internally into a narrow, 
transversely elongated, flattened process, terminated by elongated 
setigerous sete externally, into a broad flat plate bearing eight or 
ten extremely long and strong setigerous seta; five other obscurely 
marked joints follow these two. 
The next appendage (fig. 5) is not quite so long as that just 
described ; it is obscurely divided into seven joints and tapers to 
its extremity, which, like its inner edge, is provided with very long 
and strong sete. The inner sete are by far the stronger, and are 
directed forwards so as nearly to reach the mouth. 
The fourth post-oral appendage (fig. 6) is more than twice as 
long as the third. It consists of a short and strong, subcylindrical, 
basal joint, beset along its inner margin with a row of very long 
setigerous sete, and having a rounded projecting inner distal angle. 
One long and five short gradually diminishing articulations follow this. 
To this appendage succeed the five pairs of swimming feet 
(fig. 7), of which the first and the last are smaller than the inter- 
mediate ones, Each natatory limb consists of a stem (protopodite) 
and two branches (endopodite and exopodite). In the former a long 
proximal and a short distal joint are distinguishable. The proximal 
joint is united, for a considerable part of its length, to its fellow on 
the opposite side by a flattened plate, continuous above with the 
sternum, and thickened by a spatulate ridge in the middle line. 
The distal joint has the inner half of its terminal margin more 
produced than the outer. The former bears a shorter (endopodite), 
and the latter a longer, three-jointed branch (exopodite). The 
terminal articulation is the largest, and is provided with long and 
strong sete. 
The inner edges of the basal joint of the protopodite of the fifth 
pairs of these feet are beset from their distal extremities to the uniting 
plate with a series of short, but strong and pointed teeth. 
On considering the structure of Calanus with reference to the 
general plan of the Crustacea, 1 may observe, in the first place, 
that of the twenty typical somites, only eighteen, at most, appear 
to be represented. \Ve may reckon one, for the eyes; two, for the 
antennules and antenne ; four, for the four pairs of post-oral 
appendages ; and five, for the swimming feet, making, in all, eleven, 
provided with moveable appendages. Then, if the first segment 
of the abdomen be really formed, as there is much reason to believe 
it is, by the coalescence of two somites, and if the two caudal styles 
really represent another, we shall have six somites in the abdomen, 
making up the sum total to eighteen. 
