ORGANS FOR SPERM-TRANSFER 271 
The pleopods of the second, third, fourth and fifth somites 
of both males and females are represented at the time of hatch- 
ing and all alike have the appearance seen in fig. 26 which is mag- 
nified 75 diameters and represents the anterior face of the third 
left pleopod of a male'18 mm., in July, when in the fifth larval 
stage. The pleopod is flat and translucent; the endopodite (/n.) 
is longer than the exopodite (#v.) and both are fringed by long 
setae that are really plumes, though not so figured. Both endo- 
podite and exopodite are obscurely jointed and the protopodite 
has a short annular segment as well as a long main segment. 
Through the thin shell may be seen the muscles, represented by the 
dotted lines. At the base are three large and one minute muscles; 
two of the main three are posterior and one anterior, and appar- 
ently the movement of the entire appendage would be a more 
powerful backward swing and weak forward recovery, as in swim- 
ming. Within the main segment of the protopodite are three 
long muscles that would seem to aid in bending the appendage at 
its base, while distally there are two muscles which both go to the 
exopodite to move it. The endopodite is left with only intrinsic 
muscles to move it at its base and with a long branched muscle 
that can act only to bend the endopodite itself. The exopodite 
has also intrinsic muscles at its base as well as the muscles of the 
protopodite to move it. There is likewise a long branched muscle 
to bend the exopodite. 
In the early stages the second appendage of the male is quite like 
this third pleopod, but in a male of 21 mm. (probably in the same 
larval stage as the male having the third appendage shown in fig. 
26) we find the pleopod of the second somite modified as in fig. 
27, that is, there has been added to it the excrescence seen on the 
median side of the endopodite. This is to become the triangle or 
appendix masculina of the adult. 
The first discovered trace of this outgrowth was seen in a larva 
of the fourth stage, 11 mm. long, in July. This first beginning 
of the triangle is the slight elevation (a) seen in fig. 25, on the side 
of the endopodite. This figure represents only that part of the 
endopodite which is not well jointed and forms a sort of base 
beyond which is the more flabelliform distal part, (fig. 26). It 
JOURNAL OF MORPHOLOGY, VOL, 22, NO, 2 
