No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 69 
A similar case is shown in the single embryo in Fig.52. A 
comparison of this embryo with the preceding leaves little 
doubt that the constriction has taken place between the third 
and fourth segments. The cephalic lobes are much degener- 
ated, and the chelicerae are absent. The modification of the 
remaining appendages is easily determined from the lettering. 
In Figs. 54 and §5 are two other illustrations of transverse 
fission accompanied by degeneration. In Fig. 55 the anterior 
portion of the thorax, consisting of three pairs of appendages, 
but without cephalic lobes, oesophagus, or neuromeres, is widely 
separated from the posterior portion, which consists solely of a 
conical projection representing either an enlarged caudal lobe, 
or else the last pair of fused, thoracic appendages. One or two 
small pits in front of it indicate, probably, where the other 
posterior thoracic appendages have disappeared. In Fig. 54 is 
a curious modification not easily explained. It appears to be 
due to a transverse constriction separating the thorax, the only 
part that is left, intotwo portions. The two pairs of appendages 
of the anterior part have fused to form a median row, the 
fusion being indicated by the great length of the two twisted 
median appendages. The appendages were apparently absent 
on the right side of the posterior portion, so that it is spirally 
twisted. 
In Fig. 53 an earlier stage of the same process is shown. 
There has been a partial constriction between the third and fourth 
pairs of thoracic appendages. In the anterior portion the third 
pair have fused, the second are still widely separated, while 
the first pair and most of the cephalic lobes are absent. In the 
posterior portion the fourth pair are fused, and the right fifth 
and sixth appendages, including their neuromeres, are absent. 
The fifth and sixth appendages of the left side are very large, 
and owing to the unequal bilateral development, thrown spirally 
toward the right. The distortion of the axial line is also shown 
by the position of the telopore, 72. 
Still other embryos where median fusion has played a con- 
spicuous part, but which still show evidences of transverse 
fission, are shown in Figs. 56, 57, and 58. In Fig. 58 the ante- 
rior projection probably represents the remnant of the fused 
