No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. 55 
length and irregularity in shape, which is very unusual, indicat- 
ing that they fused nearly simultaneously and to an equal degree. 
In Fig. 93 the left-hand embryo, at its anterior end, is medi- 
anly fused and partially degenerated. The minute pit oc. 
probably represents the remnant of the fused second pair of 
appendages. 
In Fig. 95 fusion and degeneration result in the formation 
of a slipper-shaped embryo in which nothing is left but a single 
median appendage and a pit that may represent the last of an 
appendage, or perhaps the fused dorsal organs. 
In Fig. 98 is represented an extremely interesting condition, 
due in part to transverse constriction or fission. The constric- 
tion, followed by fusion, and finally by degeneration, takes place 
between the third and fourth pairs of appendages, and dimin- 
ishes from that point in both directions. In Fig. 103, embryo 
A, the same process is carried a little farther, all the anterior 
appendages being fused to form a row of four median projec- 
tions which preserve approximately the relative proportions of 
the three pairs of appendages from which they arose. In the 
same figure, fusion and degeneration are carried further in 
embryo 4, and further yet in embryo C, where nothing is left 
but the fused dorsal organ at c, and the last two fused thoracic 
appendages. 
In the triple embryo shown in Fig. 104, all three individuals 
are in essentially the same condition as embryo C of Fig. 103. 
In Fig. 102 we have a triple monster in which embryo 4 is 
nearly normal. JZ has undergone transverse fission, as in Fig. 
go, and the anterior half has undergone degeneration till nothing 
is left but a single median projection representing an imper- 
fectly fused pair of appendages. Embryo C is represented by 
the fused sixth pair of legs and by traces of the abdomen. 
It is an interesting fact that in nearly all cases when there 
has been undoubted antero-posterior fusion and degeneration, 
the abdomen with its appendages is nearly normal and very 
well preserved. This is somewhat surprising, as the abdomen 
is usually much abbreviated and often entirely absent in em- 
bryos that show a tendency to abnormality in other respects 
than in multiple fission or in median fusion. 
