No: 1.]] VARIATIONS IN LIMULUS POLYPHEMUS. II5 
brain and optic ganglion farther than usual. Otherwise the right side appears to 
be normal. 
The left sixth thoracic appendage is a small, three-jointed organ, showing 
distinctly the characteristic flabellum near its lateral margin. The fifth is a mere 
papilla, and the third and fourth are entirely absent. The second is relatively 
large, and seems to be partially invaginated at the tip. The left chelicera is 
absent. In the left half of the abdomen the appendages are partly fused, forming 
a great three-lobed mass. 
It is, therefore, plain that while the left half of the thorax is greatly reduced, as 
a whole, it shows in addition a diffuse transverse atrophy with its line of greatest 
intensity between the third and fourth appendages, and diminishing gradually in 
front and back of this line. We thus have a case of hour-glass atrophy, confined 
to the left half of the body. (Compare Fig. 29.) 
The nervous system is not sharply outlined. The parts of the cephalic lobes 
are run together, and a great dark fold extends diagonally backwards over the 
right half. The left is much smaller, and has been carried backward by the 
contraction of the left side. The oesophagus and mouth are small and inconspic- 
uous. It is not clear from surface views whether a part of the left nerve-cord is 
absent or not. 
The lateral eyes and dorsal organs were in their normal positions on the right 
side, but could not be seen on the left. 
The marginal fold is normal on the right. On the left it disappears near the 
optic ganglion. There is a large, laterally directed fold opposite the left sixth 
appendage that consists, in part at least, of the marginalfold. In the abdomen the 
left marginal fold is normal, and it meets the fold just described at a sharp angle, 
but does not appear to be continuous with it. Between the two folds is a very 
deep triangular depression. This condition of the marginal fold is the usual one 
when there has been atrophy of the anterior quarter of the same side. 
In the anal region the marginal fold is greatly thickened, forming a conspicuous 
U-shaped boundary to the posterior part of a deep depression. 
Fic. 37, X 40. This embryo is, in part, in an advanced stage of develop- 
ment, corresponding to that seen in embryos about ready to hatch. There are 
only a very few markedly abnormal embryos found in this late stage. 
The right side is nearly perfect and normal. The ganglionic fold over the 
right half of the brain is large, as in the preceding figure. The right nerve- 
cord, which was very well developed and plainly outlined, is bent sharply at 
the junction of the thorax and abdomen. This bend, as seen in the drawing, 
is due, to a small extent only, to the foreshortening produced by looking down 
on the upturned abdomen. Almost the entire left half of the cephalic lobes 
are absent, but the left nerve-cord, as far as the beginning of the abdomen, 
is present in its normal condition. All the left thoracic appendages have 
disappeared completely, leaving no trace behind except in the third segment, 
where there is a shallow depression, seen edgewise in the figure, with a small papilla 
projecting from its centre. This papilla probably represents the last trace of the 
third thoracic appendage. The left nerve-cord seems to terminate abruptly at 
the posterior end of the abdomen, without uniting with its abdominal part. The 
anterior end of the latter turns off sharply to the left, toward a large, dark, 
conical elevation. To the left of it is a short, conical projection, with its apex 
directed forwards. 
