No. 1.] VARIATIONS IN LIMULUS POLYPHEMUS. 59 
is permanent, and if very marked seems to lead to the final 
disappearance of the embryo. 
(III) Adsence of tndividual organs. 
A. Atrophy of the thorax. The most frequent defect in the 
thorax 1s the absence of the entire cheliceral segment, a fact of 
striking significance tn connection with its diminutive size 
throughout the entire group of arachnids. 
Further degeneration may do away, one at a time, with the 
succeeding thoracic segments in their order from before back- 
wards, but usually leaving two, or more frequently, the three 
posterior ones, nearly intact. This mode of degeneration may 
not be preceded by the median fusion previously described. The 
missing organs are either absent from the start or else degen- 
erate very early. Meantime the cephalic lobes and oesophagus 
may persist, but in variously modified conditions, as shown in 
Pigs: 26," 27; 33, 34, 35, 62, 67. These facts show that the 
anterior part of the thorax 1s the weakest part and most likely 
to be absent or to degenerate, and that this weakness gradually 
diminishes towards the posterior end. 
B. When the cephalic lobes show partial degeneration, the 
reduction seems to take place first along thetr anterior margins, 
and to progress backwards independently of anterior degeneration 
in the thorax. The law of degeneration of the cephalic lobes as 
here stated is not so clearly shown as in the case of the thorax, 
on account of the difficulty of distinguishing the parts. But 
ievisecms. toy hold good in Figs. 18, 20, 21, 22, 27, 47.) Tims 
perfectly certain, however, that the cephalic lobes and oesophagus 
may be preserved after every trace of one or more segments behind 
them has disappeared, as in Fig. 47 and in Fig. 25. 
The area where degeneration of the anterior margin of the 
cephalic lobes has taken place, and which has not been covered 
by the contracting marginal fold, is nearly always flattened and 
depressed, as shown in Figs. 10, II, 16, 20, 27, etc. 
C. In the abdomen, the same law of degeneration seems to hold 
good, t.e. the most anterior abdominal appendages and neuro- 
meres are the first to degenerate, the posterior ones being the 
most persistent. It should be borne in mind that the normal 
growth of the abdominal appendages is similar to that of the 
