No. I.] VARIATIONS IN LIMULUS POLYPHEMUS. 63 
Limulus, as shown by their more frequent absence and median 
fusion, is comparable with the small size and frequent median 
fusion of the first three pairs of post-oral appendages in insects 
and other arthropods. 
(c) There is at the anterior end of the abdomen a region of 
condensation, specialization, and degeneration, forming what I 
have called the vagus region of scorpions and Limulus, and which, 
in a paper on “ The Origin of Vertebrates from Arachnids,” 
I showed was probably of wide distribution in the arachnida, 
including the trilobites and related forms. In scorpions, it 
consists of four very much condensed metameres, and in 
Limulus of two or more, which in both forms are provided with 
rudimentary appendages, nearly or quite fused. 
(2) Finally, the very wide distribution of well-developed 
terminal appendages through all groups of arthropods is a 
manifestation of the same law of growth. 
In conclusion, therefore, it would appear that the division of 
the body of arthropods into successive regions composed of several 
segments 1s not due primarily to specialization or adaptation, 
either by or for any particular use, or through disuse, but to 
some form determining forces that govern growth. The same 
factors in all probability produce in a similar way metamertc 
segmentation, transverse fission in annelids, and determine 
the length of the body in a given individual. 
(IV) The complete degeneration of the whole embryo is an 
obscure process, and probably varies considerably in different 
individuals. A continuation of the various local degenerations 
previously described has, without doubt, gradually produced 
the misjointed fragments of embryos seen in Pls. VI and 
VII. Théy admirably illustrate Empedoclian fancies. The 
embryos themselves give some indication of the various ways 
in which they have degenerated. They also indicate that the 
final stages of degeneration lead to a tolerably uniform con- 
dition. 
With the disappearance of all the appendages, the embryo 
may, in the class of cases we shall now consider, be reduced to 
a mere pit or sac, yet preserving certain features which show 
