30 A TMEV [VoL. XII. 
some future theories of development, find their proper place 
and partial explanation. 
It seemed inadvisable to add further to this paper by detailed 
reference to the voluminous literature bearing only indirectly 
on the facts here set forth. 
So little is known about variations in arthropod embryos, 
and especially arachnids, and the facts we present are so differ- 
ent in character from those already known, that no injustice 
will be done previous workers along similar lines by not refer- 
ring in detail to their publications, The only reference to an 
abnormality in Limulus that I have been able to find is in an 
article on “ Diploteratology, An Essay on Compound Human 
Monsters,” by Geo. J. Fisher, Albany, 1868. A good figure 
is there given of an adult (?) animal with a double caudal spine 
and a symmetrically forked abdomen. 
DESCRIPTION OF THE DIFFERENT CLASSES OF VARIATION. 
I. INVAGINATION OF APPENDAGES. 
This remarkable modification is of comparatively common 
occurrence in forms which are in other respects more or less 
abnormal. It is confined, so far as I have observed, to the 
thoracic appendages, and most commonly affects the middle 
ones of the series. 
It may begin after the appendage is fully formed as a minute, 
slit-like depression at its distal end, Figs. 10, 11, th. ap, 
th. ap. The slit is always transverse to the long axis of the 
body, and appears in the stained specimens as a fine line in 
the middle of a clear band devoid of nuclei. When the invagi- 
nation is complete, the whole appendage is carried inward, so 
that in its place is an opening leading into a deep tube with a 
flattened, conical lumen. 
The third or fourth appendages on either or both sides may 
be invaginated, Figs. 10, 11, or, as in one case —the only one 
observed —all the thoracic appendages, with the exception of 
the first and sixth pairs, may be invaginated, the infolding being 
