THE HORSE FOOT CRAB. 269 
most triangular, the lower side being slightly rounded, the 
upper sharply edged, while a section of the tail of this young 
specimen would be almost ovoidal. The tail of the young 
is also more distinctly marked with lines of segmentation 
than is that of the adult. As it travelled on the mud before 
this moult, it made tiny rows of toe-tracks, leaving a plain 
unmarked space between the rows. Now it moves with tail 
depressed, and makes a medial line dividing the toe-tracks 
into two series. 
Alas, at this point, when I had become intensely interested, 
a serious illness, against which I had offered a dogged de- 
termination to keep at work, peremptorily settled the matter 
by taking from me the use of my eyes. 
It will be noticed thus far that the observations here re- 
corded, are almost entirely morphological, and not physi- 
ologieal. Professor E. D. Cope has given us a lucid phrase, 
"expression point." He says of development, "while the 
change is really progressing, the external features remain 
unchanged at other than those points, which may be called 
expression points.” It seems to me that “expression points" 
of generic significance have been pointed out four times in 
these reniarks. Twice in the ovum I thought there was an 
"expression point" of a trilobed genus; and in the larval 
stage, I thought Pterygotus and Eurypterus were shadowed 
forth 
And in the metamorphoses of the larval state there are 
remarkable changes with reference to functional necessities. 
Already mention has been made of the moult at which the 
animal receives its articulated tail. Now in the life of Lim- 
ulus this tail is as indispensable as is the Alpine stock to the 
Swiss mountaineer. It is constantly liable by the least agi- 
tation, or obstruction, to be turned on its back, when but 
for its tail it would be as helpless as a tortoise in the same 
position. It is then that it deflects the tail, and inserts this 
sharp spine into the mud or sand, and after a few perse- 
vering efforts succeeds in turning itself over. So feeble are 
