546 
J. S. KINGSLEY. 
of the epiblast of the head, 1 and as having a common phylo- 
genetic origin, namely, from an Annelid ancestor. 
Such being the case, I regard the dorsal surface of the 
cephalothorax of Limulus as but the greatly expanded upper 
portion of the head, and believe that the segments indicated by 
the six pairs of appendages below are without proper terga. 
If we follow Packard and Lankester and recognise the dorsal 
surface of the shield as composed of six (or seven) coalesced 
terga, then the fact that the eyes are borne on the fifth or 
fourth and fifth 2 of these segments is difficult of explanation. 
This vagueness of expression as to the position of the eyes results 
from the fact that the dorsal epiblast of the cephalothorax 
never segments or shows any traces of division into somites, 
while in different specimens the boundary between the fourth 
and fifth mesoblastic somites is not constant in position as 
regards the eye, but at times it passes in front of the eye and 
at others beneath this organ, so that half of it is in one seg- 
ment and half over the other. The ocelli are placed over the 
first of these segments. It seems to me that these facts can 
be explained only in the way indicated at the beginning of this 
paragraph. 
A nearly parallel case is found in the carapax of the Decapod 
Crustacea. As usually described the carapax of the lobster or 
crayfish is regarded as composed of the coalesced terga of the 
segments visible below, and the obliquely transverse section 
which crosses it (the “ cervical suture ”) is held to indicate the 
line of division between head and thorax. In the Brachyura 
the homologous suture is usually sought in the depressions 
surrounding the cardiac region of the carapax. The true 
explanations of these various structures was pointed out by 
Dana (' 52 , pp. 23 — 28) over thirty years ago, and their total 
neglect by all subsequent students is a partial excuse for their 
mention here. The carapax of the lobster is wholly composed 
1 The peculiar lateral eyes of Euphausia are not included. 
2 Dr. Packard, in his text (’ 72 , p. 165), speaks of the compound eyes 
appearing “ on the third segment of the cephalothorax,” but his figures 
(pi. v, figs. 24, 24a) show them in their proper position. 
