NOTES ON THE EMBRYOLOGY OF L1MULUS. 
551 
The same agreements and differences are seen in (6) the pos- 
session of an entostermite with the same shape, position, and his- 
tological structure ; (7) in the backward extension of the tergum 
of the head, so as to form tlie dorsum of the cephalothorax ; 
(8) in the distribution of the eyes ; (9) in the existence of 
a blood colored blue by the presence of hsemocyanin (10) in 
the branching and anastomosing spermatic duct (Benham, ’ 83 ) ; 
(11) in the mode of formation of the mesoblast, its segmenta- 
tion, and the early extension of the body cavity into the limbs ; 
(12) in the mode of formation and structure of the alimentary 
canal and its appendages; 1 (13) the elongate stomodseum and 
the short proctodseum ; (14) in the origin of the sternal artery 
and its intimate association with the ventral nervous cord ; 
(15) the early closure of the duct of fifth (the only) segmental 
organ; (16) the absence of the second segmental organ (an- 
tennal gland). 
Besides these points there are a number of other points in 
which the two agree, and which are but rarely paralleled in the 
Crustacea, and then not in groups which anyone would think 
of placing near Limulus. Among these may be mentioned the 
possession of vibratile spermatozoa (paralleled in the bar- 
nacles 2 ), the concentration of the nervous system around the 
oesophagus (also in Brachyura), a brain which never supplies 
any of the appendages. 
This array of similarities is almost conclusive, but we must 
turn to the other side of the question, and seek the arguments 
against the association of Limulus with the Scorpions. Dr. 
Packard is by far the most able advocate of the Crustacean 
affinities of Limulus, and in his latest paper (’ 82 ), in which he 
deals with this subject (confessedly a reply to Lankester’s 
“ Limulus an Arachnid”), we find the following summary of 
the arguments against placing Limulus in the Arachnidan 
phylum. In the quotation I have inserted numerals for con- 
venience of reference below : “ At the outset it will be remem- 
1 These points will be discussed in the second part of this paper now in 
preparation. 
2 They also have a slight motion in some other Crustacea. 
