LIMULUS AN ARACHNID. 685 
the two animals compared, viz. Limulus and Scorpio, are 
found, on inspection, to be identical in general form and 
relation from one end of the body to the other. 
It is not possible, it should be observed, to maintain both 
positions. If the identification with the parts of the Scorpion 
is maintained, then all assimilation of the appendages and 
regions of the body of Limulus to those of a Podophthalmous 
or of a Copepodous Crustacean must be abandoned. There 
is no contact whatever between Limulus and Astacus until 
a common ancestral form is reached which exhibited in the 
most generalised condition the segmentation and appen- 
dages which are the common inheritance of all Arthropoda. 
It appears to me quite impossible to assume that this 
ancestral form had the characters of the Podophthalmous 
Crustacea. Such differentiation and numerical grouping of 
appendages as are seen in that highly developed Crustacean 
order are of late appearance, and accordingly such forms as 
Astacus and Homarus should not be made use of as standards 
of comparison representative of the Crustacea, but less differ- 
entiated examples must be sought. On the other hand, 
when we find it possible to establish a series of agreements 
between a form of doubtful affinities, such as Limulus, and 
a highly differentiated Arthropod, such as the Scorpion, the 
closeness of the genealogical connection thereby proved is 
greater in proportion as the differentiation of the forms 
compared is high, and as the number of points of agreement 
are numerous. 
The two authors who have had the facts in reference to 
Limulus and Scorpio most fully before them (since some 
of the more important were established by their own re- 
searches), and yet have not drawn the conclusion from those 
facts to which it seems to me that they necessarily lead, are 
MM. Alphonse Edwards and Dr. A. 8. Packard. M. 
Alphonse Milne-Edwards, although he showed that the 
cerebral ganglion of Limulus was unlike that of the Crus- 
tacea, could not admit of its assimilation to that of the 
Scorpion, not being acquainted, apparently, with Metschni- 
koff’s observations on the development of the latter animal ; 
and although he recognises the similarity of the perineural 
arterial system of Limulus to the supraneural or “spinal” 
arterial system of Scorpio, yet he is led away from the 
assimilation of the two animals by holding to the strange 
notion that the chilaria of the King Crab placed just in front 
of its genital operculum are the homological equivalents of 
the pectiniform appendages of the Scorpion placed just 
behind its genital operculum. M. Milne-Edwards places the 
