LIMULUS AN ARACHNID. 647 
the Hexapoda and of the Myriapoda with the other large 
groups of Arthropoda, there is probably more hope of a 
definite indication being obtained as the result of a critical 
study and comparison of the structure of the eyes than 
from any other source. The eyes of Arthropods are elaborate 
in the histological details of their structure, and at the same 
time have not been inherited from a common ancestor in one 
and the same elaborate form by all the members of the group, as 
have been the eyes of craniate Vertebrata for example. Ac- 
cordingly we may expect that the elaboration of the eye has 
taken a somewhat different course in different lines of descent 
within the limits of the Arthropod phylum, and we should 
be justified in concluding a common line of descent for 
classes of Arthropods showing identity in numerous details 
of the optical structure, which details had been ascertained 
not to be acommon inheritance from the primeval Arthropod 
ancestor. 
Whatever may be the conclusion arrived at in the future 
in reference to the affinities of Hexapoda and Myriapoda, 
the result of the recognition of the intimate relationship of 
Scorpio and Limulus must be, I think, to break up the 
artificial group of ‘* Arthropoda Tracheata ” by the separation 
of the Scorpions, Spiders, and Mites, from any special con- 
nection with it. 
Phylum.—APvPEnDICcULATA. 
Branch 3.—Arthropoda (Gnathopoda). 
Class.—Arachnida. 
Arthropoda developed from ancestral forms, in which a 
‘ prosoma’ formed by the union of the prostomium and six 
anterior segments was sharply marked off from the rest of 
the body, both by the confluence of its terga to form a 
carapace and by the special character and size of its appen- 
dages. ‘The six pairs of appendages (including the foremost 
of the whole series) were arranged round the mouth, and all 
subservient to the purpose of prehension and mastication of 
food. In the later developed forms of Arachnida either 
the number of these appendages may be reduced (Euryp- 
terina, Trilobita), or the functional relation to the mouth of 
the more posterior of the six pairs may be lost. Whatever 
their number, the foremost pair is free from a jaw-like 
enlargement of the coxa. The palps of all six pairs of 
appendages exhibit a wide range of adaptational form, as 
prehensile, tactile, ambulatory, natatory, or fossorial organs. 
