LIMULUS AN ARACHNID. 629 
which (like that of the embryo Scorpion and Spider) supplies 
only eyes and integument with nerves, and not any appendage. 
In all Crustacea, except some Phyllopoda, such an archi- 
cerebrum does not exist ; but even in young stages the brain 
is found to supply at least one pair of appendages as well 
as the eyes. 
11. It agrees with Scorpio in the concentration of the 
origins of nerves supplying the anterior part of the abdomen, 
in the cephalothorax in the form of a nervous collar, per- 
forated by the pharynx. Sucha nerve-collar has its parallel 
in Crustacea among the brachyurous Decapoda, which, how- 
ever, are in other respects the Crustaceans which least 
resemble Limulus. 
The points in which Limulus agrees with the Crustacea 
and differs from Arachnida are three only. They are as 
follows : 
1. Limulus agrees with many Crustacea, and differs from 
Arachnida, in that its respiratory organs are adapted to an 
aquatic in place of an aérial medium. 
2. Limulus agrees with Crustacea, and differs from Arach- 
nida, in that it possesses a pair of groups of eyes, in which 
the association of the individual eyes of each group is so close 
as to constitute a compound eye. 
3. Limulus agrees with Crustacea (excepting some Iso- 
poda ?),and differs from Arachnida, in mot possessing glandular 
ceca (the Malpighian tubules) growing out from the 
proctodzum. 
The first of these agreements is purely one of functional 
adaptation. The lamelligerous organs of Scorpio and the 
Spiders act upon atmospheric oxygen, as might be expected 
in animals living on dry land. The fact that the corre- 
sponding organs of Limulus respire the oxygen dissolved 
in sea water, as do the gills of Crustacea, does not even 
remotely tend to establish a morphological agreement between 
Limulus and Crustaceans. All attempts to associate organ- 
isms in one genealogical group on account of an agreement 
in the ultimate mode of performing such functions as respira- 
tion and locomotion, without reference to the exact nature 
of the organs by which those functions are performed, are 
liable to serious error. We cannot, as a principle, associate 
in genealogical classification all animals that breathe air, 
or all animals that breathe water, or all animals that fly, 
or all animals that swim, or all animals that walk. On 
the contrary, we must hold the actual structure and ana- 
tomical relations of organs to be the only guide to the genetic 
affinities of the animals which possess them, quite irrespec- 
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