1885.] The Inter-Relationships of Arthropods. 561 
development of the king-crab. The introduction of this form 
into this group seems to necessitate a new term for the whole, and 
I have adopted the name Acerata for the arachnids and the 
merostomes, in reference to the absence of antennz. It is but a 
slight modification of the word Acera, used many years ago by 
Latreille for the spiders alone. The term Arachnida still retains 
its former significance. 
As here limited, the Acerata may be defined as arthropodous 
animals with the body divided into two regions (cephalothorax 
and abdomen), the cephalothorax bearing six pairs of primitively 
post-oral appendages. The number of abdominal appendages 
vary, but four or more are modified for respiratory purposes ; 
respiration being performed by gills, “lungs” or trachez, the 
homology between these three types of organs being easily traced. 
The genital ducts empty at the base of the seventh (first abdom- 
inal) appendages and paired segmental organs open, in the young, 
at the base of the fifth pair of limbs, but lose their excretory duct 
in the adult. The genital glands are branched and the branches 
communicate through numerous anastomoses. The liver is large 
and voluminous. The development is direct, no metamorphosis 
being introduced. 
Some of these points may require explanation, and while I 
would refer the reader to the paper on the embryology of Limu- 
lus for details, I may here mention a few facts. My studies on 
the development of the gills of the king-crab when compared 
with those of Metschnikoff on the scorpion and those of Salen- 
sky on the spiders, show that the lungs of the one and the gills 
of the other are (as was suggested by Lankester) perfectly 
homologous. They arise as foldings at the base of appendages, 
occupying the same position serially in both Limulus and the 
scorpions. Leydig showed, some thirty years ago, that the 
tracheze and pulmonary sacs of the spiders were homologous 
organs, and in later years the same has been pointed out by Bert- 
kau and Macleod. One very important point should here be 
noted. In the spiders the stigmata or external openings of the 
trachez or pulmonary sacs never occur elsewhere than on the 
abdomen, and they always perforate the sternal plates. In the 
hexapods they occur in all parts of the body, and always on the 
sides and never on the ventral surface. 
The so-called coxal glands or, as I regard them, segmental 
