242 THE ORIGIN OF VERTEBRATES 
the foremost mesosomatic segments by numbers (8-13). I have 
placed the four endognaths and the nerves going to them close 
together, and made them small, mere tentacles, in recognition of the 
character of these appendages in Eurypterus, and have indicated the 
position and size of the large ectognath, with its separate nerve, 
by (6). If among the ancient Eurypterus-like forms, which were 
living at the time when vertebrates first appeared, there were some 
in which the ectognaths also had dwindled to a pair of tentacles, 
then such animals would possess a prosomatic chamber formed by 
a metastoma or accessory lip, within which were situated five pairs 
of short tactile appendages or tentacles. If the vertebrate were 
derived from such an animal, then the trigeminal nerve, as the 
representative of these prosomatic appendage-nerves, ought to be 
found to supply the muscles of this accessory lip and of these five 
pairs of tentacles in the lowest vertebrate. 
This prosomatic or oral chamber, as it might be called, was limited 
posteriorly by the fused metastoma (7) and operculum (8), so that if 
in the same imaginary animal one imagines that the gill-chambers, 
instead of being separate, are united to form one large respiratory 
chamber, then, in such an animal, a prosomatic oral chamber, in 
which the prosomatic appendages worked, would be separated from 
a mesosomatic respiratory chamber by a septum composed of the 
conjoined basal portions of the mesosomatic operculum and the 
prosomatic metastoma, as indicated in the diagram. In this septum 
the nerves to the last prosomatic appendage (equivalent to the last 
part of the trigeminal in the vertebrate) and to the first mesosomatic 
(equivalent to the thyroid part of the facial) would run, as shown in 
the figure, close together in the first part of their course, and would 
separate when the ventral surface was reached, to pass headwards 
and tailwards respectively. 
THE CoxaL GLANDS. 
One more characteristic of these appendages requires mention, 
and that is the excretory glands situated at the base of the four 
endognaths known as the coxal glands. These glands are the main 
excretory organs in Limulus and the scorpions, and extend into the 
basal segments or cox of the four endognaths, not into those of the 
ectognaths or the chilaria (or metastoma). Hence their name, coxal 
