PROSOMATIC SEGMENTS OF LIMULUS 255 
dorsal muscles. Among the muscles of the appendages the system of large 
tergo-coxal muscles is especially apparent. 
From these considerations it follows that the number of segments in this 
region in the vertebrate ought to be seven; that the musculature supplied by 
the trigeminal nerve ought to represent seven ventral or splanchnic segments, 
of which only the last two are likely to be conspicuous ; and that the musculature 
supplied by the eye-muscle nerves ought to be dorso-ventral in direction, which 
it is, and represent seven dorsal or somatic segments. 
A further peculiarity of this region, both in Limulus and the scorpions, 
is found in the excretory organs which are known by the name of coxal glands, 
because they extend into the basal joint, or coxa, of certain of the prosomatic limbs. 
The appendages so characterized are always the four endognaths, and it follows 
that if these four endognaths lose their locomotor power, become reduced in 
size, and concentrated together to form mere tentacles, then of necessity the 
coxal glands will be concentrated together, and tend to form a glandular mass 
in the region of the mouth; in fact, take up a position corresponding to that 
of the pituitary body in vertebrates. 
Taking all these facts into consideration, it is possible to construct a drawing 
of a sagittal section through the head-region of Eurypterus, which will 
represent, with considerable probability, the arrangement of parts in that 
animal. This can be compared with the corresponding section through the 
head of Ammoceetes. 
Now, as pointed out in the last chapter, the early stage of Ammoccetes is 
remarkably different from the more advanced stage; at that time the septum 
between the oral and respiratory chambers has not yet broken through, and the 
olfactory or nasal tube, known at this stage as the tube of the hypophysis, is 
directed ventrally, not dorsally. 
The comparison of the diagram of Hurypterus with that of the early stage 
of Ammoceetes is remarkably close, and immediately suggests not only that the 
single nose of the former is derived from the corresponding organ in the 
paleostracan, but that the pituitary body is derived from the concentrated 
coxal glands, and the lower lip from the metastoma. The further working out 
of these homologies will be discussed in the next chapter. 
In addition to the evidence of segmentation afforded by the appendages, there 
are in this region, in Limulus and the scorpion group, three other criteria of 
segmentation available to us, if from any cause the evidence of appendages fails 
us. These are— 
1. The number of neuromeres are marked out in this region of the brain - 
more or less plainly, especially in the young animal, just as they are also in 
the embryo of the vertebrate. 
2. The segmentation is represented here, just as in the mesosomatic region, 
by two sets of muscle-segments ; the one somatic, consisting of the segmentally 
arranged dorso-ventral muscles, the continuation of the group already discussed 
in connection with the mesosomatic segmentation, and the other appendicular 
characterized by the tergo-coxal muscles. These latter segmental muscles are 
especially valuable, for in such forms as Mygale, Phrynus, etc., their presence 
is indicated externally by markings on the prosomatic carapace, and thus corre- 
sponding markings found on fossil carapaces or on dorsal head-shields can be 
