252 THE ORIGIN OF VERTEBRATES 
animal as Limulus, or in the members of the scorpion group, in which 
the number of segments are known definitely by the présence of the 
appendages. In Limulus we know, from the observations of Kishi- 
nouye, that a series of coelomic cavities are formed embryologically in 
the various segments of the mesosoma and prosoma, in a manner 
exceedingly similar to their mode of formation in the head-region of 
the vertebrate, and he has shown that in the mesosoma a separate 
ccelomie cavity exists for each segment,so that just as the dorso-ventral 
somatic muscles are regularly segmentally arranged in this region, so 
are the coelomic cavities, and we should be right in our estimation 
of the number of segments in this region by the consideration of 
the numerical correspondence of these cavities with the mesomatic 
appendages. Similarly, in the vertebrate, we find every reason to 
believe that a single, separate head-cavity corresponds to each of 
the branchial segments in the opisthotic region, and therefore we 
should estimate rightly the number of segments by the division of 
the mesoderm in this region. 
In the prosomatic region of Limulus, the dorso-ventral muscles 
are not arranged with such absolute segmental regularity as in the 
mesosomatic region, and Kishinouye’s observations show that the 
écelomic cavities in this region do not correspond absolutely with 
the number of prosomatic appendages. His words are :— 
A pair of ccelomic cavities appears in every segment except the 
segments of the 2nd, 3rd, and 4th appendages, in which ccelomic 
cavities do not appear at all. At least eleven pairs of these cavities 
are produced. The eleventh pair belongs to the seventh abdominal 
segment. 
The first pair of ccelomic cavities is common to the cephalic lobe 
and the segment of the first appendage (¢.e. the cheliceree). 
The second ccelomic cavity belongs to the segment of the fifth 
appendage. It is well developed. 
The ventral portion of the second ccelomic cavity remains as the 
coxal gland. 
Consequently, if we were to estimate the number of segments in 
this region by the number of coelomic cavities we should not judge 
rightly, for we should find only four cavities and seven appendages, 
as is seen in the following table :— 
