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 presence of the 



appendages. In Limulus we know, from the observations of Kishi- 



nouye, that a series of ccelomic 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 



ccelomic cavity exists for each segment, so that just as the dorso-ventral 



somatic muscles are regularly segmentally arranged in this region, so 



are the ccelomic 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 Kishinonye's observations show that the 

 ccelomic 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 {i.e. the chelicene). 



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 ccelomic cavities we should not judge 

 rightly, for we should find only four cavities and seven appendages, 

 as is seen in the following table : — 



