THE EVIDENCE OF COELOMIC CAVITIES. In the head-region of the vertebrate, morphologists depend largely upon the embryonic divisions of the mesoderm for the estimation of the number of segments, and, therefore, upon the number of coelomic cavities in this region, the walls of which give origin to the striated muscles of the head, so that the question of the number of segments depends very largely upon the origin of the muscles from the walls of these head-cavities. It is therefore interesting to examine whether a similar criterion of segmentation holds good in such a segmented {252}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 Kishinouye, 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 coelomic 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 coelomic cavities in this region do not correspond absolutely with the number of prosomatic appendages. His words are:-- A pair of coelomic cavities appears in every segment except the segments of the 2nd, 3rd, and 4th appendages, in which coelomic 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 coelomic cavities is common to the cephalic lobe and the segment of the first appendage (_i.e._ the cheliceræ). The second coelomic cavity belongs to the segment of the fifth appendage. It is well developed. The ventral portion of the second coelomic 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:--