A pair of cœlomic cavities appears in every segment except the segments of the 2nd, 3rd, and 4th appendages, in which cœlomic 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 cœlomic cavities is common to the cephalic lobe and the segment of the first appendage (i.e. the cheliceræ).
The second cœlomic cavity belongs to the segment of the fifth appendage. It is well developed.
The ventral portion of the second cœlomic cavity remains as the coxal gland.
Consequently, if we were to estimate the number of segments in this region by the number of cœlomic cavities we should not judge rightly, for we should find only four cavities and seven appendages, as is seen in the following table:—
| LIMULUS. | VERTEBRATE. | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Segments. | Appendages. | Eurypterid appendages. | Cœlomic cavities. | Cœlomic cavities. | ||||||
 |  | 1 | Cheliceræ or 1st locomotor. | Cheliceræ | 1 | Anterior | ||||
| 2 | 2nd locomotor |  | Endognaths | 2 | Premandibular | |||||
| 3 | 3rd loco" | |||||||||
| 4 | 4th loco" | |||||||||
| 5 | 5th loco" | |||||||||
| 6 | 6th loco" | Ectognath | 3 |  | Mandibular | |||||
| 7 | Chilaria | Metastoma | 4 | |||||||
 | | 8 | Operculum | | Operculum |  | Genital | 5 | | Hyoid |
| 9 | 1st branchial | 1st branchial | 6 | |||||||
| 100 | 2nd brat" | 2nd branchial | 7 | 1st branchial | ||||||
| 110 | 3rd bran" | 3rd bran" | 8 | 2nd brat" | ||||||
| 120 | 4th bran" | 4th bran" | 9 | 3rd bran" | ||||||
| 130 | 5th bran" | 5th bran" | 100 | 4th bran" | ||||||
| 140 | 6th bran" | 110 | ||||||||
| LIMULUS. | VERTEBRATE. | |||||||
|---|---|---|---|---|---|---|---|---|
| Segments. | Appendages. | Eurypterid appendages. | C.c. | Cœlomic cavities. | ||||
| 1 | Cheliceræ or 1st locomotor. | Cheliceræ | 1 | Anterior | |||
| 2 | 2nd locomotor |
| 2 | Premandibular | ||||
| 3 | 3rd loco" | |||||||
| 4 | 4th loco" | |||||||
| 5 | 5th loco" | |||||||
| 6 | 6th loco" | Ectognath | 3 | Mandibular | ||||
| 7 | Chilaria | Metastoma | 4 | |||||
| 8 | Operculum |
| 5 | Hyoid | |||
| 9 | 1st branchial | 6 | ||||||
| 10 | 2nd brat" | 2nd branchial | 7 | 1st branchial | ||||
| 11 | 3rd bran" | 3rd bran" | 8 | 2nd brat" | ||||
| 12 | 4th bran" | 4th bran" | 9 | 3rd bran" | ||||
| 13 | 5th bran" | 5th bran" | 10 | 4th bran" | ||||
| 14 | 6th bran" | 11 | ||||||
The second cavity would in reality represent four segments belonging to the 2nd, 3rd, 4th, 5th locomotor appendages, i.e. the very four segments which in the Eurypteridæ are concentrated together to form the endognaths, and we should be justified in putting this interpretation on it, because, according to Kishinouye, its ventral portion forms the coxal gland, and, according to Lankester, the coxal gland sends prolongations into the coxa of the 2nd, 3rd, 4th, 5th locomotor appendages. Similarly in the vertebrate, we find three head-cavities in the region which corresponds, on my theory, to the prosomatic region of Limulus, (1) the anterior cavity discovered by Miss Platt, (2) the premandibular cavity, and (3) the mandibular cavity, which, if they corresponded with the prosomatic cœlomic cavities of Limulus, would represent not three segments but seven segments, as follows:—the anterior cavity would correspond to the first cœlomic cavity, i.e. the cavity of the cheliceral segments in both Limulus and the Eurypteridæ; the premandibular, to the second cœlomic cavity, representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in Limulus and the endognathal segments in the Eurypteridæ; and the mandibular to the 3rd and 4th cœlomic cavities, representing the last locomotor and chilarial segments in Limulus, i.e. the ectognathal and metastomal segments in the Eurypteridæ.
It is worthy of note that, in respect to their cœlomic cavities, as in the position and origin of their nerves in the central nervous system, the first pair of appendages, the cheliceræ, retain a unique position, differing from the rest of the prosomatic appendages.