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 Ammocoetes. Now, as pointed out in the last chapter, the early stage of Ammocoetes 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 Eurypterus with that of the early stage of Ammocoetes is remarkably close, and immediately suggests not only that the single nose of the former is derived from the corresponding organ in the palæostracan, 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.