PROSOMATIC SEGMENTS OF LIMULUS 255 



dorsal muscles. Among the muscles of the appendages the system of large 

 tergo-coxal muscles is especially apparent. 



From these considerations it follows that the number of segments in this 

 region in the vertebrate ought to be seven; that the musculature supplied by 

 the trigeminal nerve ought to represent seven ventral or splanchnic segments, 

 of which only the last two are likely to be conspicuous ; and that the musculature 

 supplied by the eye-muscle nerves ought to be dorso-ventral in direction, which 

 it is, and represent seven dorsal or somatic segments. 



A further peculiarity of this region, both in Limuhis and the scorpions, 

 is found in the excretory organs which are known by the name of coxal glands, 

 becaiise they extend into the basal joint, or coxa, of cei'tain 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 gdands 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 

 pakeostracan, but that the pituitary body is derived from the concentrated 

 coxal gdands, 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 appendag-es, there 

 are in this region, in Limulus and the scorpion g*roup. 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. 



2. The segmentation is represented here, just as in the mesosomatic region, 

 by two sets of muscle-segments ; the one somatic, consisting of the seg-mentally 

 arranged dorso-ventral muscles, the continuation of the group already discussed 

 in connection with the mesosomatic segmentation, and the other appendicular 

 characterized by the tergo-coxal muscles. These latter segmental muscles are 

 especially valuable, for in such forms as Mygale, Phrynus, etc., their presence 

 is indicated extei-nally by marking's on the prosomatic carapace, and thus corre- 

 sponding markings found on fossil carapaces or on dorsal head-shields can be 



