CHAPTER VII 
THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES 
Comparison of the trigeminal with the prosomatic region.—The prosomatic 
appendages of the Gigantostraca.—Their number and nature.—Endognaths 
and ectognath—The metastoma.—The coxal glands.—Prosomatic region 
of Eurypterus compared with that of Ammocotes.—Prosomatic segmenta- 
tion shown by muscular markings on carapace.—Evidence of ccelomic 
cavities in Limulus.—Summary. 
THE derivation of the olfactory organs of the vertebrate from the 
olfactory antenne of the arthropod in the last chapter is confirmatory 
proof of the soundness of the proposition put forward in Chapter IV., 
that the segmentation in the cranial region of the vertebrate was 
derived from that of the prosomatic and mesosomatic regions of the 
paleostracan ancestor. Such a segmentation implies a definite series 
of body-segments, corresponding to the mesomeric segmentation of 
the vertebrate, and a definite series of appendages corresponding to 
the splanchnic segmentation of the vertebrate. 
Of the foremost segments belonging to the supra-cesophageal 
region characterized by the presence of the median eyes, of the lateral 
eyes, and of the olfactory organs, a wonderfully exact replica has 
been shown to exist in the pineal eyes, the lateral eyes, and the 
olfactory organ of the vertebrate, belonging, as they all do, to the 
supra-infundibular region. 
Of the infra-cesophageal segments belonging to the prosoma and 
mesosoma respectively, the correspondence between the mesosomatic 
segments carrying the branchial appendages and the uterus, with 
those in the vertebrate carrying the branchie and the thyroid gland 
respectively, has been fully proved in previous chapters. 
There remain, then, only the segments of the prosomatic region 
to be considered, a region which, both in the vertebrate and inver- 
tebrate, is never respiratory in function but always masticatory, such 
