CHAPTER VII 



THE PROS OM A TIC 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 Ammocoetes. — Prosomatic segmenta- 

 tion shown by muscular markings on carapace. — Evidence of coelomic 

 cavities in Limulus. — Summary. 



The derivation of the olfactory organs of the vertebrate from the 

 olfactory antennae 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 

 palseostracan 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-oesophageal 

 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 prosoraa and 

 mesosoma respectively, the correspondence between the mesosomatic 

 segments carrying the branchial appendages and the uterus, with 

 those in the vertebrate carrying the branchiae 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 



