The Origin of Vertebrates

by Walter Holbrook Gaskell

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 Ammocœtes.—Prosomatic segmentation shown by muscular markings on carapace.—Evidence of cœlomic cavities in Limulus.—Summary.

The derivation of the olfactory organs of the vertebrate from the olfactory antennæ 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 palæostracan 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-œsophageal 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-œsophageal 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 branchiæ 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 invertebrate, is never respiratory in function but always masticatory, such mastication being performed in Limulus and its allies by the muscles which move the foot-jaws or gnathites, which are portions of the prosomatic appendages specially modified for that purpose, and in the vertebrates by the masticatory muscles, which are always innervated by the trigeminal or Vth cranial nerve. This comparison implies that the motor part of the trigeminal nerve originally supplied the prosomatic appendages.

The investigations of van Wijhe and of all observers since the publication of his paper prove that in this trigeminal region, as in the vagus region, a double segmentation exists, of which the ventral or splanchnic segments, corresponding to the appendages in the invertebrate, are supplied by the trigeminal nerves, while the dorsal or somatic segments, corresponding to the somatic segments in the invertebrate, are supplied by the IIIrd or oculomotor and the IVth or trochlear nerves—nerves which supply muscles moving the lateral eyes.

In accordance, then, with the evidence afforded by the nerves of the branchial segments, it follows that the muscles supplied by the motor part of the trigeminal ought originally to have moved the appendages belonging to a series of prosomatic segments. On the other hand, the eye-muscles ought to have belonged to the body-part of the prosomatic segments, and must therefore have been grouped originally in a segmental series corresponding to the prosomatic appendages.

The evidence for and against this conclusion will be the subject of consideration in this and the succeeding chapters. At the outset it is evident that any such comparison necessitates an accurate knowledge of the number of the prosomatic segments in the Gigantostraca and of the nature of the corresponding appendages.