This interpretation of the meaning of the transformation process is in accordance with all the previous evidence both from the side of the palæostracan as from the side of the vertebrate, for it signifies that a dwindling process has taken place in the foremost of the original prosomatic appendages—the cheliceræ and the endognaths; while, on the contrary, the ectognath and the metastoma have continued to increase in importance right into the vertebrate stage. This process is simply a continuation of what was already going on in the invertebrate stage, for whereas in Eurypterus and other cases the cheliceræ and endognaths had dwindled down to mere tentacles, the ectognath was the large swimming appendage, and the metastoma was on the upward grade from the two insignificant chilaria of Limulus.
The transformation of these foremost appendages into a suctorial apparatus is very common among the arthropods, as is seen in the transformation of the caterpillar into the butterfly, and it is in accordance with the evidence that the main mass of that suctorial apparatus should be formed from appendages corresponding to the ectognath and metastoma rather than from the four endognaths. In all probability the nucleus masticatorius of the trigeminal nerve with its innervation of the great muscles of mastication is evidence of the continued development of the musculature of these two last prosomatic appendages, just as the descending root of the Vth demonstrates the further disappearance of all that belongs to the foremost prosomatic appendages. As yet, however, as far as I know, the musculature of the head-region of Petromyzon has not been brought into line with that of other vertebrates, and until that comparative study has been completed it is premature to discuss the exact position of the masticating muscles of the higher vertebrates.
The analysis of these tentacular segments belonging to the trigeminal nerve presents greater difficulties than that of any of the other cranial segments, owing to the deficiency of our knowledge of what occurs at transformation. Light is required not only on the origin of the new muscles but also on the origin of the new and elaborate cartilages which are newly formed at this time.
Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we are not yet in a position to analyze the trigeminal supply in Petromyzon into its component appendage elements, an analysis which ought ultimately to enable us to determine from which appendage-muscles the masticating muscles in the higher vertebrates have arisen. As far as the muscles are concerned, she gives me the following information:—
The tongue-nerve supplies in Ammocœtes the rudimentary muscles which pass laterally from the base of the large ventral tentacle to the wall of the throat, and even in Ammocœtes must possess some power of moving that tentacle.
At transformation these muscles proliferate and develop enormously, and form the bulk of the large basilar muscle which surrounds the throat ventrally and laterally, and is the most bulky muscle in the suctorial apparatus.
The velar or mandibular nerve supplies in Ammocœtes the muscles of the lower lip. In Petromyzon it supplies also the longitudinal muscles of the tongue. The tongue-cartilage first develops in the region of the median ventral tentacle, and there the longitudinal tongue-muscles first begin to develop, not from the rudimentary muscles in the tongue but from those in the lower lip region.
In Ammocœtes the tentacular nerve supplies the rudimentary muscles in the tentacles and the muscles of the upper lip. The latter disappear entirely at transformation, and in Petromyzon the tentacular nerve supplies the circular, pharyngeal, and annular muscles, which are derived from the rudimentary tentacular muscles.
For the convenience of my reader I append here a table showing my conception of the manner in which the endognathal and ectognathal segments of the Palæostracan are represented in Ammocœtes. It shows well the uniform manner in which all the individual segmental factors have been fused together to represent the appearance of a single segment (van Wijhe's first segment) in the case of the four endognathal segments, but have retained their individuality in the case of the ectognathal segment.
| V. Wijhe's segments. | Eurypterid segments. | Appendages. | Appendage nerves. | Skeletal elements. | Somatic motor nerves. | Dorso- ventral segmental muscles. | Cœlomic cavities. | Coxal glands. | ||
|---|---|---|---|---|---|---|---|---|---|---|
| Eurypterid. | Ammocœtes. | |||||||||
| 1 | 2 3 4 5 |  | 4 Endognaths | 4 Tentacles | 1 Tentacular to 4 tentacles | 1 Tentacular bar to 4 tentacles | 1 Oculomotor supplying 4 muscles | Sup. inf. int. rectus and inf. oblique | 1 Premandibular fusion of 4 | 1 Pituitary body; fusion of 4 coxal glands |
| 2 | 6 | 1 Ectognath | 1 Tongue | 1 Tongue nerve | 1 Tongue bar | 1 Trochlearis supplying 1 muscle | Sup. oblique | 1 Mandibular | ||