| V. Wijhe's segments. | 1 | 2 |
| Eurypterid segments. | 2 3 4 5 | 6 |
| Eurypterid Appendages. | 4 Endognaths | 1 Ectognath |
| Ammocœtes Appendages. | 4 Tentacles | 1 Tongue |
| Appendage nerves. | 1 Tentacular to 4 tentacles | 1 Tongue nerve |
| Skeletal elements. | 1 Tentacular bar to 4 tentacles | 1 Tongue bar |
| Somatic motor nerves. | 1 Oculomotor supplying 4 muscles | 1 Trochlearis supplying 1 muscle |
| Dorso- ventral segmental muscles. | Sup. inf. int. rectus and inf. oblique | Sup. oblique |
| Cœlomic cavities. | 1 Premandibular fusion of 4 | 1 Mandibular |
| Coxal glands. | 1 Pituitary body; fusion of 4 coxal glands |
The Tubular Muscles.
The only musculature innervated by the trigeminal nerve which remains for further discussion, consists of those peculiar muscles found in the velum, known by the name of striated tubular muscles. This group of muscles has already been referred to in Chapter IV., dealing with respiration and the origin of the heart.
It is a muscular group of extraordinary interest in seeking an answer to the question of vertebrate ancestry, for, like the thyroid gland, it bears all the characteristics of a survival from a prevertebrate form, which is especially well marked in Ammocœtes. I have already suggested in this chapter that the homologues of these muscles are represented in Limulus by the veno-pericardial group of muscles. I will now proceed to deal with the evidence for this suggestion.
The structure of the muscle-fibres is peculiar and very characteristic, so that wherever they occur they are easily recognized. Each fibre consists of a core of granular protoplasm, in the centre of which the nuclei are arranged in a single row. This core is surrounded by a margin of striated fibrillæ, as is seen in Fig. [122]. Such a structure is characteristic of various forms of striated muscle found in various invertebrates, such as the muscle-fibre of mollusca. It is, as far as I know, found nowhere in the vertebrate kingdom, except in Ammocœtes. At transformation these muscles entirely disappear, becoming fattily degenerated and then absorbed.
Fig. 122.—A Tubular Muscle-fibre of Ammocœtes.
A, portion of fibre seen longitudinally; B, transverse section of fibre (osmic preparation); the black dots are fat-globules.
For all these reasons they bear the stamp of a survival from a prevertebrate form. This alone would not make this tissue of any great importance, but when in addition these muscles are found to be arranged absolutely segmentally throughout the whole of the branchial region, then this tissue becomes a clue of the highest importance.
As mentioned in Chapter IV., the segmental muscles of respiration consist of the adductor muscle and the two constrictor muscles—the striated constrictor and the tubular constrictor. Of these muscles, both the muscles possessing ordinary striation are attached to the branchial cartilaginous skeleton, whereas the tubular constrictors have nothing to do with the cartilaginous basket-work, but are attached ventrally in the neighbourhood of the ventral aorta.