This simple alteration of the branchiomeric unit from a gill-pouch to an appendage, which may or may not bear branchiæ, immediately sheds a flood of light on the segmentation of the head-region, and brings to harmony the chaos previously existing. Let us, then, follow out its further teachings. Next anteriorly to the glossopharyngeal and vagus nerves comes the facial nerve; a nerve which supplies the hyoid segment, or, rather, according to van Wijhe the two hyoid segments, for embryologically there is evidence of two segments. As already mentioned, the facial nerve is usually included in the trigeminal or pro-otic group of nerves, the opisthotic group being confined to the glossopharyngeal and vagus. This inclusion of the facial nerve into the pro-otic group of nerves forms one of the main reasons why this group has been supposed to have originally supplied gill-pouch segments, for the hyoid segment is clearly associated with branchiæ.
When, however, we examine Ammocœtes (cf. Figs. 63 and 64) it is clear that the foremost of the segments forming the respiratory chamber, which must be classed with the rest of the mesosomatic or opisthotic segments, is that supplied by the facial nerves.
An examination of this respiratory chamber shows clearly that there are six pairs of branchial appendages or diaphragms, which are all exactly similar to each other. These are those already considered, the foremost of which are supplied by the IXth or glossopharyngeal nerves. Immediately anterior to this glossopharyngeal segment is seen in the figures the segment supplied by the VIIth or facial nerves. It is so much like the segments belonging to the glossopharyngeal and vagus nerves as to make it certain that we are dealing here with a branchial segment, composed of a pair of branchial appendages similar to those in the other cases, except that the cartilaginous bar is here replaced by a bar of muco-cartilage and the branchiæ are confined to the posterior part of each appendage. The anterior portion is, as is seen in Fig. [74], largely occupied by blood-spaces, but in addition carries the ciliated groove (ps. br.) called by Dohrn 'pseudo-branchiale Rinne.' This groove leads directly into the thyroid gland, which is a large bilateral organ situated in the middle line, as seen in Fig. [80] and Fig. [85]. As shown by Miss Alcock, the facial nerve supplies this thyroid gland, as well as the posterior hyoid branchial segment, and, as pointed out by Dohrn, there is every reason to consider this thyroid gland as indicative of a separate segment, especially when van Wijhe's statement that the hyoid segment is in reality double is taken into account.
Fig. 74.—Ventral half of Head-region of Ammocœtes.
Somatic muscles coloured red. Branchial and visceral muscles coloured blue. Tubular constrictor muscles distinguished from striated constrictor muscles by simple hatching. Tent., tentacles; Tent. m.c., muco-cartilage of tentacles; Vel. m.c., muco-cartilage of the velum; Hy. m.c., muco-cartilage of the hyoid segment; Ps. br., pseudo-branchial groove; Br. cart., branchial cartilages; Sp., space between somatic and splanchnic muscles; Th. op., orifice of thyroid; H., heart.
The evidence, then, of Ammocœtes points directly to this conclusion: The facial nerves represent the foremost of the mesosomatic group of nerves, and supply two segments, which have amalgamated with each other. The most posterior of these, the hyoid segment, is a branchial segment of the same character as those supplied by the vagus and glossopharyngeal nerves; represents, therefore, the foremost pair of branchial appendages. The anterior or thyroid segment, on the other hand, differs from the rest in that, instead of branchiæ, it carries the thyroid gland with its two ciliated grooves. If this segment, which is the foremost of the mesosomatic segments, also indicates a pair of appendages which carry the thyroid gland instead of branchiæ, then it follows that this pair of appendages has joined together in the mid-line ventrally and thus formed a single median organ—the thyroid gland. If, then, we find that the foremost of the mesosomatic appendages in the Palæostraca was really composed of two pairs of appendages, of which the most posterior carried branchiæ, while the anterior pair had amalgamated in the mid-line ventrally, and carried some special organ instead of branchiæ, then the accumulation of coincidences is becoming so strong as to amount to proof of the correctness of our line of investigation.
The First Mesosomatic Segment in Limulus and its Allies.
What, then, is the nature of the foremost pair of mesosomatic appendages in Limulus. They differ from the rest of the mesosomatic appendages in that they do not carry branchiæ, and instead of being separate are joined together in the mid-line ventrally to form a single terminal plate-like appendage known as the operculum. On its posterior surface the operculum carries the genital duct on each side.
So also in the scorpion group, the operculum is always found and always carries the genital ducts.