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. {187}[Illustration: FIG. 74.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES. 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.]