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.] {163}[Illustration: FIG. 64.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES. _Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._, median serrated flange of velar folds.] {164}[Illustration: FIG. 65.--SECTION THROUGH BRANCHIAL APPENDAGE OF AMMOCOETES. _br. cart._, branchial cartilage; _v. br._, branchial vein; _a. br._, branchial artery; _b.s._, blood-spaces; _p._, pigment; _S._, sense-organ; _c._, ciliated band; _E., I._, external and internal borders; _m. add._, adductor muscle; _m.c.s._, striated constrictor muscle; _m.c.t._, tubular constrictor muscle; _m._ and _m.v._, muscles of valve.] [Illustration: FIG. 66.--SECTION THROUGH BRANCHIAL APPENDAGE OF LIMULUS. _br. cart._, branchial cartilage; _v.br._, branchial vein; _b.s._, blood-spaces formed by branchial artery; _P._, pigment; _m_1_, posterior entapophysio-branchial muscle; _m_2_, anterior entapophysio-branchial muscle; _m_3_, external branchial muscle.] The work of Miss Alcock has shown that the segmental branchial nerve supplies solely and absolutely such an appendage or branchial {165}segment, and does not supply any portion of the neighbouring branchial segments. The nerve-supply in Ammocoetes gives no countenance to the view that the original unit was a branchial pouch, the two sides of which each nerve supplied, but is strong evidence that the original unit was a branchial appendage, which was supplied by a _single_ nerve with both motor and sensory fibres. Any observer having before him only this picture of the respiratory chamber of Ammocoetes, upon which to base his view of a vertebrate respiratory chamber, would naturally look upon the branchial unit of a vertebrate as a gilled appendage projecting into the open cavity of the anterior part of the alimentary canal or pharynx. This is not, however, the usual conception. The branchial unit is ordinarily described as a gill-pouch, which possesses two openings or slits, an internal one into the lumen of the alimentary canal, and an external one into the surrounding medium. This view is based upon embryological evidence of the following character:--