This singular branchial basket undoubtedly bears a superficial resemblance to the branchial arches of Fishes, but in any comparison of the two structures it is well to bear in mind that the branchial rods of the Lamprey are situated along the outer edges of the inter-branchial septa, and are therefore external to the gill-sacs and branchial arteries, and further, that they are developed in the somatic mesoblast of the embryonic protovertebrae, whereas true branchial arches are situated at the inner margins of the septa, internal to the gill-clefts and branchial arteries, and have their origin from the splanchnic layer of the mesoblast. So far as their position is concerned, the rods agree rather with the {220}extra-branchial cartilages of an Elasmobranch than with the more deeply-seated branchial arches. [Illustration: FIG. 125.—Side view of the skull of _Bdellostoma_; the gill-apertures and their cartilages have been omitted. A, Auditory capsule; B, B′, B″, the anterior, middle, and posterior segments of the lingual bar; _b^1_, cartilage connecting the hyoid arch with the second branchial arch; _br^1_, _br^2_, first and second branchial arches; _c.c_, coronal cartilage; _Cr_, cranium; _D_, dental plate; _dt_, median dorsal tooth; _Ex.n.c_, external part of the naso-pituitary canal; _Hp_, hypophysial plate; _Hy_, hyoid arch; _N_, subnasal cartilage; _nc_, neural canal; _Nt_, notochord; _OC_, olfactory capsule; _PL_, palatine portion of the palato-quadrate cartilage _PQ_; _S_, supra-pharyngeal plate supporting the velum; _t_, tendon of the retractor mandibuli muscle; _t^1_, _t^2_, _t^3_, tentacular cartilages; _t^4_, cartilage supporting mouth lobe; _tr_, trabecula; _V^1_, rod connecting _S_ with the inner surface of the hyoid arch of its side; _V_, outer lateral rod which joins _V^1_; 1, 2, 3, fenestrae. (Modified from Ayers and Jackson.)] While the skull of the Myxinoid Cyclostomes[194] is constructed on the same general lines as that of the Lamprey, it is in some respects more primitive. It is also clear that in other features the skull has undergone marked specialisation on lines of its own, and in some points again it seems to deviate less from the more normal Craniate type. Of the more obvious differences, as illustrated by the skull of _Bdellostoma_ (Figs. 125-127), it will be sufficient here to mention the following: (i.) The more primitive condition of the chondrocranium, the roof and side walls of the cranial cavity being entirely membranous. (ii.) The non-development of a suctorial buccal funnel and the presence of oral tentacles, associated with the absence of the complex system of labial cartilages and the substitution of a special tentacular skeleton. (iii.) The special modifications induced by the length and physiological importance of the naso-pituitary canal and by its communication with the pharynx after perforating the pituitary fontanelle in the cranial floor.