from a cartilaginous rod (parachordal cartilage) lying on each side of the notochord and possibly representing a fused row of dorsal arcualia. The originally separate parachordals and trabeculae become connected to form a trough-like, primitive cranium, complete or nearly so laterally and ventrally but open dorsally. With the primitive cranium there are also connected cartilaginous capsules developed round the olfactory and auditory organs. There also become fused with the hinder end of the cranium a varying number of originally distinct neural arches.
(C) Visceral Arches.—The skeleton of the visceral arches consists
essentially of a series of half-hoops of cartilage, each divided
in the adult into a number of segments and connected with its
fellow by a median ventral cartilage. The skeleton of arches
I. and II. (mandibular and hyoidean) undergoes modifications
of special interest (figs. 14 and 15). The lower portion of the
mandibular arch becomes greatly thickened to support the lower
or hinder edge of the mouth. It forms the primitive lower jaw
or “Meckel’s cartilage.” Dorsal to this an outgrowth arises
from the anterior face of the arch which supports the upper
or anterior margin of the mouth: it is the primitive upper jaw
or palato-pterygoquadrate cartilage. The portion of the arch
dorsal to the palato-pterygo-quadrate outgrowth may form the
suspensorial apparatus of the lower jaw, being fused with the
cranium at its upper end. This relatively primitive con-arrangement
(protostylic, as it may be termed) occurs in Dipneusti
among fishes (cf. fig. 14). More usually this dorsal part of the
Fig. 16.—Fore-limb
of Ceratodus.
mandibular arch becomes reduced, its
place being occupied by a ligament (pre-spiracular)
uniting the jaw apparatus to the
chondrocranium, the upper jaw being also
attached to the chondrocranium by the
ethmopalatine ligament situated more
anteriorly. The main attachment, however,
of the jaws to the chondrocranium
in such a case, as holds for the majority
of fishes, is through the enlarged dorsal
segment of the hyoid arch (hyomandibular)
which articulates at its dorsal end with
the chondrocranium, while its ventral end
is attached to the hinge region of the jaw
by stout ligamentous bands. A skull in
which the jaws are suspended in this
manner is termed a hyostylic skull (e.g.
Scyllium in fig. 15).
In Notidanus (fig. 15, B) there is a large direct articulation of the upper jaw to the chondrocranium in addition to the indirect one through the hyomandibular: such a skull is amphistylic. In Heterodontus the upper jaw is firmly bound to the cranium throughout its length, while in Holocephali (fig. 15, C) complete fusion has taken place, so that the lower jaw appears to articulate directly with the cranium (“auto stylic” condition). In Dipneusti[1] (Lepidosiren and Protopterus) the cartilaginous upper jaw never develops (except in its hinder quadrate portion) beyond the condition of a faint rudiment, owing doubtless to its being replaced functionally by precociously developed bone.
(D) Appendicular Skeleton.—The skeleton
of the free part of the limb is attached to
the limb girdle which lies embedded in
the musculature of the body. Each limb girdle is probably
to be looked upon as consisting, like the skeleton of the visceral
arches, of a pair of lateral half-hoops of cartilage. While in
Pleuracanthus the lateral halves are distinct (and segmented
like the branchial arches), in living Selachians generally the
two halves are completely fused ventrally with one another.
The part of the girdle lying dorsal to the articulation of the limb
is termed scapular in the case of the pectoral limb, iliac in the
Fig. 17.—a, Skeleton of pectoral
limb of Pleuracanthus. (From
Gegenbauer, after Frisch.) b,
Skeleton of pectoral limb of
Acanthias. (After Gegenbauer.)
case of the pelvic, while the
ventral portions are known
respectively as coracoid and
ischio-pubic.
In most Teleostomes the primitive pelvic girdle does not develop; in the Dipneusti it is represented by a median unpaired cartilage.
The skeleton of the free limb is probably seen in its most archaic form amongst existing fishes in the biserial archipterygium of Ceratodus (fig. 16). This is indicated by the relative predominance of this type of fin amongst the geologically more ancient fishes. The biserial archipterygium consists of a segmented axial rod, bearing a praeaxial and a postaxial series of jointed rays.
In Protopterus and Lepidosiren the limbs are reduced and the lateral rays have less (Protopterus) or more (Lepidosiren) completely disappeared.
- ↑ Cf. W. E. Agar, Trans. Roy. Soc. Edin. xlv. (1906), 49.