Section I—Skeleton

Study a prepared specimen of the dogfish skeleton. It may be immersed in fluid, in a sealed glass jar, or set in clear acrylic blocks and available only for visual inspec­tion. Separate specimens of the head skeleton may be available for closer inspection.

Examine a specimen of a dogfish skeleton (Figure 3.1). Anteriorly, the skeleton of the head includes the chondrocranium and Splanchnocranium. The vertebral column extends from the chondrocranium to the tail and supports the anterior dorsal fin, posterior dorsal fin, and caudal fin. The pectoral girdle, supporting the pec­toral fins, lies just posterior to the splanchnocranium; the pelvic girdle, supporting the pelvic fins, lies further posteriorly. Note that the pectoral and pelvic girdles are isolated components, without direct attachment to the rest of the skeleton.

Chondrocranium

The chondrocranium is the large single element of the head skeleton (Figure 3.2). It surrounds and provides support for the brain and sense organs. The scoop-like rostrum projects anteriorly and contains the precerebral cavity. The cavity communicates posteriorly with the cranial cavity by way of the precerebral fenestra. The rostrum is supported ventrally by the rostral carina. Paired rostral fenestrae lie on either side of the carina. The paired nasal capsules lie laterally to the base of the rostrum. Paired openings, the nares, may be preserved on the ventral surface of each capsule.

Posterior to the capsules lies the optic region of the chondrocranium.

Behind the orbits is the squared otic region, which contains the otic capsules that house the inner ears. The basal plate is the wide, flattened ventral part. The carotid foramen lies at the very anterior end of the basal plate. On the dorsal surface, near the junction of the postorbital processes and otic regions, the chondrocra­nium bears a large, median depression, the endolym­phatic fossa. Within the fossa are two pairs of openings. The smaller, anterior two are the endolymphatic foram­ina; the posterior pair the perilymphatic foramina. The hyomandibular foramen pierces the chondrocranium at the anteroventral part of the otic region.

The occipital region forms the posterior part of the chondrocranium. The large median opening is the foramen magnum. An occipital condyle lies on either side and just below it. The paired condyles articulate with the first vertebra. A vagus foramen lies lateral to each condyle. A glossopharyngeal foramen lies at each ventrolateral corner of the occipital region.

Splanchnocranium

The splanchnocranium includes the seven visceral arches: the mandibular arch, hyoid arch, and five branchial arches (Figures 3.3 and 3.4). Each arch com­prises various segments. The mandibular and hyoid arches are highly modified for their use as jaw elements. The branchial arches support the interbranchial septa (see Figure 3.20) and lie between successive gill slits.

The mandibular arch, the largest of the arches, forms the jaws. Paired palatoquadrate cartilages fuse anteri­orly to form the upper jaw, which articulates with the chondrocranium. The palatoquadrate cartilage bears two prominent dorsal projections. Anteriorly, the orbital process contacts the medial wall of the orbit. Posteriorly, the adductor mandibulae process serves for the attachment of the jaw-closing musculature, the adductor mandibulae, which is considered later. The ventral half of the mandibular arch forms the lower jaws, which consists of paired Meckel’s cartilages fused anteriorly. The slender labial cartilage, sup­porting the labial folds (see page 37), is attached to Meckel’s cartilage, but may not be preserved in your specimen.

The hyoid arch is modified to support the jaws (or mandibular arch). The dorsal segment of the hyoid arch, on each side, is the hyomandibular, which abuts against the otic capsule. A ceratohyal articulates with the other end of the hyomandibular. The ceratohyals from either side articulate ventrally with the median basihyal. Liga­mentous attachments exist between the mandibular arch and hyoid arch, which, through articulation with the otic regions, acts as a support or suspensor of the jaw.

The remaining arches are branchial arches. Each arch is formed, in dorsal to ventral order, from paired pharyn- gobranchials, epibranchials, and ceratobranchials. Ven­trally the arches are completed by three paired hypobranchials and two median, unpaired basi- branchials. The pharyngobranchials of visceral arches 6 and 7 are fused.

FIGURE 3.1 Skeleton of the shark in left lateral view.

FIGURE 3.2 Chondrocranium of the shark.

FIGURE 3.3 Visceral arches of the shark in left lateral view.

FIGURE 3.4 Hyoid arch and branchial arches of the shark in ventral view. Note that mandibular arch, illus­trated in Figure 3.3, is absent here.

Vertebrae and Fins

There are two vertebral types: trunk and caudal (Figures 3.5 and 3.6). Sections of these types are usually set in acrylic blocks for examination. Although the vertebral column has a dorsal passageway in both the trunk and tail, a ventral passageway is present only in the tail. Each segment of the vertebral column is formed largely from an hourglass-shaped vertebral body or centrum. Notochordal tissue can be observed within the centrum in sagittal section. Dorsal to each centrum is a triangu­lar neural plate, the base of which sits on the centrum. The tips of the neural plates bear a low ridge termed the neural spine. Between successive neural plates are

FIGURE 3.5 Anterolateral view of sagittally sectioned and whole articulated trunk vertebrae of the shark.

FIGURE 3.6 Vertebrae of the shark in anterolateral view. a. Trunk vertebra. b. Caudal vertebra.

FIGURE 3.7 Skeletal elements of the anterior dorsal fin of the shark in left lateral view.

intercalary plates, also triangular but inverted so that neural and intercalary plates together form the neural arch. The arch helps form a passageway, the neural canal (for passage of the spinal cord), above the centra. Each plate is pierced by a foramen for the root of a spinal nerve.

Caudal vertebrae also bear a vertebral arch. In addition, the ventral surface of each centrum bears, on either side, a plate of cartilage that forms with its opposite number the hemal arch. The hemal canal, for the caudal artery and vein, passes within the arch. A thin horizontal par­tition, the hemal plate, may separate the passage for the artery dorsally and the vein ventrally. A hemal spine extends ventrally from the hemal arch.

The vertebral column helps support the anterior and posterior dorsal fins, and the caudal fin. The anterior (Figures 3.1 and 3.7) and posterior (Figure 3.1) dorsal fins are similar in structure. Each has a large proximal basal pterygiophore, to which the fin spine is anchored anteriorly. More distally are a series of radial pterygio- phores and, finally, the ceratotrichia, which are fibrous dermal rays, rather than cartilaginous elements. The caudal fin (Figure 3.8) is of the heterocercal type—asym­metric, with the vertebral axis curving into the dorsal half of the fin. It is supported proximally by the hemal arches and radial pterygiophores.

The paired appendages, the pectoral and pelvic fins, are supported by cartilaginous girdles. The pectoral girdle (Figure 3.9) includes a stout, U-shaped cartilage to which the fins attach on either side. The portion between the fins is the coracoid bar, and the part extend­ing dorsally past the attachment of each fin is the scapu­lar process. The area that articulates with the fin is the glenoid surface. A separate slender element, the supras­capular cartilage, attaches dorsally to each scapular process. The pectoral fin (Figures 3.9 and 3.10) has proximal basal and radial pterygiophores, followed by ceratotrichia. Three basals are recognized: the proptery-

FIGURE 3.8 Skeletal elements of the caudal fin of the shark in left lateral view.

gium, mesopterygium, and metapterygium, in anterior to posterior order. The glenoid surface of the fin is borne mainly by the mesopterygium. The radials of the pec­toral fin are rod-like structures, more regularly shaped than in the median dorsal fins, and are arranged in rows.

The pelvic girdle (Figure 3.11) consists of a single element, the puboischiadic bar. The pelvic fins articu­late on either end of the bar at the acetabular surfaces. An iliac process lies near each acetabular surface. The pelvic fin of the female and male are similar in bearing a short propterygium and a stout, elongated metaptery­gium that extends posteriorly (Figure 3.11). A row of cylindrical radial pterygiophores articulates mainly with the metapterygium and supports the ceratotrichia. The pelvic fin of the male differs in bearing a clasper, formed from modified radials, that functions in the intromission of sperm into the cloaca of the female. The clasper extends posteriorly from the metapterygium and bears a hook and spine distally (Figure 3.11a).

Key Terms: Skeleton abducens foramen acetabular surfaces adductor mandibulae process

anterior dorsal fin antorbital process antorbital shelf basal plate basal pterygiophore basapophysis basibranchials basihyal basitrabecular processes

branchial arches caudal fin centrum (vertebral

body) ceratobranchials ceratohyal ceratotrichia chondrocranium clasper coracoid bar endolymphatic foramina

(sing., foramen)

FIGURE 3.9 Pectoral girdle and skeletal elements of pectoral fins of the shark in anterior view.

FIGURE 3.10 Skeletal elements of the left pectoral fin of the shark in dorsal view.

endolymphatic fossa

(plur., fossae) epibranchials epiphyseal foramen fin spine foramen magnum glenoid surface glossopharyngeal

foramen

hemal arch hemal canal hemal plate hemal spine hook of clasper hyoid arch hyomandibular hyomandibular foramen hypobranchials iliac process intercalary plates

(interneural arch) labial cartilage

mandibular arch Meckel’s cartilage

(mandibular cartilage) mesopterygium metapterygium nares (sing., naris) nasal capsules neural canal (vertebral

canal)

neural plate neural spine occipital condyle oculomotor foramina

(sing., foramen) optic foramen optic pedicle orbital process otic capsules palatoquadrate

cartilages pectoral fins pectoral girdle

FIGURE 3.11 Skeletal elements of the pelvic fin of the shark in dorsal view. a. Left fin of the male. b. Right fin of the female.

pelvic fins

pelvic girdle perilymphatic foramina pharyngobranchials posterior dorsal fin postorbital process precerebral cavity precerebral fenestra propterygium puboischiadic bar radial pterygiophores rib

rostral carina

rostral fenestra (plur., fenestrae)

rostrum

scapular process spine of clasper splanchnocranium superficial ophthalmic foramina supraorbital crest suprascapular cartilage trigeminofacial foramen trochlear foramen vagus foramen vertebral arch vertebral column