Fig. 162.—Petromyzon marinus. Transverse section through the branchial region (semi-diagrammatic). br.m, Branchial membrane; d.ao, dorsal aorta; d.c, dorsal cartilage of the branchial basket; d.m, dorsal muscles; e.a, external aperture of a gill-sac; f.t, fibrous tissue enclosing neural canal; h, i, lateral longitudinal cartilages of the branchial basket; i.a, internal aperture of a gill-sac; i.ju, inferior jugular vein; ju, jugular vein (anterior cardinal); my, spinal cord; nc, notochord; n.ca, neural canal; n.p, neural process; oes, oesophagus; p.br, peri-branchial lymph sinus; r.m.t, retractor muscle of the tongue; r.t, respiratory tube or branchial canal; s, circum-oesophageal lymph sinus; v.ao, ventral aorta; v.c, ventral cartilage of branchial basket; v.m, ventral muscles. (From T. J. Parker.)
The branchial lamellae are represented by a series of vascular horizontal and parallel ridges radiating outwards along the roof, floor, and lateral walls of each gill-sac, and invested by an epithelium which is partially ciliated. The inter-branchial septa are much thicker than in Elasmobranchs, and include not only the walls of adjacent sacs and the branchial muscles, but also contain cavernous peribranchial lymph-sinuses. The cartilaginous branchial skeleton is situated wholly external to the gill-sacs, the so-called branchial arches lying between the external apertures of the sacs, and directly beneath the superficial skin, or, in other words, on the outer margins of the inter-branchial septa, and not on the inner, as is invariably the case with the branchial arches of Fishes.
Fig. 163—Dissection of Myxine glutinosa from the left side. au.c, Auditory capsule; br.ap, left branchial aperture; br.b, rudiment of branchial basket; br.s.1, first gill-sac; c.br.t, common branchial tube; cn.c, cornual cartilage; gul, gullet; ht, heart; lg.m, lingual muscles; m.v.c, median ventral cartilage; na.t, nasal tube; nch, notochord; n.t, neural tube; oes.ct.d, oesophageo-cutaneous duct; p.l.c, posterior lateral cartilage; sb.oc.a, subocular arch; sp.c, spinal cord; st.p, styloid process. (After W. K. Parker, from Parker and Haswell's Zoology.)
In the Hag-Fish (Myxine) (Fig. 163), there are usually six, very rarely seven, pairs of gill-sacs, all of which open directly into the pharynx, and not into a branchial canal as in the Lampreys. On the other hand, Myxine is unique in having the outer extremities of its gill-sacs produced into a corresponding number of tubular canals which, after a longer or shorter course obliquely backwards and outwards, unite to form on each side a ventrally-situated external aperture (Fig. 163). In the same genus a short canal, or oesophageo-cutaneous duct, passes from the pharynx behind the last gill-sac of the left side, and opens externally with the common external branchial aperture of that side.
In Bdellostoma there are usually six or seven pairs of gill-sacs, but some species have ten or even fourteen pairs.[[283]] They agree with those of the Lamprey in having independent external apertures, but resemble the corresponding organs in Myxine in opening directly into the pharynx. An oesophageo-cutaneous duct is also present.[[284]]
In the Holocephali there are but four branchial clefts, the fifth cleft being closed. Spiracles are absent in the adult, although present in the young of Chimaera. The branchial lamellae resemble those of Elasmobranchs, but the inter-branchial septa are somewhat shorter, so that the lamellae project slightly beyond their outer margins (Fig. 164, B). A hyoidean hemibranch is present. A noteworthy feature is the development of a cutaneous fold from the outer surface of the hyoid arch, which grows backwards over the gill-clefts, and, uniting above and below with the body-wall, terminates in a free posterior margin, just behind the last gill-cleft. By the growth of this opercular fold the gills become enclosed in a spacious branchial cavity, and the clefts communicate with the exterior through a slit-like opening between the free margin of the fold and the body-wall.
The reduction in the extent of the inter-branchial septa which is initiated in the Holocephali is carried to a still further extent in the Teleostomi. Commencing with the Chondrostei, and passing thence to the more specialised Teleostei, the septa become gradually reduced in length, and the branchial lamellae project freely beyond their outer margins to an increasing extent.
This modification, least marked in Acipenser (Fig. 164, C) and Polyodon, attains its maximum in the Teleosts (Fig. 164, D and E), where the branchial lamellae take the form of a double series of free filaments disposed along the convex outer margin of each branchial arch, and attached by their bases only to the reduced and inconspicuous septa. As a general rule each of the first four arches supports two hemibranchs,[[285]] forming a biserial gill or holobranch. In shape the branchial filaments are usually somewhat triangular, and consist of an axial supporting cartilage or bone, invested superficially by a highly vascular mucous membrane. As in most of the preceding groups the fifth branchial arch is gill-less. All Teleostomi possess a well-developed movable operculum, supported by a more or less complete series of opercular bones, with or without the addition of branchiostegal rays (Fig. 161, B). The size of the external branchial aperture varies considerably. Usually the hinder and lower margins of the operculum are free, and then the aperture is spacious. Not infrequently, however, the more or less extensive fusion of the ventral and hinder edges of the operculum with the body-wall reduces the aperture to a narrow slit, as in the Eels and some Siluridae, or to a small upwardly directed pore, as in the "Sea-Horse" (Hippocampus). In the Symbranchidae the branchial apertures close dorsally, but fuse ventrally, leaving a single median orifice on the under side of the throat.