† Entirely extinct.
The Fishes included in the Teleostomi were formerly arranged in two groups: the Ganoidei, including the Crossopterygii, Chondrostei, and the Holostei, with their numerous fossil allies; and the Teleostei. Living Ganoids agree with one another, and differ from Teleosts in possessing an intestinal spiral valve and a conus arteriosus. It is difficult, however, to separate the two groups, inasmuch as in each group there are living forms which tend to approximate to the other; and numerous fossil genera, of whose soft parts nothing is known, are in many respects intermediate between the two. The position and relationships of the Palaeospondylidae, Ostracodermi, Antiarchi, and Arthrodira are very uncertain. The Palaeospondylidae have been included in the Cyclostomata, or at all events have been regarded as more or less closely related to that group, while the absence of paired fins and the apparent want of jaws have suggested that the Ostracodermi occupy an intermediate position between the Cyclostomata and the Gnathostomata.[[120]] On the other hand, the Arthrodira are either regarded as an independent group of Fishes, or are included amongst the Dipnoi. In the latter case, the Dipnoi are divided into the Arthrodira and the Sirenoidei, the last mentioned group including Neoceratodus, Protopterus and Lepidosiren, and their extinct allies.
CHAPTER VI
EXTERNAL CHARACTERS OF CYCLOSTOMATA AND OF FISHES
EXTERNAL CHARACTERS—COLORATION—POISON GLANDS AND POISON SPINES—PHOSPHORESCENT ORGANS.
Fig. 91.—Petromyzon marinus. A, ventral; B, lateral; and C, dorsal, view of the head. br.cl.1, First branchial cleft; buc.f, buccal funnel; eye, the eye; mth, mouth; na.ap, nasal aperture; p, papillae; pn, pineal area; t1, t2, t3, teeth of buccal funnel; t4, teeth on the tongue. (From Parker and Haswell, after W. K. Parker.)
In all the Cyclostomata the body is Eel-like in shape, the head and trunk being nearly cylindrical, and the tail somewhat flattened from side to side. In Petromyzon the head terminates in a ventrally-directed, funnel-like cavity—the buccal funnel—in the roof of which the relatively small mouth is situated (Fig. 91, A.). The margin of the funnel is fringed by a series of short papillae, but in the Hag-Fishes (Myxine and Bdellostoma), where a buccal funnel is not developed, longer tentacle-like structures are present on each side of the mouth. On the upper surface of the head is the single median nostril, or naso-pituitary aperture, placed between the eyes in the Lampreys (Fig. 91, B, C), but at the anterior margin of the head in Myxine and its allies (Fig. 92). In the living Lampreys a semi-transparent area of skin may be noticed behind the nasal organ, which coincides with the position of the more deeply-seated parietal eye. On each side of the body, commencing a short distance behind the eye, is a series of small and almost circular branchial clefts (Petromyzon, Bdellostoma). In Myxine, however, the clefts of each side have a single common external aperture, situated on the ventral side of the body and some distance behind the head (Fig. 92, A). At the junction of the trunk with the tail is the anus, behind which is the papilla which carries the urino-genital aperture at its extremity. There are no paired limbs or vestiges of such organs. Median fins are represented in the Lampreys by an anterior dorsal fin and a posterior dorsal fin, the latter being continuous with the caudal fin which fringes the upper and lower margins of the protocercal tail. In Myxine a caudal fin only is present, surrounding the extremity of the tail.
Fig. 92.—Head of Myxine glutinosa (A), and of Bdellostoma forsteri (B), from beneath. br.ap, Left external branchial aperture; br.cl.1, first branchial cleft; mth, mouth; na.ap, nasal aperture; oes.ct.d, oesophageo-cutaneous duct. The smaller openings in A are those of mucous glands. (From Parker and Haswell, after W. K. Parker.)