PTEROBRANCHIA, a zoological group established by Ray Lankester in 1877. It contained at that time the single genus Rhabdopleura, a minute animal dredged by Sars off the Lofoten Islands, and by Norman off the Shetlands. Rhabdopleura was at first regarded as an aberrant Polyzoon, but with the publication of the Challenger Report (Cephalodiscus) in 1887, it became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta. The connexion of the Pterobranchia with the Polyzoa is in the highest degree questionable.
Rhabdopleura is no doubt of world-wide distribution, since it has been recorded in various localities from Greenland to South Australia, usually in water of not less than forty fathoms. Cephalodiscus, which for many years was known solely as the result of a single dredging by the “Challenger” from 245 fathoms in the Straits of Magellan, has recently been found in entirely different parts of the world, as for instance between Japan and Korea at 100 fathoms, at about half that depth off the south-east coast of Celebes, and between tide-marks on the coast of Borneo. It appears to be common in the neighbourhood of Cape Town, while the recent Antarctic expeditions have shown that it occurs in various localities from the Falkland Islands to the Antarctic circle. No less than twelve species, referred to three sub-genera (Demiothecia, Idiothecia, Orthoecus), have now been described; but it is at present uncertain whether more than a single species of Rhabdopleura is valid, although several specific names have been suggested for specimens from different localities.
Both genera are characterized by their habit of secreting a tubular gelatinoid investment, the “coenoecium,” composed of a number of superposed lamellae, doubtless the result of its intermittent secretion, mainly though perhaps not exclusively, by the proboscises of the zooids. In Rhabdopleura each zooid forms its own delicate tube composed of a characteristic series of distinct rings. In Cephalodiscus the coenoecium is more massive, and may contain a continuous irregular cavity in which the zooids live (Demiothecia), or may be secreted in such a way that each zooid has its own independent tube (Idiothecia, Orthoecus).
The zooids are a modification of the type of structure known in Balanoglossus, from which they differ principally in the following respects: (i.) The alimentary canal, instead of being straight, has a U-shaped flexure, the dorsal line between the mouth and the anus being short. (ii.) The proboscis (fig. 1, b), known as the “buccal shield,” is a large organ, strongly flattened in an antero-posterior direction, its ventral lobe usually concealing the mouth. (iii.) The collar is produced dorsally into arms (one pair in Rhabdopleura, four to eight pairs in Cephalodiscus), each of which bears numerous ciliated tentacles, the organs by which the microscopic food-particles are conveyed to the mouth. (iv.) The third division of the body, the metasome, is prolonged ventrally into a relatively enormous outgrowth containing the loop of the alimentary canal, beyond which projects a stalk (fig. 1, c), of a length varying with the state of contraction and perhaps with the species. (v.) The stalk gives rise to buds, by which the colonial habit is acquired. While in Rhabdopleura the buds remain in organic continuity with the parent, in Cephalodiscus they become free at an early stage, and the coenoecium accordingly contains a number of separate individuals. In the living Cephalodiscus a zooid can crawl by means of its proboscis over the gelatinous processes of the outer side of the coenoecium, a position which it can assume owing to the very great extensibility of the stalk, the proximal suctorial end of which remains attached to the inner surface of some part of the coenoecium (Andersson, 1907).
|(From a drawing by Professor McIntosh.)|
Fig. 1.—Zooid of Cephalodiscus dodecalophus.
|d,||Arms and tentacles.|
|e,||Ventral edge of proboscis.|
|f,||Its dorsal edge.|
In correspondence with the fundamental constitution of the zooid, each of the three segments has its own body-cavity separated from the others. The main proboscis-cavity (fig. 2, b.c.1) is unpaired, and opens to the exterior by the two proboscis pores (p.p.). It contains a closed vesicle regarded by Schepotieff as a right proboscis-cavity and in any case representing the pericardium of Balanoglossus, the glomerulus of which is also probably represented. The collar-cavity (b.c.2) is paired, although its ventral mesentery is not complete. It extends into the arms, which originate in the bud (fig. 1) as dorsal outgrowths of the collar. The ventral and lateral parts of the anterior margin of the collar constitute the so-called operculum (op.), a structure which not only acts as a lower lip, but must be important in separating the food-current produced by the cilia of the tentacles from the external apertures of the collar-canals and gill-slits. The collar-canals (fig. 3, c.p.) are a pair of ovoid organs which open from the collar-cavity to the exterior, their external pores lying immediately behind the base of the operculum.
Fig. 2.—Median (sagittal) section of Cephalodiscus dodecalophus.
|op.,||Operculum, or ventral lip.|
|b.c1.,||Body-cavity of proboscis.|
|n.s.,||Central nervous system.|
While it is not improbable that the collar-pores and the proboscispores may evacuate excretory substances, there can be little doubt that their primary function is to regulate the turgidity of the segment to which they respectively belong. A pair of gill-slits (fig. 3, g.s.), which do not occur in Rhabdopleura, open immediately behind the collar-pores. It is probable that they serve to strain off the superfluous water which is introduced into the mouth during the process of feeding. An anterior median diverticulum of the pharynx (fig. 2, nch.), growing forwards in the septum between the proboscis-cavity and the collar-cavities, and supported dorsally by the median mesentery of the collar, is the representative of the so-called notochord or stomochord of Balanoglossus; and if the view that this organ is really a notochord is well founded, it may be regarded as the homologue of the anterior end of the Vertebrate notochord.
The metasome contains nearly the whole of the alimentary canal, in which pharynx (fig. 2, ph.), oesophagus (oes.), stomach (st.) and intestine (int.) may be distinguished. The remarkable position of the anus (a) on the dorsal side has already been alluded to. The metasomatic cavities are divided by dorsal (fig. 3, d.mes.) and ventral mesenteries, the latter following the outer curvature of the loop of the alimentary canal. The most conspicuous blood vessel possessed by Cephalodiscus is the dorsal vessel (d.b.v.). A ventral vessel occurs on the anterior side of the metasome and forms a loop extending down the entire length of the stalk, while a “heart” projects into the cavity of the pericardium, probably connected on the ventral side of the notochord with the ventral vessel, and on its dorsal side with the dorsal vessel. At their opposite ends the dorsal and ventral vessels are probably connected with one another by means of a splanchnic sinus surrounding the stomach. The original specimen of C. dodecalophus contained exclusively female zooids, in which a single pair of ovaries (figs. 2, 3, ov.) lie in the metasomatic cavities, and open to the exterior dorsally by short, highly pigmented oviducts (fig. 2, ovd.). In C. nigrescens and in some other species a zooid may contain a pair of ovaries, a pair of testes, or an ovary and a testis, although the males, females and hermaphrodites do not differ from one another in external characters. In C. sibogae (Celebes) the single colony known is of the male sex. The reproductive individuals have undergone an extraordinary simplification of the organs concerned with the collection and digestion of food. Thus the arms are reduced to a single pair and possess no tentacles, there is no definite operculum, and the alimentary canal is vestigial. The testes, which correspond in position with the ovaries of a female Cephalodiscus, constitute the greater part of the animal. Associated with these males are neuter zooids, which usually possess no functional reproductive organs, but have in other respects the structure of an ordinary female Cephalodiscus. It appears probable that there is a vascular connexion between these and the male individuals, which thus derive their nutriment from the neuters. The reproductive organs of Rhabdopleura have but seldom been observed. They resemble those of Cephalodiscus in structure and in position, except that in each sex the gonad occurs on the right side of the body only (Schepotieff, 1906).
Fig. 3.—Section transverse to the long axis of Cephalodiscus dodecalophus (morphologically a frontal section).
|b.c3.,||Body cavity of metasome.|
|c.p.,||Collar-canal, above which is seen the operculum.|
|plch,||Vacuolated tissue of pharyngeal wall, the so-called “pleurochords” of Masterman.|
The eggs of Cephalodiscus possess a large amount of yolk, and it is practically certain that there is no pelagic larval form. The embryos are hatched in an early stage, but their metamorphosis has not been observed. The early development appears to resemble that of the large-yolked species of Balanoglossus. In the bud-development, the three-segmented condition is extremely conspicuous, and a striking feature is the great relative size of the proboscis (fig. 1). A considerable part of the alimentary canal is said to be derived from the ectoderm in the buds of both Cephalodiscus and Rhabdopleura. Schepotieff (1907) states that in the young buds of the latter the central part of the alimentary canal is developed from cells which are apparently not of ectodermic origin.
The affinity of the Pterobranchia to the Enteropneusta may be regarded as definitely established. Considering the wide differences between the two groups in the size and external characters, and in the mode of life, including the mode of feeding, it is indeed surprising that in every important organ the two groups should show a fundamental morphological identity. Their relations to Phoronis are doubtful (see Phoronidea). The question of their affinity to other divisions of the animal kingdom depends principally on the views which are held with regard to the relationships of the Enteropneusta and Phoronidea respectively. The suggestion has been made by Allmann and recently upheld by Schepotieff that Rhabdopleura is related to some of the Graptolites.
Bibliography.—(1) Andersson, “Die Pterobranchier,” Wiss. Ergebn.-Schwed. Südpolar Exp. (1907) vol. v.; (2) Fowler, “Rhabdopleura,” Proc. Roy. Soc. (1893), lii. 132; Festschr. Leuckarts (1892), p. 293-; art. “Hemichorda,” Ency. Brit. (1902), suppl. vols. xxix. p. 249; Quart. Journ. Mic. Sci. (1905), xlviii. 23; (3) Harmer, “Appendix to report on Cephalodiscus,” Challenger Rep. (1887), vol. xx. pt. lxii. p. 39; “Pterobranchia,” Sibona Rep. (1905), Monogr. vol. xxvi. bis.; (4) Lankester, “Rhabdopleura,” Quart. Journ. Mic. Sci. (1884), xxiv. 622; art. “Polyzoa,” Ency. Brit., 9th ed. (1885), xix. 430, 434; “Cephalodiscus nigrescens,” Proc. Roy. Soc. (1905), B. lxxvi. 400; (5) M‘Intosh, “Report on Cephalodiscus,” Challenger Rep. (1887), vol. xx. pt. lxii.; (6) Masterman, “Cephalodiscus,” Quart. Journ. Mic. Sci. (1898), xl. 340 (1903), vol. xlvi. 715; “Cephalodiscus: Budding,” &c., Trans. Roy. Soc. Edin. (1900), vol. xxxix. 507; (7) Ridewood, “Cephalodiscus,” Mar. Invest. S. Africa (1906), vol. iv. 173; National Antarctic Exp., Nat. Hist., ii. (1907); Quart. Journ. Mic. Sci. (1907), vol. ii., 221; (8) G. O. Sars, “Rhabdopleura,” Christiania Univ. Program. (1869), vol. i.; (9) Schepotieff, “Rhabdopleura,” Zool. Jahrb. Abt. Anat. (1906), vol. xxiii., 463; (1907), vol. xxiv., 193; “Cephalodiscus” (1907), vol. xxiv. 553; “Rhabdopleura and Graptolites,” Neues Jahrb. f. Mineral (1905), Bd. ii. p. 79.
(S. F. H.)