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PTEROBRANCHIA

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Originally appearing in Volume V22, Page 616 of the 1911 Encyclopedia Britannica.
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PTEROBRANCHIA , a zoological See also:

group established by See also:Ray Lankester in 1877. It contained at that See also:time the single genus Rhabdopleura, a See also:minute See also:animal dredged by Sara off the Lofoten Islands, and by See also:Norman off the Shetlands. Rhabdo pleura was at first regarded as an aberrant Polyzoon, but with the publication of the Challenger See also:Report (Cephalodiscus) in 1887, it became clear that Cephalodiscus, the second genus now included in the See also:order, had See also:affinities in the direction of the Enteropneusta. The connexion of the Pterobranchia with the See also:Polyzoa is in the highest degree questionable. Rhabdopleura is no doubt of See also:world-wide See also:distribution, since it has been recorded in various localities from See also:Greenland to See also:South See also:Australia, usually in See also:water of not less than See also: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 See also:Magellan, has recently been found in entirely different parts of the world, as for instance between See also:Japan and See also:Korea at too fathoms, at about See also:half that See also:depth off the south-See also:east See also:coast of See also:Celebes, and between See also:tide-marks on the coast of See also:Borneo. It appears to be See also:common in the neighbourhood of Cape See also:Town, while the See also:recent See also:Antarctic expeditions have shown that it occurs in various localities from the See also:Falkland Islands to the Antarctic circle. No less than twelve See also:species, referred to three sub-genera (Demiothecia, Idiothecia, Orthoecus), have now been described; but it is at See also: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 See also: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 proboscides of the zooids. In Rhabdo pleura each zooid forms its own delicate See also:tube composed of a characteristic See also: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 See also:independent tube (Idiothecia, Orthoecus). The zooids are a modification of the type of structure known in See also:Balanoglossus, from which they differ principally in the following respects: •(i.) The alimentary See also:canal, instead of being straight, has a U-shaped flexure, the dorsal See also:line between the mouth and the anus being See also:short.

(ii.) The See also:

proboscis (fig. 1, b), known as the " buccal See also:shield," is a large See also:organ, strongly flattened in an (From a See also:drawing by See also:Professor McIntosh.) Fin. 1. Zooid of Cephalodiscus dodecalophus. a, a, Buds. d, Arms and tentacles. b, Proboscis. e, Ventral edge of proboscis. c, Stalk. f, Its dorsal edge. antero-posterior direction, its ventral See also:lobe usually concealing the mouth. (iii.) The See also:collar is produced dorsally into arms (one pair in Rhabdopleura, four to eight pairs in Cephalodiscus), each of which bears numerous ciliated tentacles, the See also:organs by which the microscopic See also:food-particles are conveyed to the mouth. (iv.) The third See also:division of the See also:body, the metasome, is prolonged ventrally into a relatively enormous outgrowth containing the See also:loop of the alimentary canal, beyond which projects a stalk (fig. 1, c), of a length varying with the See also: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 See also:

parent, in Cephalodiscus they become See also:free at an See also:early See also:stage, and the coenoecium accordingly contains a number of See also:separate individuals. In the living Cephalodiscus a zooid can crawl by means of its proboscis over the gelatinous processes of the See also:outer See also:side of the coenoecium, a position which it can assume owing to the very See also:great extensibility of the stalk, the proximal suctorial end of which remains attached to the inner See also:surface of some See also:part of the coenoecium (Andersson, 1907). In See also:correspondence with the fundamental constitution of the zooid, each of the three segments has its own body-cavity separated from the others. The See also:main proboscis-cavity (fig. 2, b.c.i) 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 See also: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 See also:complete. It extends into the arms, which originate in the bud (fig. I)asdorsal 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 See also:lower See also:lip, but must be important in separating the food-current produced by the See also:cilia of the tentacles from the See also: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 See also:base of the operculum.

PP (After Harmer.) a, Anus. Body-cavity of proboscis. Of collar. b.c3., Of metasome. int., See also:

Intestine. m., Mouth. nch., Notochord. n.s., Central See also:nervous See also:system. oes., See also:Oesophagus. While it is not improbable that the collar-pores and the proboscis-pores may evacuate excretory substances, there can be little doubt that their See also:primary See also: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 See also:strain off the superfluous water which is introduced into the mouth during the See also: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.), See also:

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.See also:mes.) and ventral mesenteries, the latter following the outer curvature of the loop of the alimentary canal. The most conspicuous See also:blood See also: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 " See also: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 See also:original specimen of C. dodecalophus contained exclusively See also:female zooids, in which a single pair of ovaries (See also:figs. 2, 3, ov.) See also:lie in the See also:meta-somatic cavities, and ofen to the exterior dorsally by short, highlypigmented 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 See also:

males, See also:females and hermaphrodites do not differ from one another in external characters. In C. sibogae (Celebes) the single See also:colony known is of the male See also: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 See also:ordinary female Cephalodiscus. It appears probable that there is a vascular connexion between these and the male individuals, which thus de-rive their nutriment from the neuters. The reproduc- tive organs of Rhabdopleura (After Masterman.) have but seldom been ob- served. They resemble FIG. 3.-See also:Section transverse to the those of Cephalodiscus in See also:long See also:axis of Cephalodiscus dodecalophus structure and in position, (morphologically a frontal section). except that in each sex the b.ca., Body cavity of metasome. gonad occurs on the right c.p., Collar-canal, above which is side of the body only seen the operculum.

(Schepotieff, 1906). d.b.v., Dorsal blood-vessel. The eggs of Cephalodiscus d.mes.,Dorsal mesentery. possess a large amount of e p„ Proboscis. yolk, and it is practically g.s., Gill-slit. certain that there is no See also:

ant., Intestine. pelagic larval See also:form. The l.c.c., See also:Left collar-cavity. embryos are hatched in an in., Mouth. early stage, but their meta- ov., Ovary, morphosis has not been plch, Vacuolated See also:tissue of pharyngeal observed. The early de- See also:wall, the so-called " pleuro- velopment appears to re- chords ".of Masterman. semble that of the large- r,c.c., Right collar-cavity. yolked species of See also:Bala- noglossus. In the bud- development, the three-segmented See also:condition is extremely conspicuous, and a striking feature is the great relative See also:size of the proboscis (fig.

I). 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 See also:

young buds of the latter the central part of the alimentary canal is See also:developed from cells which are apparently not of ectodermic origin. The See also:affinity of the Pterobranchia to the Enteropneusta may be regarded as definitely established. Considering the wide See also:differences between the two See also:groups in the size and external characters, and in the mode of See also: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 See also:PHORONIDEA). The question of their affinity to other divisions of the animal See also:kingdom depends principally on the views which are held with regard to the relationships of the Enteropneusta and Phoronidea respectively. The See also:suggestion has been made by Allmann and recently upheld by Schepotieff that Rhabdopleura is related to some of the See also:Graptolites. Cephalodiscus," Quart. Journ. .Mic. Sci.

(1898), xl. 340; op., Operculum, or ventral lip. ov., Ovary. ovd., Oviduct. ph., Pharynx. p.p., Proboscispore. p.s., Proboscis. st., Stomach. stk., Stalk. (1903), vol. xlvi. 715; " Cephalodiscus: Budding," &c., Trans. See also:

Roy. See also:Soc.

Edin. (1900), vol. xxxix. 507; (7) Ridewood, " Cephalodiscus " See also:

Mar. Invest. S. See also:Africa (1906), vol. iv. 173; See also:National Antarctic Exp., Nat. Hist., ii. (1907); Quart. Journ. Mic. Sci.

(19o7), vol. ii., 221; (8) G. O. Sars, " Rhabdopleura," See also:

Christiania Univ. Program. (1869), vol. 1.; (9) Schepotieff, " Rhabdopleura," Zool. Jahrb. See also:Abt. Anat. (1906), vol. See also:xxiii., 463; (1907), vol. See also:xxiv., 193; " Cephalodiscus " (1907), vol. xxiv. 553 ; " Rhabdopleura and Graptolites," Neues Jahrb. f. See also:Mineral (19o5), Bd. ii. p.

79. (S. F.

End of Article: PTEROBRANCHIA

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