Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
PHORONIDEA
Online EncyclopediaEncyclopedia Home :: PER-PIG
del.icio.us it!
|
PHORONIDEA , a zoological See also:order, containing a single genus Phoronis, which is known to be of practically See also:world-wide See also:distribution, while there are many records of its larva, Actinotrocha, from localities where the adult has not been found. Phoronis is often gregarious, the tubes which it secretes being sometimes intertwined in an inextricable See also:mass. These associations of individuals can hardly be the result of the See also:metamorphosis of a corresponding number of larvae, but are probably due to a spontaneous fragmentation of the adult animals, each such fragment developing into a See also:complete Phoronis (De Selys-Longchamps). The See also:animal is from a See also:quarter of an See also:inch to six inches (P. australis) in length. The See also:free end of the See also:long vermiform See also:body ends in a horseshoe-shaped " lophophore," or tentacle-bearing region (fig. 1, a), which strikingly resembles that of the Phylactolaematous See also:Polyzoa (see POLYZOA). In some See also:species (See also:figs. 2, 3) the two ends of the lophophore are rolled into spirals. An oral view of this region (fig. 2) shows: the mouth (m), continuous on either See also:side with the groove between the two See also:series of tentacles; the anus (a), in the See also:middle See also:line, at no See also:great distance from the mouth; a transversely elongated epistome ep), between the mouth and the anus; and, in the concavity of the lophophore, the apertures of the nephridia (n.o.) which, according to De Selys-Longchamps, open into the two large sensory or glandular "lophophoral See also:organs ' the orifices of which are seen at gl. The mouth leads into the See also:oesophagus, which extends straight down the body nearly to the aboral end or " See also:ampulla," where itdilates into a See also:stomach, from which the ascending See also:limb of the U-shaped alimentary See also:canal passes directly to the anus. The coelomic body-cavity is divided by a transverse septum (fig. 3, s) which lies near the bases of the tentacles. The praeseptal or lophophoral coelom is continued into each of the tentacles and into the (After See also:Allman.) a, The horseshoe-shaped lopho- f, Efferent See also:vessel. See also:pore. g, One of the two efferent lopho- b, Mouth. phoral vessels, uniting to c, See also:Optical See also:section of the epistome See also:form f. (seen immediately below the h, Dorsal or afferent vessel. end of the reference-line). i, Body-See also:wall. d, Oesophagus. k. Fused bases of the tentacles. e, See also:Intestine. epistome. The postseptal coelom is partially divided by a ventral mesentery which is attached along the entire length of the See also:convex side of the See also:loop of the alimentary canal (a, a") and by two lateral mesenteries (a') which further connect the oesophagus with the V at. Prb i D (After Benham.) a, Anus. m, Position of the mouth. D, Posterior See also:surface. n.o., Nephridial surface. ep, Epistome. n.o., Nephridial opening. g1, Lophophoral See also:organ. o.t., Bases of See also:outer tentacles. :.t., Bases of inner tentacles. V. Anterior surface. body-wall. Each nephridium is provided with either one or two funnels which open into the postseptal See also:division of the coelom (ne.f). The See also:nervous See also:system lies in the epidermis, externally to the See also:basement-membrane. A See also:general See also:nerve-plexus probably exists over considerable parts of the skin, and there are See also:special nervous concentrations in the region of the epistome and along a See also:double See also:crescent (N) which follows the parietal See also:attachment of the coelomic septum. The See also:part which lies at the See also:base of the epistome is morphologically dorsal in position. It is said by See also:Schultz (11) to develop, in specimens which are regenerating the lophophoral end, from an invagination of the ectoderm ; and in this See also:condition is compared by him with the hollow central nervous system of some Enteropneusta and of Vertebrates. This comparison is not admitted by De Selys-Longchamps. The vascular system contains numerous red See also:blood-corpuscles. The See also:principal blood-channels are two See also:longitudinal vessels which run down the entire length of the body, and are known as the " afferent " vessel (af) and the " efferent " vessel (ef) respectively, from their relation to the tentacles. According to researches in 1907 by De Selys-Longchamps, the blood is driven by the afferent vessel (af) to a crescentic lophophoral vessel (d.v.) which supplies the tentacles. Each of these contains a single blindly cc. (From See also:Fowler, after Benham.) seen from the See also:left side. a, Oesophageal (ventral) mesen- N, See also:Post-oral nerve-See also:tract at tery. base of lophophore. a', Right lateral mesentery. ne.d., Duct of nephridium. a Intestinal mesentery. ne.f., Larger nephridial See also:funnel. af, Afferent vessel. ne.o., See also:External opening of ne- an, Anus. phridium. D, Posterior surface. ce, Oesophagus. d.v., Afferent lophophoral vessel. ot., Bases of outer tentacles. ef, Efferent vessel. R, Intestine. ep, Epistome. r.v., Right efferent lophophoral gl, Lophophoral organ. vessel. it, Bases of inner tentacles. s, Coelomic septum. m, Mouth. V, Anterior side. ending vessel which bifurcates at its base (see fig. 3). One of these branches communicates with the afferent lophophoral vessel, while the other one opens into the crescentic efferent lophophoral vessel (r.v.). From this the blood passes into two lateral 'vessels which See also:pierce the coelomic septum (s.), the right vessel proceeding on the anterior side of the oesophagus, as shown in fig. 3, to effect a See also:union with the left one, and thus to constitute the See also:main efferent vessel, which gives off numerous caecal branches as it passes down the body. Hence the blood returns once more to the afferent vessel through a splanchnic sinus which surrounds the stomach. The circulation is maintained by the rhythmical contraction of the afferent vessel and by less See also:regular contractions of some of the other vessels. The reproductive organs See also:lie on the left side, near the aboral end, both ovary and testis being See also:present in the same individual in some of the species. They are said to be See also:developed from the coelomic epithelium which covers the efferent vessel or its caeca. The reproductive cells pass to the exterior by means of the nephridia. See also:Reproduction by budding does not occur, although spontaneous fragmentation of the body, followed by complete regeneration of each of the pieces, is known to take See also:place. Regeneration of the tentacular end of the animal is of frequent occurrence. Developnient and See also:Affinities.—The eggs of Phoronis are small and usually undergo their See also:early development attached to the tentacles of the adult. The attachment is probably effected (Masterman) by the secretion of the lophophoral organs (fig. 2, gl.). After the formation of an invaginate gastrula the larval form is rapidly acquired. On quitting the shelter of the See also:parent tentacles the embryo becomes a pelagic larva, known as Actinotrocha (fig. 4) characterized by the See also:possession of a line of tentacles See also:running obliquely See also:round the body. Locomotion is effected principally by means of a posterior See also:ring of See also:cilia surrounding the anus. The mouth (o) is in front of the tentacles, on the ventral side, and is overhung by a See also:mobile praeoral See also:hood, in which is the principal part of the nervous system. An oblique septum which follows the bases of the tentacles and corresponds with that of the adult animal divides the body-cavity into two portions. The postseptal division is a coelomic space, partially subdivided by a ventral mesentery. The praeseptal cavity is a vascular space, since it is in free communication with the dorsal vessel of the larva, and it persists in part as the two lophophoral vascular crescents of the adult. It contains two tufts of See also:peculiar excretory cells, described by See also:Goodrich (5) as " solenocytes," which surround the See also:blind ends of a pair of nephridia. These pass backwards through the septum and open to the exterior ventrally. After the Actinotrocha has led a pelagic See also:life for some See also:time it develops a large ventral invagination of its body-wall (fig. 4, 2, iv.). At the metamorphosis, this See also:sac is everted and the alimentary canal is See also:drawn into it in the form of a loop (fig. 4, 3, 4). Most of the praetentacular region and the larval tentacles See also:separate off, being then taken into the alimentary canal, where they are digested. The relations of the surfaces after the metamorphosis are clearly very different from those which obtained in the larva. The dorsal surface of the adult is the one between the mouth and the anus, while the median ventral line is the one which corresponds with the convexity of the alimentary canal. This view of the surfaces is, however, disputed by De Selys-Longchamps, who regards the aboral extremity of the adult as the posterior end. The development of Phoronis was supposed by Caldwell (2) to furnish the explanation of the relations of the surfaces in See also:Brachiopoda, Polyzoa and perhaps the Sipunculoid See also:Gephyrea, in which the ontogenetic See also:evidence is less clear. Caldwell's views were accepted by Lankester (8) in the 9th edition of this See also:work, the Phylum Podaxonia being there instituted to include the See also:groups just mentioned, together with the See also:Pterobranchia. The peduncle of the Brachiopoda was supposed to correspond with the everted ventral sac of Actinotrocha, but the question is complicated by the want of any complete investigation of the development of the Brachiopoda, and by the See also:absence of the anus in the See also:majority of the genera. There is, however, a considerable amount of re-semblance between the lophophore of Phoronis australis, with its spirally See also:twisted ends, and that of a typical Brachiopod; nor do the structural details of the adult Brachiopods forbid the view that they may be related to Phoronis. The See also:comparative study of the development does not support the See also:hypothesis that the Polyzoa (q.v.) are comparable with Phoronis. In Pedicellina, the only Polyzoon in which the alimentary canal of the larva is known to become that of the first adult individual, the line between the mouth and anus is ventral in the larva; and since there is no reversal of the curvature of the See also:digestive loop during the metamorphosis it must be regarded as ventral in the adult. There are, indeed remarkable similarities between the external characters of the Phylactolaematous Polyzoa and the Phoronidea, and notably between their lophophores. The supposed occurrence of a pair of nephridia in certain Phylactolaemata, in a position corresponding with that of the nephridia of Phoronis, must also be mentioned, 3, Commencement of the See also:meta-morphosis. 4, Later See also:stage in the metamorphosis: a, anus; iv, ventral invagination; o, mouth. although it has been maintained that the "nephridia " of Phylactolaemata are merely ciliated portions of the body-cavity and not indeed nephridia at all. But a serious objection to the comparison is that the development of Phylactolaemata can be explained by supposing it to be a modification of what occurs in other Polyzoa, while it appears to have no relation whatever to that of Phoronis. Most observers consider that Actinotrocha is a highly modified Trochosphere, and this would give it some claim to be regarded as distantly related to the Entoproct Polyzoa and to other groups which have a Trochosphere larva. A ,A a 1 AB, Anteroposterior See also:axis. DV, Dorsoventral axis. I, 2, Actinotrocha. Phoronis has long been regarded as a possible ally of Rhabdopleura (see PTEROBRANCHIA); and Masterman (lo) has attempted to demonstrate the existence in Actinotrocha of most of the structures which occur in the Pterobranchia. According to his view the praeoral hood of Actinotrocha (cf. fig. 4) corresponds with the " See also:proboscis " of Pterobranchia; the succeeding region, as far as the bases of the tentacles, with the See also:collar; and the post-tentacular region with the metasome. Masterman's more detailed comparisons have for the most part been rejected by other morphologists. One of the most formidable difficulties in the way of the See also:attempt to reduce Actinotrocha to the Pterobranchiate type of structure Is the condition of the coelom in the former. There is indeed a perfectly definite transverse septum which divides the body-cavity in the region of the tentacle-bases. Even if it be admitted that the postseptal space may be the metasomatic cavity, the praeseptal space can hardly be regarded as coelomic in nature, since it is in continuity with the vascular system; while Masterman's conclusion that the cavity of the praeoral hood (the supposed proboscis-cavity) is separated from that of the supposed collar has received no See also:confirmation. In spite of these difficulties it must be conceded that the dorsal flexure of the alimentary canal of the Pterobranchia is very Phoronis-like. It has, moreover, been shown (see especially Goodrich, 5) that shortly before its metamorphosis, Actinotrocha develops a coelomic space which lies immediately in front of the oblique septum, and gives rise later to the cavity of the lophophore and tentacles. Regarding this as a collar-cavity, it becomes possible to agree with Masterman that the region shown in fig. 4, I. between the tentacles and the praeoral hood, is really a collar the coelom of which develops relatively See also:late. It will be noticed that the lophophore of Phoronis is, on this See also:assumption, a derivative of the collar just as it is in the Pterobranchia. The epistome of the adult Phoronis cannot well be the proboscis since its cavity is continuous with the lophophoral coelom, and because the praeoral hood of Actinotrocha is entirely lost at the metamorphosis. It is possible that this See also:consideration will See also:account for the want of an anterior body-cavity in Phoronis. Since the proboscis is a purely larval organ in this genus, it may be supposed that the coelomic space which properly belongs to it fails to develop, but that the praeoral hood itself is none the less the morphological representative of the proboscis. In spite of the criticisms which have been made on the conclusion that Phoronis is allied to the Pterobranchia, it is thus possible that the view is a See also:sound one, and that the Phoronidea should take their place, with the Enteropneusta and the Pterobranchia, as an order of the Hemichordata. (8) Lankester, See also:art. " Polyzoa," Ency. Brit. xix. 430, 433 (1885); (9) De Selys-Longchamps, See also:Arch. Biol. xviii. 495 (1902); Wiss. Meeresunt. (N. F.) vi. See also:Abt. Helgoland (1903), Heft i.; Mem. classe sci. acad. belgique, vol. i. (1904); See also:Fauna u. See also:Flora G. v. Neapel, 3o Monogr. (1907); (10) Masterman, Quart. Journ. Mic. See also:Soc. xl. 281 (1898); xliii. 375 (1900); (II) Schultz, Zeitschr. wiss. Zool. lxxv. 391, 473 (1903); (12) See also:Shearer, Mitth. zool. Stat. Neapel, xvii. 487 (1906) ; (13) See also:Shipley, Cambr. Nat. Hist. ii. 450 (1896). (S. F. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] PHORMIUM |
[next] PHORORHACOS |