Online Encyclopedia

collar. By the waves of contrac- tion executed by the proboscis accompanied by inflation of the collar, progression is effected, some- times with marvellous rapidity. The third body region or See also:

TRUNK (Fr. tronc, Lat. truncus, cut off, maimed)

trunk may attain a See also:

GREAT

great length; one or two feet, or even more, and is also muscular, but the truncaI muscles are of subordinate import- ance in locomotion, serving princi- pally to promote the peristaltic contractions of the body by which the See also:

food is carried through the gut. The See also:

FUNCTION

function of alimentation is closely associated with that of locomotion, somewhat as in the burrowing See also:

EARTHWORM

earthworm; in the ex- cavation of its burrows the sand is passed through the body, and any nutrient See also:

MATTER

matter that may ad- here to it is extracted during its passage through the See also:

INTESTINE (Lat. intestinus, internal, usually in neuter plural intestina. from intus, within)

intestine, the exhausted sand being finally ejected through the vent at the orifice of the burrow and appearing at See also:

low (New See also:

CALEDONIA

Caledonia), from ante with this manner of feeding, above; about life See also:

SIZE

size. the mouth is kept permanently open and prevented from collapsing by a pair of skeletal cornua belonging to a sustentacular apparatus (the nuchal See also:

SKELETON

skeleton), the body of which lies within the narrow See also:

NECK (O. Eng. hnecca; the word appears in many Teutonic languages; cf. Dutch ride, Ger. Nacken; in O. E. the common word was heals; cf. Ger. Hals)

neck of the proboscis; the latter is inserted into the collar and surrounded by the anterior See also:

FREE

free flap of this segment of the body. When first discovered by J. F. See also:

ESCHSCHOLTZ, JOHANN FRIEDRICH (1793—1831)

Eschscholtz at the See also:

Marshall Islands in 1825, Balanoglossus was described as a worm-like animal belonging to the Echinoderm See also:

order of Holothurians or sea-cucumbers. In 1865 Kowalevsky discovered that the See also:

ORGANS

organs of respiration consist of numerous pairs of gill-slits leading from the See also:

DIGESTIVE

digestive See also:

canal through the thickness of the body-See also:

wall to the exterior.

On this See also:

account the animal was subsequently placed by See also:

GEGENBAUR, CARL (1826-1903)

Gegenbaur in a See also:

SPECIAL

special class of Vermes, the Enteropneusta. In 1883–1886 See also:

BATESON (BATSON or BETSON), THOMAS

Bateson showed by his embryological researches that the Enteropneusta exhibit chordate (vertebrate) See also:

AFFINITIES

affinities in respect of the coelomic, skeletal and See also:

NERVOUS

nervous systems as well as in regard to the See also:

RESPIRATORY

respiratory See also:

SYSTEM

system, and, further, that the gill-slits are formed upon a See also:

plan similar to that of the gill-slits of See also:

AMPHIOXUS, or LANCELET

Amphioxus, being subdivided by tongue-bars which depend from the dorsal See also:

BORDERS, THE

borders of the slits. Coelom and See also:

PORE

Pore-canals.—In See also:

correspondence with the tri-regional differentiation of the body in its See also:

EXTERNAL

external configuration, the coelom (body-cavity, perivisceral cavity) is divided into three portions completely separated from one another by septa:—(1) proboscis-coelom, or first body-cavity; (2) the collar-coelom, or second body-cavity;(3) truncal coelom, or third body-cavity. Of these divisions of the coelom the first two communicate with the exterior by means, of a pair of ciliated pore-canals placed at the posterior end of their respective segments. The proboscis-pores are highly variable, and frequently only one is See also:

PRESENT

present, that on the See also:

LEFT

left See also:

side; sometimes the pore-canals of the proboscis unite to open by a See also:

COMMON

common median orifice, and sometimes their communication with the proboscis coelom appears to be occluded, and finally the pore-canals may be quite vestigial. The collar-pores are remarkable for their constancy; this is probably owing to the fact that they have become adapted to a special function, the inhalation of See also:

WATER

water to render the collar turgid during progression. There are reasons for supposing that the truncal coelom was at one See also:

time provided with pore-canals, but supposed vestiges of these structures have only been described for one genus, Spengelia,in which they See also:

lie near the anterior end of the truncal coelom. Enteron.—Not only is the coelom thus subdivided, but the enteron (gut, alimentary canal, digestive See also:

TUBE (Lat. tuba)

tube) itself shows indications of three See also:

main subsections in continuity with one another: (1) proboscis-gut (Eicheldarm, stomochord, vide infra) ; (2) collar-gut (buccal cavity, See also:

THROAT (O. Eng. protu, prote or }Prota, possibly from preotan, to press, whence threat, or, with loss of initial s, connected with strut, to swell)

throat) ; (3) truncal gut extending from the collar to the vent. Stomochord.—The proboscis-gut occurs as an outgrowth from the anterior dorsal wall of the collar-gut, and extends forward into the basal (posterior) region of the proboscis, through the neck into the proboscis-coelom, ending blindly in front. Although an integral portion of the gut, it has ceased to assist in alimentation, its epithelium undergoes vacuolar differentiation and See also:

HYPERTROPHY (Gr. inrEp, over, and rpo(A, nourishment)

hypertrophy, and its lumen becomes more or less vestigial. It has, in fact, become metamorphosed into a resistant supporting structure resembling in some respects the notochord of the true Chordata, but probably not directly comparable with the latter structure, being related to it solely by way of substitution. On account of the presence and mode of origin (from the gut-wall) of this organ Bateson introduced the See also:

TERM

term See also:

HEMICHORDA, or HEMICHORDATA

hemichorda as a phyletic name for the class Enteropneusta.

As the proboscis-gut appears to have undoubtedly skeletal properties, and as it also has topographical relations with the mouth, it has been designated in See also:

ENGLISH

English by the non-committal term stomochord. It is not a See also:

SIMPLE

simple diverticulum of the collar-gut, but a complex structure possessing paired lateral pouches and a ventral convexity (ventral caecum) which rests in a concavity at the front end of the body of the nuchal skeleton (fig. 3). In some See also:

SPECIES

species (Spengelidae) there is a See also:

long capillary vermiform See also:

EXTENSION (Lat. ex, out ; tendere, to stretch)

extension of the stomochord in front. The nuchal skeleton is a non-cellular laminated thickening of See also:

BASEMENT

basement-membrane underlying that portion of the stomochord which lies between the above-mentioned pouches and the orifice into the throat. At the point where the stomochord opens into the buccal cavity the nuchal skeleton bifurcates, and the two cornua thus produced pass obliquely backwards and downwards embedded in the wall of the throat, often giving rise to projecting ridges that See also:

BOUND, or BOUNDARY (from O. Fr. bonde, Med. Lat. bodena or butina, a frontier line)

bound a dorsal groove of the collar-gut which is in continuity with the wall of the stomochord (fig. 3) Nervous System.—At the See also:

BASE

base of the epidermis (which is in general ciliated) there is over the entire See also:

SURFACE

surface of the body a layer of See also:

NERVE (Lat. nervus, Gr. vevpov, a bowstring)

nerve-See also:

fibres, occurring immediately outside the basement-membrane which separates the epidermis from the subjacent musculature. The nervous system is thus essentially epidermal in position and diffuse in See also:

DISTRIBUTION (Lat. distribuere, to deal out)

distribution; but an interesting concentration of nerve-cells and fibres has taken See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place in the collar-region, where a medullary tube, closed in from the outside, opens in front and behind by anterior and posterior neuropores. This is the collar nerve-tube. Sometimes the central canal is wide and uninterrupted between the two neuropores; in other cases it becomes broken up into a large number of small closed medullary cavities, and in others again it is obsolete. In one See also:

FAMILY

family, the Ptychoderidae, the medullary tube of the collar is connected at intermediate points with the epidermis by means of a variable number of unpaired outgrowths from its dorsal wall, generally containing an axial lumen derived from and in continuity with the central canal. These hollow roots terminate blindly in the dorsal epidermis of the collar, and See also:

ACE (derived through the Lat. as, from the Tarentine form of the Gr. etc)

ace the nervous layer of the latter in See also:

DIRECT

direct connexion with the fibres of the nerve-tube.

The exact significance of these roots is a matter for See also:

SPECULATION

speculation, but it seems possible that they are epiphysial structures remotely comparable with the epiphysial (pineal) complex of the craniate vertebrates. In accordance with this view there would be also some See also:

PROBABILITY (Lat. probabilis, probable or credible)

probability in favour of regarding the collar nerve-tube of the Enteropneusta as the See also:

EQUIVALENT

equivalent of the cerebral vesicle only of Amphioxus and the Asridian See also:

TADPOLE

tadpole, and also of the See also:

PRIMARY

primary fore-See also:

BRAIN (A.S. braegen)

brain of vertebrates. Special thickenings of the diffuse nervous layer of the epidermis occur in certain regions and along certain lines. In the neck of the proboscis the fibrous layer is greatly thickened, and other intensifications of this layer occur in the dorsal and ventral See also:

MIDDLE

middle lines of the trunk extending to the posterior end of the body. The dorsal epidermal nerve-See also:

TRACT (from Lat. tractare, to treat of a matter, through Provencal tractat and Ital. trattato)

tract is continued in front into the ventral wall of the collar nerve-tube, and at the point of junction there is a circular commissural thickening following the posterior rim of the collar and affording a special connexion between the dorsal and ventral nerve tracts. From the ventral surface of the cellar nerve-tube numerous motor fibres may be seen passing to the subjacent musculature. These fibres are not aggregated into roots. rods in the tongue-bars of Balanoglossus are See also:

double; (d) the tongue-See also:

bar in Balanoglossus does not fuse with the ventral border of the cleft, but ends freely below, thus producing a continuous U-shaped cleft. The meaning of this singular contrast between the two animals may be that we have here an instance of an interesting gradation in See also:

EVOLUTION

evolution. From serving primitively as the essential organ of the cleft the tongue-bar may have undergone reduction and modification, becoming a secondary bar in Amphioxus, subordinate to the primary bars in size, vascularity and development; finally, in the craniate vertebrates- it would then have completed its involution, the See also:

SUGGESTION

suggestion having been made that the tongue-bars are represented by the thymusprimordia. Gill-pouches and Gill-pores.—Only rarely do the gill-slits open freely and directly to the exterior (fig. 1).

In most species of Balanoglossus each gill-slit may be said to open into its own atrial chamber or gill-pouch; this in its turn opens to the exterior by a See also:

minute gill-pore. There are, therefore, as many gill-pouches as there are gill-slits and as many gill-pores as pouches. The gill-pores occur on each side of the dorsal aspect of the worm in a See also:

LONGITUDINAL

longitudinal See also:

SERIES (a Latin word from serere, to join)

series at the base of a shallow groove, the branchial groove. The respiratory current of water is therefore conducted to the exterior by different means from that adopted by Amphioxus, and this difference is so great that the theory which seeks to explain it has to postulate See also:

RADICAL (Lat. radix, a root)

radical changes of structure, function and See also:

topography. Excretory and Vascular Systems.—It seems likely that the coelomic pore-canals were originally excretory organs, but in the existing Enteropneusta the pore-canals (especially the collar canals) have, as we have seen, acquired new functions or become vestigial, and the function of See also:

EXCRETION (Lat. ex, out of, cernere, cretum, to separate)

excretion is now mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom. The vascular system itself is quite peculiar, consisting of lacunae and channels destitute of endothelium, situated within the thickness of the basement-membrane of the body-wall, of the gut-wall and of the mesenteries. The See also:

BLOOD

blood, which is a non-corpuscular fluid, is propelled forwards by the contractile dorsal See also:

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

vessel and collected into the central blood-sinus; this lies over the stomochord, and is surrounded on three sides by a closed vesicle, with contractile walls, called the pericardium (Herzblase). By the pulsation of the pericardial vesicle (best observed in the larva) the blood is driven into the glomerulus, from which it issues by efferent vessels which effect a junction with the ventral (sub-intestinal) vessel in the trunk. The vascular system does not readily lend itself to morphological comparison between such widely different animals as Balanoglossus and Amphioxus, and the reader is therefore referred to the See also:

MEMOIRS

memoirs cited at the end of this See also:

ARTICLE (from Lat. articulus, a joint)

article for further details. Reproductive System.—The sexes are See also:

SEPARATE

separate, and when mature are sometimes distinguished by small See also:

DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences of colour in the genital region. Both male and See also:

FEMALE

female gonads consist of more or less lobulated hollow sacs connected with the epidermis by See also:

SHORT, FRANCIS JOB (1857– )

short ducts. In their disposition they are either uniserial, biserial or multiserial.

They occur in the branchial region, and also extend to a variable distance behind it. In exceptional cases they are either confined to the branchial region or excluded from it. Whenthey are arranged in uniserial or biserial rows the genital ducts open into or near the branchial grooves in the region of the pharynx and in a corresponding position in the See also:

POST

post-branchial region. An important feature is the occurrence in some species (Ptychoderidae) of paired longitudinal pleural or lateral folds of the body which are See also:

MOBILE

mobile, and can be approximated at their free edges so as to See also:

close in the dorsal surface, embracing both the median dorsal nerve-tract and the branchial grooves with the gill-pores, so as to See also:

FORM (Lat. forma)

form a temporary See also:

PERI, JACOPO (1561–16 ?)

peri-branchial and medullary tube, open behind where the folds cease. On the other See also:

hand, they can be spread out horizontally so as to expose their own upper side as well as the dorsal surface cts, posterior limit of collar. dv, dorsal vessel passing into central sinus (bs). ev, efferent vessel passing into ventral vessel (vv). epr, epiphysial tubes. st, stomochord. vs, ventral septum of proboscis. sk, body of nuchal skeleton. m, mouth. th, throat. tb, tongue-bars.

tc, trunk coelom. of the body (fig. 1). These folds are called the genital pleurae because they contain the bulk of the gonads. Correlated with the presence of the genital pleurae there is a pair of vascular folds of the basement membrane proceeding from the dorsal wall of the gut in the postbranchial portion of the branchio-genital region, and from the dorsal angles made by the pleural folds with the body-wall in the pharyngeal region ; they pass, in their most fully See also:

DEVELOPED

developed See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition, to the free border of the genital pleurae. These vascular membranes are called the lateral septa. Since there are many species which do not possess these genital pleurae, the question arises as to whether their presence or their See also:

ABSENCE (Lat. absentia)

absence is the more See also:

PRIMITIVE

primitive condition. Without attempting to See also:

ANSWER (derived from and, against, and the same root as swear)

answer this question categorically, it may be pointed out that within the limits of the family (Ptychoderidae) which is especially characterized by their presence there are some species in Gill-slits.—The See also:

possession of gill-slits is as interesting a feature in the organization of Balanoglossus as is the presence of tracheae in See also:

PERIPATUS

Peripatus. These gill-slits occupy a variable extent of the anterior portion of the trunk, commencing immediately behind the collar-trunk septum. The branchial bars which constitute the borders of the clefts are of two kinds:—(1) Septal bars between two contiguous clefts, corresponding to the primary bars in Amphioxus; (2) Tongue- bars. The See also:

CHIEF (from Fr. chef, head, Lat. caput)

chief resemblances . --See also:

DENS, PETER (1690-1775)

dens between Balanoglossus and II Innl nu MII mI --d3.

. Amphioxus in respect of _ '. ~IVI nul nnoa lal•n the gill-slits may be stated briefly as follows:—(a) the presence of two kinds of ==-tb branchial bars in all species and also of small See also:

CROSS

cross-bars (synapticula) in many species; 0) numerous gill-slits, from See also:

FORTY

forty to more than a See also:

HUNDRED

hundred pairs; (y) the addition of new gill-slits by fresh perforation at the posterior end of the pharynx throughout life. The chief differences are, that (a) the Yr tongue-bar is the essential _=--'vim organ of the gill-slit in Balanoglossus, and exceeds be, coelom. gp, gill-pore. while in Amphioxus the tb, tongue-bars. dn, dorsal nerve. See also:

REVERSE

reverse is the See also:

case; (b) the ds, mesentery. dv, vessel. bar contains a large pr, See also:

ridge. ce, See also:

OESOPHAGUS (Gr. o'lvca=I will carry, and r/)ayeiv, to eat)

oesophagus. coelomic space in Balano- glossus, vessel. vs, mesentery. glossus, but is solid in Am- phioxus; ventral nerve. phioxus; (c) the skeletal tongue- II I I I I I I I I I I IyY,: a, Arrow from proboscis-cavity (pc) passing to left of peri- cardium (per) and out through proboscis pore-canal. b', arrow from central canal of neurochord (cnc) passed out through anterior neuropore. b2, See also:

DITTO (from the Lat. dictum, something said, Ital. detto, aforesaid)

ditto; through posterior neuro- pore. c, arrow intended to pass from 1st gill-pouch through collar pore-canal into collar-coelom (cc). which the genital pleurae are quite obsolete, and yet lateral septa occur (e.g. Ptychodera ruficollis), seeming to indicate that the pleural folds have in such cases been secondarily suppressed. Development.—The development of Balanoglossus takes place according to two different schemes, known as direct and indirect, correlated with the occurrence in the See also:

GROUP

group of two kinds of ova, large and small. Direct development, in which the adult form is achieved without striking See also:

METAMORPHOSIS

metamorphosis by a See also:

GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

gradual See also:

SUCCESSION (Lat. successio, from succedere, to follow after)

succession of stages, seems to be confined to the family Balanoglossidae. The remaining two families of Enteropneusta, Ptychoderidae and Spengelidae, contain species of which probably all pursue an indirect course of development, culminating in a metamorphosis by which the adult form is attained. In these cases the larva, called Tornaria, is pelagic and transparent, and possesses a complicated ciliated seam, the longitudinal ciliated See also:

BAND

band, often See also:

DRAWN

drawn out into convoluted bays and lappets.

In addition to this ciliated band the form of the Tornaria is quite characteristic and unlike the adult. The Tornaria larva offers a certain similarity to larvae of Echinoderms (sea-urchins, See also:

STAR

star-fishes, and sea-cucumbers), and when first discovered was so described. It is within the See also:

BOUNDS, BEATING THE

bounds of possibility that Tornaria actually does indicate a remote See also:

affinity on the See also:

PART

part of the Enteropneusta to the Echinoderms, not only on account of its external form, but also by See also:

REASON (Lat. ratio, through French raison)

reason of the possession of a dorsal water-pore communicating with the anterior body-cavity. In the direct development Bateson showed that the three divisions of the coelom arise as pouches constricted off from the archenteron or primitive gut, thus resembling the development of the mesoblastic somites of Amphioxus. It would appear that while the direct development throws See also:

LIGHT

light upon the special plan of organization of the Enteropneusta, the indirect development affords a See also:

CLUE

clue to their possible derivation. However this may be, it is sufficiently remarkable that a small and circumscribed group like the Enteropneusta, which presents such a comparatively See also:

UNIFORM

uniform plan of See also:

composition and of external form, should follow two such diverse methods of development. Distribution.—Some See also:

THIRTY

thirty species of Balanoglossus are known, distributed among all the See also:

PRINCIPAL

principal marine provinces from Green-See also:

LAND

land to New See also:

ZEALAND (also SEALAND Or SEELAND; Danish Sjaelland)

Zealand. The species which occurs in the English Channel is Ptychodera sarniensis. The Ptychoderidae and Spengelidae are predominantly tropical and subtropical, while the Balanoglossidae are predominantly See also:

ARCTIC (Gr. "ApKroc, the Bear, the northern constellation of Ursa Major)

arctic and temperate in their distribution. One of the most singular facts concerning the See also:

GEOGRAPHICAL

geographical distribution of Enteropneusta has recently been brought to light by Benham, who found a species of Balanoglossus, sensu stricto, on the See also:

COAST (from Lat. costa, a rib, side)

coast of New Zealand hardly distinguishable from one occurring off See also:

JAPAN

Japan. Finally, Glandiceps abyssicola (Spengelidae) was dredged during the " Challenger " expedition in the See also:

ATLANTIC

Atlantic Ocean off the coast of See also:

Africa at a See also:

DEPTH

depth of 2500 fathoms.

End of Article: BALANOGLOSSUS

Additional information and Comments

There 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.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
BALANCE OF TRADE

[next]
BALARD, ANTOINE JEROME (1802-1876)