Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/113

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BALANOGLOSSUS 85 structures remotely comparable with the epiphysial (pineal) glomerulus, a vascular complex placed on either side of the complex of the craniate vertebrates. In accordance with this anterior portion of the stomochord, projecting into the proboscisview there would be also some probability in favour of regarding coelom. The vascular system itself is quite peculiar, consisting the collar nerve-tube of the Enteropneusta as the equivalent of of lacunse and channels destitute of endothelium, situated within the cerebral vesicle only of Amphioxus and the Ascidian tadpole, the thickness of the basement-membrane of the body-wall, of the and also of the primary fore-brain of vertebrates. gut-wall, and of the mesenteries. The blood, which is a nonSpecial thickenings of the diffuse nervous layer of the corpuscular fluid, is propelled forwards by the contractile dorsal epidermis occur in certain regions and along certain lines. In vessel and collected into the central blood-sinus; this lies over the neck of the proboscis the fibrous layer is greatly thickened, the stomochord, and is surrounded on three sides by a closed and other intensifications of this layer occur in the dorsal and vesicle, with contractile walls, called the pericardium (“ Herzventral middle lines of the trunk extending to the posterior end blase ). By the pulsation of the pericardial vesicle (best observed of the body. The dorsal epidermal nerve-tract is continued in in the larva) the blood is driven into the glomerulus, from whence front into the ventral wall of the collar nerve-tube, and at the it issues by efferent vessels which effect a junction with the venpoint of junction there is a circular commissural thickening tral (sub-intestinal) vessel in the trunk. The vascular system following the posterior rim of the collar and affording a special does not readily lend itself to morphological comparison between connexion between the dorsal and ventral nerve-tracts. From such widely different animals as Balanoglossus and Amphioxus, the ventral surface of the collar nerve-tube numerous motor and the reader is therefore referred to the memoirs cited at the fibres may be seen passing to the subjacent musculature. These end of this article for further details. fibres are not aggregated into roots. Reproductive System.—-'The, sexes are separate, and when mature Gill-slits.—The possession of gill-slits is as interesting a are sometimes distinguished by small differences of colour in the feature in the organization of Balanoglossus as is the presence of genital region. Both male and female gonads consist of more or tracheae in Peripatus. These gill-slits occupy a variable extent less lobulated hollow sacs connected with the epidermis by short of the anterior portion of the trunk, commencing immediately ducts. In their disposition they are either uniserial, biserial, behind the collar-trunk septum. The branchial bars which or multiserial, . They occur in the branchial region, and also constitute the borders of the clefts are of two kinds (1) Septal extend to a variable distance behind it. In exceptional cases bars between two contiguous clefts, corresponding to the they are either confined to the branchial region or excluded primary bars in Amphioxus; (2) Tongue-bars. The chief re from it. When they are arranged in uniserial or biserial rows semblances between the genital ducts open into or near the branchial grooves in the — —dji. Balanoglossus and region of the pharynx, and in a corresponding position in the jj' Amphioxus in respect post-branchial region. An important feature is the occurrence in -a-v. 0f the gill-slits may some species (Ptychoderidse) of paired longitudinal pleural or be stated briefly as lateral folds of the body which are mobile, and can be approxifollows :—(a) the pres mated at their free edges so as to close in the dorsal surface, emence of two kinds of bracing both the median dorsal nerve-tract and the branchial branchial bars in all grooves with the gill-pores, so as to form a temporary perispecies and also of branchial and medullary tube, open behind where the folds cease. small cross-bars (syn On the other hand, they can be spread out horizontally so as to apticulse) in many expose their own upper side as well as the dorsal surface of the body species ; (/3) numerous (Fig. 1). These folds are called the genital pleurae because they gill-slits, from forty to contain the bulk of the gonads. Correlated with the presence more than a hundred of the genital pleurae there is a pair of vascular folds of the basepairs ; (y) the addition ment membrane proceeding from the dorsal wall of the gut in the of new gill-slits by post-branchial portion of the branchio-genital region, and from the fresh perforation at dorsal angles made by the pleural folds with the body-wall in the the posterior end of pharyngeal region; they pass, in their most fully developed the pharynx through- condition, to the free border of the genital pleurae. These out life. The chief vascular membranes are called the lateral septa. Since there are Fig. 2.—Structure of branchial region, be, differences are. that many species which do not possess these genital pleurae, the quescoelom; gp, gill-pore; tb, tongue-bars; dn, (a) the cm e-bar is Ule ton dorsal nerve; ds, mesentery; dv, vessel; pr .nolle Dar 18 tion arises as to whether their presence or their absence is the ridge; as, oesophagus ; w, vessel; vs, mesentery • the essential organ of more primitive condition. Without attempting to answer this, vn, ventral nerve. the gill-slit in Balano- question categorically, it may be pointed out that within the , . , . , . glossus, andreverse exceeds family (Ptychoderidae) which is especially characthe septal bars m bulk, while in Amphioxus the is limits ofbythe their presence there are some species in which the the case ; (b) the tongue-bar contains a large coelomic space in terized genital pleura are quite obsolete, and yet lateral septa occur {e.g. Balanoglossus, but is solid in Amphioxus; (c) the skeletal ruficollis), seeming to indicate that the pleural folds, rods in the tongue-bars of Balanoglossus are double ; (d) the Ptychodera in such cases been secondarily suppressed. tongue-bar in Balanoglossus does not fuse with the ventral have Development.—The development of Balanoglossus takes place border of the cleft, but ends freely below, thus producing a according to two different schemes, known as direct and indirect, continuous U-shaped cleft. The meaning of this singular con- correlated with the occurrence in the group of two kinds of ova, trast between the two animals may be that we have here an and small. Direct development, in which the adult form is instance of an interesting gradation in evolution. From serving large achieved without metamorphosis by a gradual succession primitively as the essential organ of the cleft the tongue-bar ot stages, seems tostriking be confined to the family Balanoglossidaj. The may have undergone reduction and modification, becoming a remaining two families of Enteropneusta, Ptychoderidae and secondary bar in Amphioxus, subordinate to the primary bars in contain species of which probably all pursue an insize, vascularity, and development; finally, in the craniate verte- Spengclidae, course of development, culminating in a metamorphosis by brates it would then have completed its involution, the suggestion direct the adult form is attained. In these cases the larva, called having been made that the tongue-bars are represented by the which Tornaria, is pelagic and transparent, and possesses a complicated thymus-primordia. seam, the longitudinal ciliated band, often drawn out Gill-pouches and Gill-pores.—Only rarely do the gill-slits open ciliated convoluted bays and lappets. In addition to this ciliated freely and directly to the exterior (Fig. 1). In most species of into band the form of the Tornaria is quite characteristic and unlike Balanoglossus each gill-slit may be said to open into its own atrial the adult. . The Tornaria larva offers a certain similarity to chamber or gill-pouch ; this in its turn opens to the exterior by a larvae of Echinoderms (sea-urchins, star-fishes, and sea-cucumbers), minute gill-pore. There are, therefore, as many gill-pouches as and when first discovered was so described. It is within the there are gill-slits, and as many gill-pores as pouches. The gill- bounds of possibility that Tornaria actually does indicate a repores occur on each side of the dorsal aspect of the worm in a mote affinity on the part the Enteropneusta to the Echinolongitudinal series at the base of a shallow groove, the branchial derms, not only on account of of its external form, but also by reason poove. The respiratory current of water is therefore conducted the possession of a dorsal water-pore communicating with the to the exterior by different means from that adopted by Amphi- of body-cavity. In the direct development Bateson showed oxus, and this difference is so great that the theory which seeks anterior the three divisions of the coelom arise as pouches constricted to explain it has to postulate radical changes of structure, function that off from the archenteron or primitive gut, thus resembling the and topography. of the mesoblastic somites of Amphioxus. It would Excretory and Vascular Systems.—It seems likely that the development that while the direct development throws light upon the ccelomic pore-canals were originally excretory organs, but in the appear organization of the Enteropneusta, the indirect existing Enteropneusta the pore-canals (especially the collar special plan otaffords a clue to their possible derivation. Howcanals) have, as we have seen, acquired new functions or become development ever this may be, it is sufficiently remarkable that a small and vestigial, and the function of excretion is now mainly accom- circumscribed group like the Enteropneusta, which presents plished by a structure peculiar to the Eirteropneusta called the such a comparatively uniform plan of composition and of