84
BALANOGLOSSUS
respiratory system, and, further, that the gill-slits were formed upon a plan similar to that of the gill-slits. of Amphioxus, being subdivided by tongue-bars which depend from the dorsal borders of the slits. Coelom and Pore-canals. — In correspondence with the triregional differentiation of the body in its external configuration, the coelom (body-cavity, perivisceral cavity) is divided into three portions completely separated from one another by septa :—(l) proboscis-coelom, or first body-cavity ; (2) the collar-coelom, or second body-cavity ; (3) truncal coelom, or third body-cavity. Of Balanoglossus is the general name given to these divisions of the coelom the first two communicate with the certain peculiar opaque worm-like animals which live an exterior by means of a pair of ciliated pore-canals placed^ at the end of their respective segments. The proboscis-pores obscure life under stones, and burrow in the sand from posterior highly variable, and frequently only one is present, that on between tide-marks down to the abyssal regions of the are the left side ; sometimes the pore-canals of the proboscis unite to sea. Their colour is usually some tone of yellow with open by a common median orifice, and sometimes their comdashes of red, brown, and green, and they frequently emit munication with the proboscis-ccelom appears to be occluded, and the pore-canals may be quite vestigial. The collar-pores a pungent odour. The name has reference to the tongue- finally remarkable for their constancy ; this is probably owing to the shaped muscular proboscis by which the animal works its are fact that they have become adapted to a special function, the way through the sand. The proboscis is not the only inhalation of water to render the collar turgid during progression. organ of locomotion, being There are reasons for supposing that the truncal ccelom was at assisted by the succeeding one time provided with pore-canals, but supposed vestiges of structures have only been described for one genus, Spengeha, segment of the body, the these in which they lie near the anterior end of the truncal ccelom. buccal segment or collar. By Enter on.—Not only is the ccelom thus subdivided, but the the waves of contraction exe- enteron (gut, alimentary canal, digestive tube) itself shows cuted by the proboscis accom- indications of three main subsections in continuity with one proboscis - gut (“ Eicheldarm, ” stomochord, mde panied by inflation of the another:—(1) infra) ; (2) collar-gut (buccal cavity, throat); (3) truncal gut collar, progression is effected, extending from the collar to the vent. sometimes with marvellous Stomochord.—The proboscis-gut occurs as an outgrowth irom rapidity. The third body- the anterior dorsal wall of the collar-gut, and extends forwards the basal (posterior) region of the proboscis through region or trunk may attain into the neck into the proboscis-ccelom, ending blindly in a great length, one or two front. Although an integral portion of _ the gut, it has feet, or even more, and is also ceased to assist in alimentation, its epithelium undergoes muscular, but the truncal vacuolar differentiation and hypertrophy, and its lumen becomes muscles are of subordinate more or less vestigial. It has, in fact, become metamorphosed into resistant supporting structure resembling in some respects the importance in locomotion, anotochord of the true Chordata, but probably not directly comparserving principally to promote able with the latter structure, being related to it solely by way ot the peristaltic contractions of substitution. On account of the presence and mode of origin the gut-wall) of this organ Bateson introduced the term the body by which the food (from Hemichorda as a phyletic name for the class Enteropneusta. As is carried through the gut. the proboscis-gut appears to have undoubtedly skeletal properties, The function of alimentation and as it also has topographical relations with the mouth, it is closely associated with that has been designated in English by the non - committal Grm It is not a simple diverticulum of the collar-gut, but a of locomotion, somewhat as stomochord. structure possessing paired lateral pouches and a ventra in the burrowing earthworm ; complex convexity (ventral caecum), which rests in a concavity at the in the excavation of its front end of the body of the nuchal skeleton (Fig. 3). In some burrows the sand is passed species (Spengelidae) there is a long capillary vermiform exthrough the body, and any tension of the stomochord in front. The nuchal skeleton is a Fio. 1.—Ptychodera flava (New Gale- nutrient matter that may aci- non-cellular laminated thickening of basement-membrane underlying that portion of the stomochord which lies between the demia), from above; about life . j j • size. here to it is extracted during above-mentioned pouches and the orifice into the throat. At the its passage through the intestine, the exhausted sand being point where the stomochord opens into the buccal cavity the skeleton bifurcates, and the two cornua thus produced finally ejected through the vent at the orifice of the nuchal pass obliquely backwards and downwards embedded in the wail burrow and appearing at low tide as a worm casting. In of the throat, often giving rise to projecting ridges that bound a accordance with this manner of feeding, the mouth is dorsal groove of the collar-gut which is in continuity with the kept permanently open and prevented from collapsing by wall of the stomochord (Fig. 3). Nervous System.—kt the base of the epidermis (which is in a pair of skeletal cornua belonging to a sustentacular general ciliated) there is over the entire surface of the body a apparatus (the nuchal skeleton), the body of which lies layer of nerve-fibres, occurring immediately outside the basementwithin the narrow neck of the proboscis j the latter. is membrane which separates the epidermis from the sufiiacent inserted into the collar and surrounded by the anterior musculature. The nervous system is thus essentially epidermal in position and diffuse in distribution ; but an interesting confree flap of this segment of the body. centration of nerve-cells and fibres has taken place in the collarWhen first discovered by Eschscholtz at the Marshall region, where a medullary tube, closed in from the outside, opens Islands in 1825, Balanoglossus was described as a worm- in front and behind by anterior and posterior neuropores, this like animal belonging to the Echinoderm order of is the collar nerve-tube. Sometimes the central canal is wide uninterrupted between the two neuropores ; in other cases Holothurians or sea-cucumbers. In 1865 Kowalevsky and it becomes broken up into a large number of small closed discovered that the organs of respiration consist of medullary cavities, and in others again it is obsolete. In one numerous pairs of gill-slits leading from the digestive family, the Ptychoderidse, the medullary tube of_ the collar is canal through the thickness of the body-wall to the connected at intermediate points with the epidermis by means ot variable number of unpaired outgrowths from its dorsal wall, exterior. On this account the animal was subsequently agenerally containing an axial lumen derived from and in placed by Gegenbaur in a special class of Vermes, the continuity with the central canal. These hollow roots terminal Enteropneusta. In 1883-86 Bateson showed by his blindly in the dorsal epidermis of the collar, and place the embryological researches that the Enteropneusta exhibit nervous layer of the latter in direct connexion with the fibres oi nerve-tube. The exact significance of these roots is a matter chordate (vertebrate) affinities in respect of the coelomic, the for speculation, but it seems possible that they are epiphysial skeletal, and nervous systems as well as in regard to the
undeveloped and indebted countries. So also of other countries. Each belongs naturally to one group or another. (10) The excess of imports or exports may vary indefinitely at different times according as a creditor country is receiving or lending at the time, or according as a debtor country is borrowing or paying off its debts at the time, but the permanent characteristics are always to be considered. (k. Gn.)
