Structure of Dolichoglossus Pusillus

ALMA EVANS

The animals were studied from serial sections cut in several planes. The stains used were carmine, hematoxylin and eosin. The hematoxylin seemed to show the tissues more clearly. A graphic reconstruction was attempted, but did not prove satisfactory because of the individual artificial foldings and contractions. The drawings were obtained by the use of a camera lucida. The general drawings, Figs. [1]–[9] inclusive, are not filled in in great detail. The special drawings are shown at greater magnification with more of an attempt to show the actual condition.

Dolichoglossus is a soft worm-like animal with ciliated surface. It is divided into three distinct regions: the proboscis, a long club-shaped organ; the collar, a fold in the surface just behind the proboscis, and the trunk, a long cylindrical portion posterior to the collar.

Dolichoglossus is a marine form living in sandy bays or sheltered places. Mucous glands in the surface epithelium secrete a sticky fluid which covers the body and to which tiny sand grains stick. The sand clinging to the mucous coated surface forms a fragile temporary tube in which the animal is usually secluded. The animals in the living condition are bright orange or red but lose their color very soon after preservation in alcohol or formalin.

The proboscis cavity extending the entire length of the organ is surrounded by a network of connective tissue supported by longitudinal bands of plain muscle. This cavity is supposed to communicate with the exterior by a very small opening, the proboscis pore, but this did not show in the specimens examined. The heart, proboscis gland and notochord are located in the posterior part of the proboscis.

The collar contains the central nervous system, part of the notochord, the dorsal blood vessel, ventral and dorsal mesenteries, mouth opening and anterior part of the alimentary canal.

The trunk contains the alimentary canal, dorsal and ventral blood vessels, dorsal and ventral nerves, the gill-slits, the reproductive bodies, dorsal and ventral mesenteries and muscle bands.

The nervous system consists of three parts: the central, located in the collar region, Fig. [5]; the sub-epidermic network extending over the entire body just under the surface epithelium, Figs. [1] –[7]; and the dorsal and ventral strands which are thickenings of the sub-epidermic network extending throughout the trunk, Figs. [1] and [7]. There is also quite a decided thickening of the sub-epidermic network at the base of the proboscis, Figs. [5], [6].

The vascular system consists of two parts, the central and the peripheral. The central is made up of the heart, a thin-walled vesicle at the base of the proboscis just dorsal to the notochord, and connected with it the proboscis gland, a plexus of capillaries just anterior to the notochord. Fig. [5]. The peripheral system is composed of a ventral and a dorsal vessel. The dorsal starts at the heart and continues just ventral of the dorsal nerve throughout the length of the body. Figs. [1], [5], [7]. The ventral vessel extends from the posterior border of the collar to the anal end. It is connected with the dorsal vessel by a circular vessel in the posterior edge of the collar.

The mouth is situated ventrally at the base of the proboscis, within the collar, and opens directly into the straight alimentary canal. The latter is a straight tube extending from the mouth opening to the anus. Figs. [5], [1], [7], [9].

The alimentary canal in the anterior part of the collar gives off a diverticulum, which grows forward and supports the proboscis. Because this diverticulum has the vacuolated appearance of the notochordal tissue of higher animals, it has been regarded as a notochord. It is largest at the base of the proboscis immediately anterior to the heart. Figs. [5], [6].

The paired gill-slits occupy the region just posterior to the collar. They are arranged in two longitudinal grooves in the dorsal wall. The number increases throughout life, new slits appearing just behind those already in place. I found about twenty-five to be the average number, while particular individuals had as low as eighteen and twenty and as high as thirty and thirty-one. The gills are formed in the shape of a U. A skeletal rod or gill bar separates the gills from each other. The gills are supplied with blood from the dorsal vessel. Figs. [3], [7], [8].

The sexes are distinct. The ovaries and testes are saccular organs arranged in a row along the gill and succeeding region. The sacs in other genera, for example Balanoglossus as described by Shipley, open directly on to the epidermis. I have been unable to see these openings in my preparations. Fig. [8] shows the position of the ovaries in the female; the testes in the male are in a similar location.

The surface epithelium is modified ciliated columnar, varying slightly in thickness, size of nuclei and size and shape of cell according to location. Figs. [13], [14], [15].

The epithelium forming the gills and intestine is also modified ciliated columnar. That of the gills having short narrow cells and small nuclei, and that of the intestine having longer thicker cells and large nuclei. Figs. [11], [10].

The connective tissue surrounding the proboscis cavity is of a peculiar arrangement. The connective tissue itself consists of fine strands loosely interwoven, but arranged in a definite manner. The strands form a fine network which gives a beautiful lacy appearance. Small round nuclei are quite numerous in connection with the strands. Longitudinal bands of plain muscle are very conspicuous in the connective tissue. These muscle bands are probably used in altering the size and shape of the proboscis. Figs. [4], [20], [21].

The nervous tissue consists of many fibers thickly interwoven. There are a few small nuclei scattered about among the fibers. Figs. [12], [13].

The muscle is unstriated. The fibers are very long in some places, shorter in others and always quite distinct.

(Contribution from the Zoological Laboratory of Pomona College)

REFERENCES

Assheton, Richard1918
A new species of Dolichoglossus. Zool. Anz. Bd. 33, p. 517–520.
Delage and Herouard1898
Traité De Zoologie concrète Vol. 8. Les Procordés. Balanoglossus.
Encyclopedia Britainica Balanoglossus.
Shipley, Arthur E.1893
Zoology of the Invertebrata. Balanoglossus.

EXPLANATION OF FIGURES

Fig. 1. Cross section through the gill region showing gill opening. D. N., dorsal nerve. D. V., Dorsal vessel. G. O., gill openings. A, alimentary corps. G., gill. V. N., ventral nerve. V. V. ventral vessel. N., nervous tissue. ×40.

Fig. 2. Cross section through the base of the proboscis showing diverticulum wall and proboscis gland. D., diverticulum. N., nervous tissue. P. G., proboscis gland. ×40.

Fig. 3. Longitudinal section through a gill opening. N., nervous tissue. G., gill. G. O., gill opening. ×40.

Fig. 4. Cross section through the center of the proboscis. N., nervous tissue. M. C., muscle in the connective tissue. T., connective tissue. ×90.

Fig. 5. Longitudinal section through the base of the proboscis and collar. M., mouth. C. N., central nervous system. H., heart. No., notochord. P. G., proboscis gland. N., nervous network. A., alimentary canal. D. V., dorsal ventral. ×40.

Fig. 6. Cross section through the base of the proboscis showing thickened nerve network. N., nerve network. D., diverticulum wall. H., heart. ×40.

Fig. 7. Cross section through gill region. D. N., dorsal nerve. D. B. V., dorsal blood vessel. G. B., gill vessel. V. N., ventral nerve. V. V., ventral vessel. ×40.

Fig. 8. Longitudinal section through the gill region. G., gills. B., blood. O., ovary. N., nervous network. ×40.

Fig. 9. Cross section of alimentary canal. A., wall of alimentary canal. ×40.

Fig. 10. Intestinal epithelium, modified ciliated columnar. ×400.

Fig. 11. Epithelium of the gill, modified ciliated columnar. ×400.

Fig. 12. Nervous tissue. ×400.

Fig. 13. Surface epithelium of proboscis, modified ciliated columnar. ×400.

Fig. 14. Surface epithelium of collar, modified ciliated columnar. ×400.

Fig. 15. Surface epithelium of trunk, modified ciliated columnar. ×400.

Fig. 16. Cells of testis. ×400.

Fig. 17. Ovary. ×400.

Fig. 18. Plain muscle. ×400.

Fig. 19. Epithelium of diverticulum. ×400.

Fig. 20. Connective tissue of proboscis. ×400.

Fig. 21. Muscle bands in proboscis connective tissue. ×400.