XIX. The Croonian lecture - Observations on the locomotor system of echinodermata

1. Holothuria. — When a longitudinal incision is made through the perisome of a Holothurian (Holothuria communis) there is generally seen escaping, along with the branches of the respiratory tree and genital gland, a long sacculated tube filled with a fluid, and holding in suspension a large quantity of a brick-dust coloured pigment. This tube, which may be one-and-a-half times the length of the entire animal, and from one line to half-an-inch in diameter, is the polian vesicle (Plate 79, fig. 1, a). On following it upwards it is found to open freely into a wide circular canal (Plate 7 9, fig. 1, b) a short distance from the termination of the stone canal. From this circular canal five lozenge-shaped sinuses (Plate 79, fig. 1, c) project forwards, and from each of these two large oval sinuses (Plate 79, fig. 1, d) run forwards parallel with each other, the ten oval sinuses becoming continuous with the hollow stems of the tentacles (Plate 79, fig. 1, e). In a Holothurian 8 inches in length, exclusive of the tentacles, the lozenge-shaped sinuses, which may be designated the sinuses of the circular canal, measure a quarter of an inch from above downwards and a little more from side to side. From around the pointed upper ends of the canal sinuses the five longitudinal muscular bands take their origin. When a solution of Berlin blue is injected into the polian vesicle, the circular canal and its sinuses, the oval sinuses and tentacles, the radial canals, pedicels and ampullæ are rapidly distended; but, unless the pressure be kept up for a considerable time, none of the coloured fluid penetrates into the stone canal, and either the vesicle, ring, or one of the sinuses gives way before it reaches the madreporic plate. If one of the radial canals be divided while the injection is being proceeded with, the coloured fluid at once escapes, and the tension within the polian vesicle, the circular canal, and the tentacles is diminished. If plaster of Paris be substituted for the solution of Berlin blue, a cast is readily obtained of the circular canal and its sinuses, but the plaster does not find its way either into the sinuses of the tentacles or into the radial canals. When, however, a coloured solution of gelatine is forced into the polian vesicle, the tentacles and their sinuses, the radial canals, ampullæ, and pedicels are filled, as well as the circular canal and its sinuses. Examination of specimens injected with plaster of Paris and gelatine shows the circular canal to be a quarter of an inch in diameter, and to communicate freely both with the polian vesicle and with the lozenge-shaped sinuses springing from it. The specimens injected with a gelatine mass further show that each canal sinus opens into a cæcal tube, which runs forward internal to the sinuses of the tentacles as far as a wide circum-oral space (Plate 79, fig. 2,a). This space communicates by well-defined apertures (Plate 79, fig. 3, b), with that portion of the body cavity which lies between the sinuses and the œsophagus, and which is reached through the circular apertures between the sinuses of the circular canal (Plate 79, fig. 1,f).