| OCR Text |
Show 408 MR. R. T. LEIPER ON A RECENTLY [Mar. 1, Parenchyma (Plate XXV. figs. 4, 6). The parenchyma may be divided into two portions :-(1) A cellular layer, much thicker on the ventral surface than on the dorsal, with oval nuclei lying for the most part at light angles to the cuticle and staining faintly. This is known as the "dermal layer" and gives support to the integument which is immediately superimposed. (2) Fine protoplasmic strands passing inwards from the dermal portion. These by their anastomosis form a meshwork enclosing vacuoles in wmich various free cells lie. On reaching the ovaries they unite to form a delicate capsule, whence still finer strands pass to enclose the individual ova in follicle-like spaces. Digestive Vacuole (Plate X X V . figs. 1, 2, 4). Immediately over the mouth the ovaries are separated by a large vacuole extending between the dorsal and ventral layers of the dermal parenchyma and traversed by protoplasmic strands only at the periphery. The ovaries soon reunite ventrally, but the space is continued a short distance further along the mid-dorsum and roughly occupies the anterior middle fourth of the body. In it are found particles of ingested material together with wandering cells. It represents the digestive tract, but has not an epithelial lining or any defined passage to the mouth. The food-particles are in all probability caught in the mobile protoplasm presenting at the mouth, and passed on into this digestive vacuole. Frontal Organ (Plate XXV. fig. 5). The frontal organ differs from that of all other Accela save Bbhmigia maris-albi (Sabussow), and is here represented by numerous large single-cell mucous glands lying in the parenchymatous strands and bulging into the vacuolar spaces above and around the brain. Those lying immediately under the dermal parenchym dorsally are perhaps the largest, but their ducts all pass to open like those of the remainder, each by a separate aperture, on to the ventral surface only. As it perforates the cuticle each duct dilates to form a small ampulla. Both glands and ducts stain readily and deeply with haematoxylin owing to the presence of very fine granules. The secretion is adhesive and plays an important part in locomotion. Nervous System. The nervous system, which is not well differentiated, consists of two lateral masses situated at the anterior end of the body and communicating with one another by a commissure in which lies the otocyst. From each ganglionic mass a lateral and a posterior nerve are seen to be given off. The tissue generally is finely fibrillar, enclosing small clear cells with punctate nuclei. Eye-spots are absent. |
