OCR Text |
Show 194 MB. A. D. MICHAEL ON AN [Mar. 5, having very distinct nuclei of about -004 mm. and nucleoli of about •002 m m . (fig. 28). On the outer and upper part of the anterior region of the front lobe and the outer and upper part of the posterior region of the hind lobe, on each side of the body, this layer, although existing, is less distinct aud regular; the cells are somewhat smaller and more broken ; but on their inner side in these localities will be found two or three layers of much larger polygonal cells (sm.), often as large as -04 mm., in which the nucleus cannot any longer be detected : these cells are the true sperm-mother-cells, and are usually crowded with spermatozoa in various stages of maturity according to the age of the cell. The spermogenous cells of the inner of these layers, when quite mature, burst and discharge their contents into tbe interior of the organ, which, although having the appearance of a solid mass, is seen when examined with a sufficient amplification to be a hollow viscus closely packed with sperm and secretion ; thus the whole organ forms a combination of testis and vesicula seminalis. It is a sac, the walls of which are formed of a single layer of large cells, which give birth to the true spermogenous cells ou their inner surface at certain parts of the sac. These sperm-mother-cells discharge their contents into the interior of the sac, which becomes so full that the lumen of the sac is obliterated and the whole appears like one solid mass. It is probable that the contents are mixed with other secretion, but I do not detect special accessory glands. In the vasa deferentia the cellulation of the walls becomes indistinct, and there is a slight tendency to corrugation; but in the ductus ejaculatorius w e again find the wall composed of distinct fleshy cells with clear nuclei, similar in character to those composing the outer layer of the testicular sac but smaller. The penial canal is a thin and almost structureless membrane. The Female Genital Organs. These organs so closely resemble what has been before described by Schaub, Henkin, and others, that it is not necessary to say much about them. The ovary forms a flattened ring with two oviducts leading to an unpaired canal (the vagina) as in the described species ; and, as in these descriptions, the eggs are formed upon the upper surface of the ring: the only observations which it seems desirable to make are, firstly, that the ova in Tliyas petrophilus are not quite so strictly confined to the upper surface as in the other recorded species of Hydrachnidae ; in the main part of the ring they are so confined, but in the rear part and near the insertion of the oviducts they are formed on the edges, and even on the under surface as well as the upper. Secondly, that although in the nymphs and young adults the ring form of the ovary is conspicuous, the ring being open and dorso-ventral muscles passing through it, yet that in the adult, when the eggs are mature, they are so numerous and crowded on the inner edge of the ring, that, being matured in pedunculated oocysts, they fill up the whole |