194 MJi. A, D. MICHAEL ON AX [MoT. 5, 



having very distinct nuclei of about '004 mm. and nucleoli of about 

 •002 mm. (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 and 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 dis- 

 charge their contents into the interior of the organ, which, although 

 having the appearance of a solid mass, is seen when examined with 

 a sufficient amplification to be a hollo«- 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 on 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 we 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 petro- 

 jihilits are not quite so strictly confiued to the upper surface as in 

 the other recorded species of Hydrachnidse ; 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, \^hen 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 



