The _Bilharzia hæmatobia_ may be described as a trematode helminth in which the male and female reproductive organs occur in separate individuals; the male being a cylindrical vermiform worm, measuring only half an inch or rather more in length, whilst the female is filiform, longer, and much narrower than the male, being about four fifths of an inch from head to tail; in both, the oral and ventral suckers are placed near each other at the front of the body; in the male the suckers measuring 1/100″, in the female 1/314″ in diameter; in either, the reproductive orifice occurs immediately below the ventral acetabulum. The comparatively short, thick, and flattened body of the male is tuberculated and furnished with a _gynæcophoric_ canal, extending from a point a little below the ventral sucker to the extremity of the tail; this slit-like cavity being formed by the narrowing and bending inwards of the lateral borders of the animal, the right side being more or less completely overlapped by the left margin of the body; caudal extremity pointed; intestine in the form of two simple blind canals. Female with a cylindrical body measuring only 1/312″ of an inch in thickness in front of the oral sucker; lodged in the gynæcophoric canal of the male during the copulatory act; thickness of the body below the ventral acetabulum being about 1/357″, and at the lower part 1/96″; surface almost smooth throughout; intestinal canals reunited after a short separation to form a broad, central, spirally twisted tube extending down the middle of the body; vitelligene and germigene canals combining to form a simple oviducal canal, which is continued into a simple uterine tube, finally opening near the lower margin of the ventral sucker; eggs pointed at one end, or furnished with a projecting spine near the hinder pole. The study of the structure and formation of the contents of the ova possesses great interest. When fully developed the eggs are oval, measuring from 1/180″ to 1/160″ in length, with an average transverse diameter of 1/325″. Some are a trifle larger, others smaller. Occasionally one encounters narrow specimens, and also aberrant forms presenting a pear-shaped outline. I have met with eggs not exceeding 1/250″ in their long diameter, and 1/500″ transversely, whose yolk-contents had already arrived at an advanced stage of segmentation. [Illustration: FIG. 8.--Two eggs of _Bilharzia_. _a_, With the yolk coarsely segmented; _b_, with the yolk granulated and the spine wanting. Original.]