176 Introduction to Animal Morphology. 



organ-less sac (Sporocyst or Blastocyst, Diesing), which 

 may undergo transverse fission. In the body cavity of a 

 Sporocyst, by internal gemmation one or many daughter cells 

 are produced, which resemble the parent. Within these, or 

 within a Redia, form, as internal buds, numerous small, oval 

 bodies, Cercariae, with movable, sometimes forked tails 

 (Bucephalus) ; these become free, and are expelled through 

 an opening in the " nurse," and for a time enjoy free life in 

 water or on moist ground. They have two suckers, with the 

 mouth in the foremost, which deepens into a pharynx and 

 intestine ; the water-vascular system then develops, and the 

 animal is either swallowed by the host in which it is to be 

 developed, and becomes at once mature, or it burrows into 

 the skin, or into the pulmonic cavities of freshwater Molluscs, 

 &c., and loses its tail, becomes surrounded by a cyst, and be- 

 comes a pupa, in which, when its host is eaten by some other 

 animals, and it is set free in the stomach, the sexual organs 

 develop, and the adult forms are assumed ; some encapsulate 

 themselves in plants, and are thus eaten. 



The sub-order includes four families : — i . Monostomidse — 

 one sucker (oral) ; sporocysts in Planorbis ; perfect forms in 

 water-fowl, &c. Monostomum lentis has been found in the 

 capsule of the human lens. 2. Amphistomidae — a sucker at 

 each end ; Diplostoma in the lenses of fishes' eyes ; Amphi- 

 stoma in frogs, ruminants, &c. 3. Distomidae — suckers, 2 ; 

 one circum-oral in front ; one ventral ; never terminal. This 

 includes the bristled and two proboscis-bearing Rhopalo- 

 phorus,* and the dioecious Gynoecophorus (Bilharzia) haema- 

 tobius, common (no times in 360 subjects, Griesinger) in the 

 venae portx of Egyptians and Nubians. In these, the male has 

 a large abdominal groove, with two lips, in which the cylindrical 

 female is contained. The eggs are oval, with a terminal or 

 lateral spine. Distoma is hermaphrodite, and may be divided 

 into two sub-genera: — (a), with branched intestine, in- 

 cluding the ovate D. hepaticum, of the sheep, and rarely 



* The proboscides limit the mouth at each angle. These are parasitic 

 in Opossums. The spines are in rows along the proboscis. R. horridus is 

 spined over the body. 



