LIFE-HISTORIES OF TREMATODES. 163 



have a large intestine frequently with black contents. The cercariae 

 have a body length of about 450/x and breadth of 150/x. The 



tail is usually about 350/x long. The head armature consists of 

 two alternating rows of spines with a group of four spines at each 

 lateral extremity of the head. Two germinal rudiments are pre- 

 sent. The excretory system consists of a polygonal laterally com- 

 pressed bladder from which two sinuous collecting tubules pass for- 

 wards. The body is crowded with cystogenous granules which 

 largely obscure the finer details of the excretory system. 



When the cercariae leave the snail they are easily visible to 

 the naked eye. Should they reach a "clawed toad or frog," 

 Xenopus laevis, they pierce the skin, cast their tails, extrude cysto- 

 genous granules, and encyst beneath the skin. After an interval the 

 Echinostomes leave the cysts, find their way outwards, and crawl 

 about on the surface of the body. It is possible that they may 

 undergo still further development in a second vertebrate host. 



Monostomes. 



The parthenita of a Monostome fluke were found by me in the 

 liver of Physopsis africaiia, and exhibited together with the young 

 adult forms in 1919. (See "Medical Journal of South Africa," 

 Dec, 1919, pp. 117-8.) The parasite is rare. By experiment the 

 "clawed frog," Xenopus laevis, was shown to be its vertebrate 

 host. 



The redia is about l-5mm. long, and is noticeable as having 

 no marked locomotor appendages, but the body is muscular and 

 contractile. The pharynx is oval and the intestine a simple sac. 

 The cercaria has a well-marked but small oral sucker, but the 

 acetabulum is absent. The excretory bladder is posterior and two 

 collecting tubules pass forwards, uniting near the sucker. The 

 oesophagus is very narrow, and the caecal intestinal forks at first 

 are close together but diverge more posteriorly. Large numbers 

 of unicellular cystogenous glands are present. The cercariae after 

 leaving the snail swim about actively for a short time. They do not 

 live long in water. Should they reach Xenopus laevis they pene- 

 trate the skin, especially round the eyes and at the glandular areas 

 along the lateral lines, and encyst beneath it. Occasionally some 

 cercariae penetrate deeply and encyst in the muscular tissue, and 

 in one case, in the kidney substance. The cysts are usually about 

 0-5mm. in diameter. After a time rupture of the cyst occurs, and 

 a small, actively moving Monostome, as yet somewhat imperfectly 

 developed sexually, emerges and reaches the surface of the skin 

 in some cases and in others wanders in the subcutaneous and 

 peritoneal cavities. Further work is needed on this organism and 

 some is in progress. 



In conclusion, it cannot be too strongly emphasised that work 

 on the Trematoda is long and tedious. It necessitates study of the 

 detailed morphology of larval forms and correlation of the same 

 with adult structures, as well as experimental work oh the life- 

 cycles with both the vertebrate and the invertebrate hosts. 



