LARVAL TREMATODES 
203 
Tetracotyles 
Four of the dissected snails had a species of tetracotyl, two of these snails having echinostome rediae 
and metacercariae at the same time. None of the four had sporocysts with bifid-tailed cercariae. The 
tetracotyles were few in number ranging from two to four in each snail. 
These tetracotyles (Fig. 4D) are ovoid bodies measuring about 0.4 by 0.3 mm with the anterior end 
being more blunt. They appear darker round the periphery. The ventral sucker is situated at varying 
distances from the anterior sucker up to the middle of the body of the tetracotyle. The auricular depressions 
which are on either side in front of the ventral sucker, are prominent but incompletely round. 
The clasping apparatus behind the ventral sucker is prominent with an oval (long axis in the horizontal 
plane) opening in the centre. The clasping apparatus is most prominent posteriorly in the midventral 
portion and as its margin encircles forwards and medially it gradually becomes lost. No gelatinous wall is 
present around the tetracotyl in any of these specimens. 
It is possible that these tetracotyles are the later developmental stages of the furcocercous cercariae 
noted above but none of the Lymnaea natalensis exposed to these cercariae had tetracotyles when dissected 
subsequently (see experiment described below). 
The six five snails from Malaya died in the course of two weeks, the two infected ones living longest. 
Those discharging cercariae died within three days of arrival in spite of their having been transferred 
immediately to well-aerated water in jars in a room maintained at a constant temperature of 22 °C. 
Dissection showed the presence of the mature rediae and metacercariae of the same two species of 
echinostomes, mature sporocysts packed with bifid- tailed cercariae and the tetracotyles already described. 
Only the sporocysts showed some slight evidence of motility. 
Metacercaria with Stylet (Fig. 5 a) 
In addition to the above, one of the snails had a small number of metacercariae with a stylet. These 
cysts have double cyst walls and the curled up young flukes are very active inside. The stylet was lying 
free within the cyst and rupturing the cyst wall and freeing the young flukes confirmed that it had been 
extruded. The cyst is globular and measures 0.15 mm. in diameter. The stylet has a slight thickening 
behind the tip giving it a spear-head appearance; it was 0.018 to 0.022 mm long. The metacercariae he 
doubled up so that the ventral surface of the anterior part of the body is opposed to the ventral surface of 
the posterior part. There are no eyespots. The two suckers are conspicuous as also is the pharynx, and 
behind the ventral sucker lies a mass of small spherical concretions filling up an enlarged excretory vesicle. 
Description of Agamodistome (Fig. 5B) 
The young fluke, freed by the rupture of the cyst wall, contracts and expands actively, is elongate oval 
in shape and measures 0.22 mm in length by 0.1 mm in maximum width. The ventral sucker is somewhat 
smaller than the oral sucker and is situated at about the middle of the body. The whole cuticle is spinous 
but the spines are most prominent in the anterior region. The mouth is sub terminal, the prepharynx is 
long followed by a muscular pharynx. The oesophagus is short and divides in front of the ventral sucker 
into two long simple caeca which extend to the posterior end. The excretory vesicle filled with fine 
granules tapers towards the posterior end. The main excretory ducts and the flame cell pattern were not 
made out. Similarly, it was not possible to delineate the genital rudiment lying behind the dense excretory 
vesicle. No development of vitellaria was evident. 
Experiments with Larval Trematodes 
Exposure of Cercariae to Human Skin 
About 1 c.c. of water containing furcocercous cercariae was pipetted on to the flexor 
surface of my forearm and at the end of 15 minutes nothing had happened. Similar experi- 
ments with the echinostome cercariae caused no irritation to the skin. In was noted that the 
cercariae did not encyst on blades of grass, sides of the glass container or on empty snail shells. 
Exposure of Cercariae to Snails 
In three 3" x 1" tubes, six Lymnaea natalensis bred in the laboratory of the London School 
of Hygiene and Tropical Medicine were placed. To each of these one of the three different 
types of cercariae liberated by the Malayan Lymnaea was added. At the end of two hours it 
was noted that whereas there had been a considerable diminishing in the numbers of the 
echinostome cercariae, no noticeable change was seen in the case of the furcocercous cercariae. 
These snails were transferred to separate jars with well-aerated water in a room maintained 
at a constant temperature of 22 C C. 
All these snails died within three weeks while the controls were alive and continued to 
breed. The snails were dissected as they died and metacercaria of both echinostome species 
MALAYA , No. 26. 1953 
