374 
Tetrarhynchus erinaceiis 
or if it is situated in a mesentery, the nodule will come to project into 
the body cavity and a short stalk will be formed, so that the typical 
flask-like larval cyst is formed. 
Summarising the life-history we find that (1) Tetrarhynchus 
erinaceus is sexually mature in the large intestine of various species of 
Raia, but not in other genera of fishes so far as our knowledge goes. 
These fishes act as the “ final host ” and the ripe proglottides are then 
expelled per anum with the faeces. (2) The proglottides may lead an 
independent existence for a short time at the sea bottom and are then 
probably eaten by some small fish or invertebrate—the true larval 
host. (3) This animal is then probably eaten by the collateral host 
and the Oncosphere larvae develop within the body of the latter, 
usually inhabiting the peritoneum over the alimentary canal, or in 
the mesenteries; and there they attain the Plerocercoid stage and 
degenerate. Or the larval host may be eaten by the final host and 
the larva then begins to segment and attains sexual maturity. 
IV. The scolex. 
Tetrarhynchus erinaceus as found in Irish Sea fishes is rarely more 
than about 60 mm. in total length, but the ultimate proglottides are 
continually dehiscing from the strobila and one or more may usually be 
found in the large intestine of the host. The mature Cestode will live 
at least for several hours in a dish of clean seawater. It is very mobile, 
drawing itself along by means of rhythmic movements of the bothridia. 
The latter elongate, their anterior extremities being pushed forward 
and fixed to the object over which the worm is moving; the posterior 
extremities are then drawn forward and the scolex fixes itself in a new 
position. The proboscides act apparently only as organs of attachment 
and do not take an important part in the locomotion of the worm. 
The Cestode is rapidly killed by immersion in fresh water, and it soon 
breaks up when preserved in weak formalin. When preserved in any 
of the mercuric chloride fixatives the strobila usually remains intact. 
The worm represented in PI. XIX, fig. 1 was about 52 mm. in total 
length and possessed 47 distinct proglottides. The most anterior 
segments are very indistinct: they are about 0'2 mm. in length and 
about 0’6 mm. in breadth. The terminal proglottis, which was still 
incompletely ripe in the specimen figured, was about 3 mm. in length, 
and about 1'2 mm. in breadth. The anterior segments are elliptical in 
