466 
WORMS. 
eyes, and only a couple of suckers at the hinder end of the body. After quitting the 
egg the larvae are very lively and restless in their movements, either gliding slowly 
hither and thither, or swimming with rapidity. If unable to find the fish in whose 
gills they are destined to live, they grow feeble and perish; but if successful in 
making a settlement in their necessary surroundings, they grow into the Diporpa 
(< d ), which is flattened and lancet-like in shape, and bears a small sucking-disc on 
the under surface, and a conical excresence on the back. After living some weeks 
or months in this state, and gaining nourishment by sucking blood from the fish’s 
gills, the worms begin to join together 
in pairs, one specimen seizing the 
conical excrescence of another by its 
ventral sucker, then, by means of a 
truly acrobatic feat, the second twists 
round until it is able similarly to 
attach itself to the dorsal excrescence 
of the first, and in this state an in¬ 
separable fusion takes place between 
the suckers and excrescences involved 
in the adhesion. Another remark¬ 
able trematode is Anthocotyle mer- 
lucci, parasitic on the gills of the 
whiting, which is represented in B 
of the illustration. The other worm 
represented in the same illustra¬ 
tion (A) is Dactylocotyle pollacki, a 
parasite on the gills of the pollack. 
Here the slender front end of the 
body is much longer than in the last, 
the trunk gradually expands, and is 
"O wide and squarely cut at its posterior 
extremity, upon which are four pairs 
of long, stout, stalked suckers. The 
foremost pair of these seem to corre¬ 
spond to the very large suckers of 
Anthocotyle. We now come to two 
species of the present group of 
trematodes which, by their manner of life, lead to the second division of the 
internal parasitic forms. The first of these ( Aspidogaster ), found in the interior 
of the fresh-water mussels, is little known; but our acquaintance with the 
development of the second (Polystomum) is tolerably complete. This animal, 
with a roundish body, is less than half an inch in length, and is easily recognis¬ 
able by the presence at the hinder end of the body of a large wheel-like expansion 
bearing three pairs of suckers, between the last and longest pair of which are a 
couple of strong hooks. In the adult stage this worm lives parasitically in the 
bladder of frogs. It lays its eggs in the spring, and by thrusting itself partially 
out of the frog’s body deposits them in the water. The eggs take from six to 
A, Dactylocotyle ; B, Anthocotyle (magnified). 
