4 66 



WORMS. 



A 



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 

 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). 



