THE EINTEMIES OF FISH. 511 



The fecundity of some of these -worms is considerably increased by the 

 so-called "chau.oe of generation," that is, the interpolation of generation 

 without sexual connection l)ctween the regular generation. 



From one of the innumerable eggs of the tape-worm, for instance, there 

 develops an embryo armed with six little hoohs, which finds its way 

 through the animal tissues, settles somewhere, and develops into a tape- 

 worm. In the beginning it is inclosed in a bladder, and in this state is 

 known as the " bladder- worm." Generally not until it has reached the 

 stomach or intestinal tube of some other animal, does the head get free 

 and develop the diiferent joints of the tape-worm, in which eggs develop 

 in the regular manner. 



In another kind of the helminths, the Trematodes, we also find this 

 peculiar manner of propagation. From the embryo worm-like animals 

 are developed sporocysts or redia, which again develop little worms with 

 tails, the cercaria. These very lively li ttle animals, which frequently have 

 bristles on the head, envelop themselves in a capsule and throw off their 

 organs of motion. 



If in this condition they enter the stomach of that animal which is 

 to be their definite place of abode, the cyst, as I have shown by experi- 

 ments years ago, is digested, and its contents becomes a fully-matured 

 animal of the Trematodes kind. Before they get so far they may, how- 

 ever, pass through several different animals. 



Tapeworms are very frequently found in fish, often in an entirely unde- 

 veloped condition, which shows that they were first eaten by some other 

 animal along with the animal in which they lived. 



Among these must be counted the Ligula, which is frequently found 

 in the abdominal cavity of our fresh-water fish, such as the bleak, blay, 

 crucian, salmon, trout, pike, and perch. In some i:)arts of Italy this 

 worm is called " Macaroni piatti," and is considered a great delicacy. 

 In storks, herons, gulls, and wild ducks it is found in its more devel- 

 oped form. According to Van Beneden it only gets into these birds acci- 

 dentally. 



Very similar to the Ligula is the ScMstocephalus, which often fills the 

 abdominal cavity of the stickle-backs to such an extent as almost to 

 make them burst. When 25 years ago I pursued ichthyological studies 

 here in Berlin, under the instruction of my venerable teacher. Professor 

 Peters, 1 often fed crows and ducks with these worms. These experi- 

 ments were made in my student's quarters in the third story, and were 

 therefore attended with considerable difficulties. Von Willemoes-Suhm 

 has pursued his experiments in a reversed manner, and has raised the 

 embryos with six little hooks from the eggs of the Ligula taken from a 

 diver, and from those of the ScMstocephalus taken from a guU. Among 

 these undeveloped forms we must also mention the Scolsx polymorphus, 

 which is found in many salt-water fish, and comprises different stages of 

 development. 



A very simply-formed tapeworm is found in the intestinal tube of nearly 



