LIFE-HISTORIES OF MONOGENEA 



61 



survive the dangers of their free-swimming existence, only those 

 become mature which conjugate permanently with another indi- 

 vidual. But although there are thus only half as many adult 

 Diplozoon as there were conjugating larvae (or Diporpa, as they 

 were called when they were considered distinct forms), yet the 

 total number of eggs produced is probably as great as if each 

 larva became individually mature. 



Diplozoon paradoxum lays its eggs on the gills of the Minnow, 

 which it frequently infests in great numbers. The ovum divides 

 rapidly at the expense of the yolk-cells, and in a fortnight a 

 larva ('2 mm. long) of the shape and complexity shown in Fig. 

 27, B, hatches out, which, however, succumbs if it does not 

 meet with a Minnow in five or six hours. 

 Should it survive, a dorsal papilla, a median 

 ventral sucker, and a second pair of posterior 

 suckers develop. Thus the Diporpa stage 

 is attained. These Diporpa may acquire a 

 third and even a fourth pair of suckers, and 

 continue to live three months, but they only 

 develop and mature their reproductive organs, 

 if each conjugates with another Diporpa 

 (Fig. 27, C, D), and this only occurs in a 

 small percentage of instances. Each grasps 

 the dorsal papilla of the other by its own 

 ventral sucker, thus undergoing a certain 

 amount of torsion. Where the two bodies 

 touch, complete fusion occurs, and, as shown 

 in Fig. 28, the united Diporpa (or Diplo- 

 zoon, as the product is now called) decussate, 

 each forming one limb of the X-shaped 

 Diplozoon, within which the two sets of 

 complex genitalia develop (Fig. 28). 



IV. Gyrodactylidae. Gyrodactylus (Fig. FIG. 29. Gyrodactylvs 

 29), the structure of which is in many *J 5 ***- 

 ways peculiar, produces one large egg at a back. (After v. Nord- 

 time. An embryo, in which the large and n b n r yo. * 125 ' *"*' 

 smaller hooks of the adhesive disc can be seen 

 (emb\ develops from this egg while still within the body of the 

 parent, and may give rise to yet another generation within itself. 

 The details of the process have not, however, been well ascertained. 



