426 FRESH-WATER BIOLOGY 



exhibit the same condition of hermaphroditism that was de- 

 scribed for the flukes, since all organs of both sexes are repre- 

 sented in a single proglottid. The reproductive organs of one 

 proglottid have usually no connection with those preceding or fol- 

 lowing; other organs are continuous throughout the chain. One 

 may readily observe at the side of the proglottid the main longi- 

 tudinal nerve trunks which connect with a complicated series of 

 enlargements or ganglia in the scolex. These main nerve trunks 

 are joined by cross-connectives in each proglottid. Near them and 

 parallel to them are the main canals of the excretory system, and 

 these also are joined by transverse vessels. A network of finer 

 tubes terminating in flame cells is present in each proglottid and 

 empties into the main vessels just described. The longitudinal 

 muscles of the body are sometimes continuous throughout the 

 chain and sometimes divided at the partitions. 



No trace of an alimentary system has been found at any time 

 in the entire life history of the cestode. 



As one proceeds backward along the chain one can observe the 

 gradual development of the reproductive organs. These appear 

 first as faint lines or bands in the tissue, outlining the positions of 

 the main organs; they grow more definite until at sexual maturity 

 a complete set of organs of both sexes can be demonstrated (Fig. 

 730). The organs are very similar in character and interrelation 

 to those of the trematodes. As one passes further along the chain 

 other changes take place, primarily the gradual accumulation of 

 eggs in the uterus and the coincident gradual shrinkage of other 

 reproductive organs until the latter may ultimately disappear save 

 for insignificant vestiges. Two different plans are observable 

 with regard to the production of eggs. The latter may be held for 

 a time in a uterine cavity and then discharged through a pore, or 

 they may accumulate indefinitely in a blind uterine sac until they 

 come to occupy nearly the entire volume of the proglottid. Where 

 the latter condition prevails, the last proglottids of the chain have 

 been reduced to mere egg cases that are cut off periodically, either 

 singly or in groups, and carry to the outer world masses of ova for 

 the dissemination of the species. 



Some cestodes have a free-hving stage which hatches from the 



