516 FRESH-WATER BIOLOGY 



eggs or sperm. In the female the fully developed eggs are pushed 

 into a slightly larger region in which fertilization takes place. 

 Sometimes the fertilized eggs are provided with a heavy shell and 

 are soon ejected to carry out their development in the outer world. 

 In other cases they are retained in a sac-like uterus until devel- 

 opment is more or less advanced. In certain families the entire 

 development is carried out within the uterus and the female brings 

 forth living young. The organs in these cases differ in length 

 and capacity rather than in fundamental structure. 



In the male the reproductive system consists of but a single 

 tube, emptying as already stated into the cloaca, whereas the 

 windings, of the tube lie in the body in front of this region. In 

 the female the tube may be single but is most frequently double 

 or Y-shaped. The short stem of the Y connects with the female 

 pore, the branches extend in coils into the body. One branch 

 may pass anteriad and the other posteriad or both may lie nearly 

 parallel in the same part of the body. One branch may be greatly 

 reduced and by its final disappearance give to the system the form 

 of a single tube such as is found in the male. Various intermediate 

 stages occur. 



In connection with the terminal portion of the male duct are 

 usually found pieces of cuticula shaped like hooks or needles, and 

 known as the spicules. There may be only one spicule or if two 

 are present they may be equal or unequal. Finally an accessory 

 piece furnishes in some species a link or groove in which the spicules 

 proper are held and through which they are extruded. These 

 spicules are easily seen both on account of their high refractive 

 index and because in many preserved specimens they project 

 conspicuously from the anal opening. In transparent forms the 

 student may detect under the microscope the spicule sac, dorsal 

 to the intestine, in which the spicules are housed and also special 

 sets of muscles by which they are operated. The number, 

 length, and exact shape of these organs serve as features for specific 

 diagnoses. 



The development of parasitic nematodes introduces all varia- 

 tions from extreme simplicity to some of the most complex life 

 histories known among animals. The early development is simple. 



