456 FRESH-WATER BIOLOGY 



Back toward its posterior third, the proboscis is armed with a 

 needle-like calcareous stylet (s) resting upon a solid conical basis. 

 Beside the central stylet there are two lateral pouches each con- 

 taining 2, 3, or 4 accessory stylets of size and shape similar to the 

 central stylet. The proboscis is considerably longer than the sheath 

 in which it lies coiled and to which it is attached at both ends. 

 By means of its powerful musculature it can be thrust out of the 

 anterior end of the body. This process of eversion turns the ante- 

 rior part of the proboscis inside out and brings the central stylet 

 to the end of the everted organ, where it constitutes a formid- 

 able weapon of defense or offense. After eversion the retractor 

 muscle at its posterior end withdraws the proboscis to its original 

 position. 



The mouth and proboscis open together through the rhyncho- 

 daeum (r) at the anterior end of the body. The esophagus leads 

 into a broad stomach, and this into the intestine with its numer- 

 ous lateral lobes. At the anterior end of the intestine a pair 

 of pyloric ceca (c) extend forward to the brain. The short rec- 

 tum leads to the opening at the posterior end of the body. The 

 nemerteans feed upon other worms and soft-bodied animals of 

 various kinds. 



The central nervous system consists of the four cerebral ganglia 

 and a pair of large longitudinal lateral nerves (In). These are 

 easily seen in the living worm. 



The excretory system extends the entire length of the body as a 

 series of delicate tubes with several efferent ducts leading to each 

 side of the body. The three longitudinal trunks of the blood 

 vascular system are often distinctly seen in the living animal. 



The fresh-water nemerteans are hermaphroditic, and usually 

 protandric. The gonads are arranged serially along each side of 

 the body between the intestinal lobes. Each gonad bears both 

 male and female genital products, which are discharged when 

 mature through a small pore opening on the side of the body. 

 The male sexual elements are formed first, and in the smaller and 

 evidently younger worms the gonads are filled with developing 

 spermatozoa. Later, and after the discharge of a portion of the 

 spermatozoa, the eggs begin their development. 



