THE ANIMAL KINGDOM 



405 



have auditory vesicles. The body wall contains several layers of 

 circular and longitudinal muscles. They are dioecious as a rule, 

 though a small number are hermaphroditic. 



Fig. 232. — Diagram of a proglottid, showing organs of reproduction (Based 

 on Leuckart.) d, sperm duct; e, excretory duct; n, nerve cord; o, ovary; s, 

 shell gland; t, testis; u, uterus; v, vagina; y, yolk gland. 



One of the striking features of nemerteans is the proboscis, 

 a muscular organ that can be extended from a pocket in front of 

 the mouth to a distance almost as great as the length of the 

 body. It seems to be primarily a tactile organ, though the fact 

 that it is armed with dartlike 

 stylets indicates an offensive 

 or protective function also. 

 Another feature is the contrac- 

 tility of the body. A specimen 

 of Cerebratulus lacteus, capable 

 of extending itself to 5 meters, 

 can shorten to less than a meter. 

 Many are brightly colored. 



Most nemerteans are nonpar- 

 asitic. 



Examples: Cerebratulus lac- 

 teus, a marine form, found on 

 the New England coast in the sand near low-water mark. 

 Length of body is from 2 to 6 m.; width, 25 mm. Prostoma 

 rubrum, breeds in fresh-water aquaria; reddish in color and 

 about 18 mm. in length. Carcinonemertes carcinophila, para- 

 sitic on the gills and eggs of crabs. 



Fig. 233. — A, cysticercus (bladder- 

 worm) with inverted scolex. B, cysti- 

 cercus with everted scolex. (After 

 Leuckart.) 



