IV. NEMERTINI. 



289 



2 inches); in ruminants, Moniezia, expansa (usually 7 feet, sometimes 30 

 feet or more), often fatal, M. denticulata (1 to 5 feet), the most common 

 tapeworm of cows; in dogs, Tcenia marginata (cysticercus in sheep and 

 swine), T. serrata (cysticercus in rabbits), T. echinococcus (above), T. COB- 

 nurus (cysticercus in brain of sheep, causing the disease called * stag- 

 gers '), Dipylidium cucumerina (most common, larva in the dog-louse, 

 Trichodectes); in the cat, Tcenia crassicollis (cysticercus in mice). Several 

 species occur in domestic birds, one (Drepanidotcenia infundibuliformis), 

 causing epidemics among chickens. 



Class IV. Nemertini. 



Most nemerteans are of appreciable size, some reaching a length 

 of a yard or more (Linens longissimus 90 feet !), and yet they are 

 so contractile that a specimen of our Cerebratulus lacteus, which 

 can extend itself to fifteen feet, can retract to two. Nemerteans 

 are rare in fresh water or moist earth, but are most abundant in the 

 sea, where they burrow through the mud or lie rolled up beneath 

 stones. Many are noticeable for their bright colors. Their system- 

 atic position is a problem. Frequently they are included in the 

 Plat helm in thes, but the presence of an anus, of distinct vascular 

 system, and the higher organization in other respects renders such 

 a position doubtful. 



Like some flatworms they have a solid parenchyma bounded 

 externally by a ciliated ectoderm rich in mucus cells, and inside 

 this at least two muscular layers, which, when but two are pres- 

 ent, are an outer circular and an inner longitudinal layer. They 

 differ from all other Plathelminthes in having a complete 



ps pm 



FIG. 254. Diagram of Nemertean (orig.). 5, brain; c, ciliated pit; d, dorsal nerve 

 trunk ; d/, dorsal blood-vessel ; gc, gastric caeca ; ?, intestine ; Z, lateral nerve 

 trunk; h\ lateral blood-vessel; p, proboscis retracted ; pm, proboscis muscles; 

 pn, protonephridial tube ; po, its opening ; ps, cavity of proboscis sheath. 



alimentary tract, beginning with a ventral anterior mouth and 

 continuing as a straight tube, with, usually, paired diverticula, to 

 the vent at the posterior end of the body (fig. 254). 



Especially diagnostic is the proboscis, which lies dorsal to the 

 alimentary tract and usually opens separate from the mouth. The 



