ir. XEMERTINI. 



2S9 



2 inclics); in ruminants, 2Ioniezia expansa (usually 7 feet, sometimes 30 

 feet or more), often fiital, 31. denticulata (1 to 5 feet), the most common 

 tapeworm of cows; in dogs, Taiiia inarginata (cystioercus in slieep and 

 swine), T. serrata (cystioercus in rabbits), T. ediiiiococcus (aljove), T. cii- 

 nurus (cystioercus in brain of sheep, causing the disease called ' stag- 

 gers'), Dipylidmm cucumerina {most common, larva in the dog-louse, 

 Trichoclectes)\ in the cat, T(enia crassicollis (cystioercus 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 {Liiieus lotiffissiiinis 90 feet I), and yet they are 

 so contractile that a specimen of our Cerehratuhis lacteus, which 

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

 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. Tlieir system- 

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

 Plathelminthes, 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 liave a solid j^arenchyma liounded 

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

 tliis at least two nruscular layers, which, when biit two are ])res- 

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

 differ from all other Plathelminthes in having a complete 



Fig. 2.54.— Diitgram of Nemertean (orig.). ^^ brain; <-, ciliated pit; d, dorsal nerve 

 truiilt : (//, dorsal blood-vessel; f/c, gastrir caec-a ; /, intestine; /, lateral nerve 

 trunk; /r, lateral blood-vessel; ;>, proboscis retracted; pin, proboscis musclea; 

 joi, protonephridial tube ; jm, its opening ; jj\ cavity of proboscis sheatli. 



alimentary tract, beginning with a ventral anterior mouth and 

 continuing as a straiglit 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 ojiens separate from the mouth. The 



