. A manual of zoology. Zoology. .'5 Li FLATHEL-MIXTHES Like some flatworms lliey have a solid parenchyma bounded exter- nally bv a ciliated ectoderm rich in mucus cells, and inside tliis at least two muscular layers, an outer circular and an inner longitudinal layer. They differ from all other Plalhelminthes in haNing a complete alimentary tract, beginning with a \'entral anterior mouth and continuing as a straight tube, with, usually, paired diverticula, to the vent at the posterior end of the body (fig. ^35). Especially diagnostic is the proboscis, which lies dorsal to tlie alimen- tai;j,- t
. A manual of zoology. Zoology. .'5 Li FLATHEL-MIXTHES Like some flatworms lliey have a solid parenchyma bounded exter- nally bv a ciliated ectoderm rich in mucus cells, and inside tliis at least two muscular layers, an outer circular and an inner longitudinal layer. They differ from all other Plalhelminthes in haNing a complete alimentary tract, beginning with a \'entral anterior mouth and continuing as a straight tube, with, usually, paired diverticula, to the vent at the posterior end of the body (fig. ^35). Especially diagnostic is the proboscis, which lies dorsal to tlie alimen- tai;j,- tract and usually opens in front of the mouth. The proboscis is a ps pm. pn Po Fig. 235.—Diac;rara of Nemertean (orig.). b, brain; r, ciliated pit; d. dorsal nerve trunk; di, dorsal blood-vessel; g. gastric cv"ca; i, intestine; /, lateral nerve trunk; Iv, lateral blood-vessel; p, proboscis retracted; />;», proboscis muscles; pr, protonephridial tube; po, its opening; ps. cavity of proboscis sheath. muscular tube closed at one end and at rest is infolded like the Iniger of a glove inside a closed sac, the proboscis sheath, which extends far back in the body. Its tip is bound to the posterior end of the sheath by a retractor muscle. By contraction of the sheath the proboscis is e^•erted, while it may be retracted again by the muscle. Nettle cells are not un- common in the proboscis wall, while in some forms (the older Enopla) the effectiveness of the organ is increased by the presence of a dart-like stylet at the tip (reserve stylets occur on either side, tig. 2}(^), and at the base of the stylet is a poison sac. The blood-vascular system consists of a pair of lateral tubes connected by transverse loops, and in most forms a tliird tube is present 1 ving between the intestine and the proboscis sheath. The blood is colorless; it rarely contains red or green corpuscles. The central nervous system (in some forms still in the ectoderm) con- sists of a supraa?sophageal brai
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
