. Text book of zoology. Zoology. 144 Flaty helminths. coloured. Sometimes nematocysts^ like those of the Ccelenteraj are found in the skin. Eyes in varying numher^ and sometimes auditory 'organs, are present; there is frequently a pair of short tentacles upon the front end, whilst the rest of the body is usually bare; a little sucker is, however, often present on the ventral surface. The mouth is ventral, sometimes near the anterior end, sometimes in the mid- ^s- 98. Fig. 99. ^ig^ Qj. again near the hind end; it leads into a buccal cavity, which is often fur- nished with a peculiar retractile,
. Text book of zoology. Zoology. 144 Flaty helminths. coloured. Sometimes nematocysts^ like those of the Ccelenteraj are found in the skin. Eyes in varying numher^ and sometimes auditory 'organs, are present; there is frequently a pair of short tentacles upon the front end, whilst the rest of the body is usually bare; a little sucker is, however, often present on the ventral surface. The mouth is ventral, sometimes near the anterior end, sometimes in the mid- ^s- 98. Fig. 99. ^ig^ Qj. again near the hind end; it leads into a buccal cavity, which is often fur- nished with a peculiar retractile, muscular pharynx. This is a longer or shorter canal, open at both ends, and firmly at- tached by one end to the hind part of the buccal cavity, whilst the other end can be everted through the mouth to seize the food. In other cases the so-called pharynx is simply a specially muscular portion of the walls of the buccal cavity, and is not pro- trusible. In a few cases even this is absent. The mouth communicates with. Pig. 98. Sketch of a rhabdoccelous Turbellarian {Meso- stomum splendidum) showing the digestiye tract da, and the cerebral nerve mass c ; ge genital aperture, m month, s pharynx. Enlarged.—After v. Graff. rig. 99. Sketch of a rhabdoooelous Turbellarian (Pro- vortex affinis) with the genitalia, d yolk-sac, g glands in connection with the male apparatus, ge genital aperture, 0 eye, ov orary, p penis, s pharynx (the gut is omitted), t testis, V vas deferens. Enlarged.—After v. Graff. the true gut, which, in some forms (Rhabdocoela), is a simple sac, whilst in others it is branched like the veins of a leaf (Dendroccela). Some forms (Acoela) have no digestive tract, even though the mouth is still present; food is taken in and digested in the soft parenchymatous tissue of the- body. The hermaphrodite genital apparatus opens on the ventral side, either by a common aperture or by separate pores, for the male or female glands. In some members of the group,, reproduction
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
