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, 

 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 by transverse fission occurs; some marine 

 Turbellaria undergo a metamorphosis. The larva is free- 

 swimming, and is furnished with processes which do not appear in. 

 the adult. 



