Phylum 3. Platyhelminths {Flatwormi). 



Fig. 95. 



The Platyhelminths are bilaterally symmetrical, unsegmented, 

 and usually almost fiat. The body is soft, and appendages are 

 wanting, though ventrally, muscular suckers are frequently present. 

 There is no body-cavity, all the organs are imbedded in a mass of 



soft connective tissue ; 

 Fig- 96, the anus, and also the 



vascular system are 

 usually absent (the 

 Nemertines excep- 

 ted) . The diges- 

 tive tract is either 

 a simple sac, or it is 

 branched ; it is want- 

 ing in many parasitic, 

 and even in some free- 

 living, forms. The 

 central nervous 

 system is repre- 

 sented by a paired 

 nerve-ganglion, which 

 lies anteriorly, and 

 from which the nerve.s 

 to the different parts 

 of the body issue. 

 From the hinder end 

 of the brain, several 

 longitudinal stems 

 arise, and are fre- 

 quently united by 

 delicate transverge commissures. Sometimes eyes are present, more 

 rarely auditory organs; they are situated anteriorly, are simple 

 in structure, and small in size. The excretory apparatus is 

 in the form of a much branched system of thin-walled tubes, which 



Fig. 95. Nervous system 

 of Distomum (viewed from 

 the dorsal side, the ventral 

 suckers showing through), r 

 dorsal nerve, sn lateral nerve, 

 6 ventral nerve, d digestive 

 tube, s anterior, s' posterior, 

 sucker. — After Gaffron. 



Fig. 96. Part of the 

 excretory system of 

 a Flatworm ; diagrammatic. 

 — Orig. 



