CHAPTER XIII 



THE FOOD OF ANIMALS— CARNIVOROUS UNSEGMENTED 

 WORMS AND ECHINODERMS 



One of the phyla which include typical unsegmented worms 

 next claims our attention, i.e. Flat-Worms (Platyhelmia). Of the 

 three classes which it includes, two — the Tape- Worms (Cestoda) 

 and Flukes (Trematoda) — embrace forms which are greatly 

 modified as the result of a parasitic habit. Typical instances 

 have already been briefly described (vol. i, chap, x), and further 

 details will be given in the section on Parasitism. The third 

 class, that of the Planarian Worms (Turbellaria), comprises an 

 immense number of predaceous forms, differing greatly in size, 

 shape, and colour, and found in the sea, fresh water, and among 

 damp land vegetation. The mouth is remarkably variable in 

 position, but it is always on the under surface, though in different 

 species it may be far forwards, in the middle, or a long way 

 back. A thick-walled pharynx can be protruded and used to 

 perforate the small animals serving as prey, from which the 

 juices and succulent parts are then sucked. Innumerable micro- 

 scopic rods are imbedded in the skin, from which they can be 

 shot out, serving, it would appear, both as offensive and de- 

 fensive structures, though their mode of action is not clear. 

 Locomotion is effected by creeping, and aquatic forms can also 

 swim by undulations of the flexible body. Organs of touch and 

 eyes are present, especially at the front end of the body, and 

 these are efficient enough to enable their possessors to detect 

 their prey by night as well as in the day. Three groups are 

 distinguished, which differ among other things in the shape of 

 the digestive tube, i.e. marine species, Polyclades, in which the 

 gut is much branched; marine, freshwater, and terrestrial forms, 

 Triclades, with the digestive tube divided into three main branches ; 

 marine, freshwater, and terrestrial forms, Rhabdocceles, in which 

 the gut when well-developed is a simple tube. 



