4 2-> ZOOLOGY SECT.. 



all higher groups of animals. An excretory vascular system of" 

 a peculiar kind the water-vascular system is present in all 

 members of the phylum. A body-cavity or coelome (p. 279) is not 

 present, the spaces between the various organs and the wall of" 

 the body being filled up with a peculiar form of connective-tissue- 

 termed the parenchyma. The egg is composite, the egg-shell 

 losing not only the oosperm or impregnated ovum, but a, 

 quantity <_>f ;^Ht'nt material or foot -yolk, derived in most in- 



es from a special set of glands, th^j yolk or vitelhne glands. 

 Tip- main features which distinguish vthe Platyhelminthes from 

 the Ccelenterata are, the pronounced bilateral symmetry with 

 the many secondary features which it involves, the presence 

 of- a middle embryonic layer or mesodernNi, and the n on -occurrence: 

 of fixed colonies formed by budding. 



V 



1. EXAMPLES OF THE PHYLUM. 

 i. A Fresh- water Triclad (Planaria or Dendroccelum). 1 



General Features. Species of fresh-water Planarians of the* 

 genera Planaria and Dcndroccelum are common in the mud at 

 the bottom of ponds of fresh-water in all quarters of the globe.. 

 They are small, thin, flattened worms a few millimetres in length... 

 broader at one end, the anterior, than at the other, the posterior,. 

 which is more or less pointed. The animal (Figs. 171-173) is 

 very readily recognised to be bilaterally symmetrical, with an upper 

 or dorsal and a lower or ventral surface, right and left borders,. 

 and anterior and posterior ends. The colour varies in different, 

 species and in different individuals; bat is usually gray, red,, 

 brown, or black. Movements of locomotion in the direction of the- 

 long axis of the body are recognisable in the living animal. Some- 

 times this is a steady gliding movement, which is biought about 

 by the action of a coating of vibratile cilia on the surface; some- 

 times the worm moves along somewhat after the fashion of a 

 Leech, the ventral surface of the anterior end of the ftody being 

 of a sticky adhesive character, and performing the part of the 

 anterior sucker of the Leech. 



Close to the anterior extremity on the dorsal surface ^are two 

 rounded black spots, the eyes (Fig. 172). On the ventral surface,, 

 a considerable distance behind the middle of the body, is the 

 o] icning of the mouth (Fig. 171, mo.), and further back still, near 

 the posterior pointed end, is a smaller median opening, the genital 

 aperture (Fig. 173). 



Digestive System. The mouth (Fig. 171, -mo.) leads through 

 a short mouth-cavity into a cylindrical thick-walled chamber, the 



1 The account is sufficiently general to apply to species of either of these; 

 genera. 



