222 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 ccelome (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 

 enclosing not only the oosperm or impregnated ovum, but a 

 quantity of nutrient material or food -yolk, derived in most in- 

 stances from a special set of glands, the yolk or vitdline glands. 



The main features which distinguish the Platyhelminthes from 

 the Coelenterata are, the pronounced bilateral symmetry with 

 the many secondary features which it involves,* the presence 

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

 of fixed colonies formed by budding. 



1. EXAMPLES OF THE PHYLUM. 

 i. A Fresh-water Triclad (Planaria or Dcndroccelum). 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 uj>^er 

 or dorsal and a lower or central surface, riylit 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 brought 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 body 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 

 opening 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 siifficieutly general to apply to species of either of these 



