236 ZOOLOGY SECT. 



a peculiar kind the water-vascular or protonephridial system is 

 present in nearly all members of the phylum. A body-cavity (see 

 following Sections) 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 in 

 most instances composite, the egg-shell enclosing not only the 

 oosperm or impregnated ovum, but a quantity of nutrient material 

 or food-yolk, derived, in most instances, from a special set of glands, 

 the yolk or vUelline glands. 



The main features wJiich distinguish the Platyhelminthes from 

 the Ccelenterata are-A^he^ronounced bilateral symmetry with 

 the many secondary feaTures^wliich Tt~Snvolves, the presence 

 of a middle embryonic layer or mesoderm, and the non- occurrence 

 of fixed colonies formed by budding. 



1. EXAMPLES OF THE PHYLUM. 

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



General Features. Species of fresh-water Planarians of the 

 genera Planaria and Dendroccelum 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. 182-184) 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 ; but 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. 183). On the ventral surface, 

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

 opening of the mouth (Fig. 182, mo.), and further back still, near 

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

 aperture (Fig. 184). 



Digestive System. The mouth (Fig. 183, mo.) leads through 



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

 genera. 



