150 PLATVHELMINTHES. 
or ‘‘sturdy” by pressure on the brain. Another small tape- 
worm of the dogs, Zienia echinococci, has enormous cysts, 
with secondary cysts and many “heads” (Zchinococct), which 
may occur in man, sheep or pigs. 
The members of the third class, or Zurbellaria, are 
not parasites, but are terrestrial, marine, or freshwater. 
In a number of characters they resemble the other classes. 
The body is usually flattened dorso-ventrally. There is 
no anus. ‘There are neither vascular nor respiratory 
organs. The excretory organs are of the ‘flame-cell” 
type, there is a brain with two lateral nerves, and the 
sexual organs are hermaphrodite and complex. On the 
other hand, the Zwurbel/aria usually have simple sense- 
organs and the body is usually ciliated. 
In the Platyhelminthes we see a distinct advance in 
structure when compared with the Calenterata. ‘The axial 
or, at most, bi-plano-symmetry of the latter has given way 
to plano-symmetry. The mesogloea of the Cwlenterata, 
with or without a few scattered cells, has given place to a 
definite mesoderm formed into muscles, gonads and 
parenchyma, and drained by a definite excretory system. 
PHYLUM PLATYHELMINTHES. 
1. Three-layered metazoa with bilateral symmetry (plano-symmetry) and flat- 
tened body. 
2. Alimentary canal, when present, has no anus. 
3. Mesoderm with no definite cavity and fills the space between skin and 
alimentary canal (parenchyma). 
4. Excretory organs consisting of ducts opening to exterior and blind branches 
containing ‘‘ flame-cells.” 
5. Nervous system usually with two lateral cords and an anterior paired brain. 
6. No vascular nor respiratory systems. 
7. Mostly hermaphrodite. 
Class I. Class II. Class III. 
CEsToDa. TREMATODA. TURBELLARIA, 
Type—Tenia. Type—Distomum. Type—Mesostoma. 
1. Elongated flat-worms | 1. Oval flat body with | 1. Oval flat body with 
with thin cuticle. cuticle bearing fine cilia and rhabdites. 
Hooks and suckers hooks and suckers. 
on head. 
z. No mouth nor alimen- | 2. Anterior mouth, race- | 2. Ventral mouth, simple 
tary canal. mose intestine. or branched intestine. 
3. Life-history of two | 3. Life-history often of | 3. Life-history simple. 
phases—the _ cestoid two phases. 
and the cystic. 
4. No sense-organs. 4. No sense-organs. 4. Paired eyes. 
5, Endoparasitic. 5. Endoparasitic or ecto- | 5. Free—aquatic and ter- 
parasitic. restrial. 
