230 



UNSEGMENTED WORMS 



Tetrarhynchidse. With four protrusible proboscides armed with 

 hooks, parasites of fishes. Found also in Sepia. 



e.g. Tetrarhynchtis. The finest pearls in the Ceylon pearl oyster 

 are formed round a larval Tetrarhynchus. 

 Tetraphylhdae. With four very mobile suckers. 



e.g. Echeneibothrium, Phyllo- 

 bothrium. 

 Taeniidae. With four suckers, often 

 with apical hooks, with margi- 

 -j^ nal genital apertures. 



e.g. Tcsnia. 



General Note on 

 Platyhelminthes 



The four classes, Turbellaria, 

 Trematoda, Cestoda, and Temno- 

 cephaloidea, constitute the Platy- 

 helminthes or Flat -worms — an in- 

 teresting group, because its members 

 illustrate so well the progressive 

 degeneration associated with increas- 

 ing parasitism, and also because of 

 the relatively great simplicity. The 

 four classes are nearly related, for 

 forms like Temnocephala connect 

 Turbellaria and Trematoda, and the 

 " monozoic " Cestodes like Archi- 

 getes, Amphilina, CaryophyllcBus, 

 and Gyrocotyle connect Trematoda 

 and Cestoda. It is probable that 

 both Cestodes and Trematodes arose 

 from a Turbellarian stock. 



Among the most striking of the 

 Platyhelminth characters are the 

 nature of the excretory and repro- 

 ductive organs and the condition of 

 the mesoderm. The excretory system, 

 with its longitudinal trunks, its 

 ramifying canals, and " fiame-cells," 

 is characteristic. The reproductive 



Fig. 125. — Diagrams of 

 bladder-worms. 



I. The ordinary Cysticercus type, 



with one head (H.). 

 II. The Coenurus type, with many 

 heads. 

 III. The Echinococcus type, with , , j- ■ • 



many heads, and with brood organs are complex, show division 

 capsules producing many of labour, and are furnished with 

 ^^^^s- ducts of their own, unconnected with 



the excretory system — a condition 

 not common in worms. The presence of shells around the eggs is 

 another point of interest. It becomes of great importance to the 

 parasitic flukes and tape-worms, but occurs also in the free-living 

 Turbellaria. The formation of yolk cells from a specialised part of the 

 ovary (yolk gland) is also noteworthy. There is no true body cavity, 



