MANUAL OF NATURAL HISTORY. 361 



Mouth with a single, terminal, retractile pro- 

 boscis, armed with recurved hooks ; body 

 elongate, cylindrical, sac-like. 



II. ORDER — Cestoid-Worms (Cestoidea). 



Body often flattened, sometimes composed of a 

 number of segments ; mouth variously armed with 

 hooks and suckers. 



1. Family. — True Tape-Worms (Tseniidae). Head 



with four, lateral, suctorial pits ; neck elon- 

 gated ; body composed of numerous segments 

 united together in a linear series. 



2. Family. — Armed Tape-Worms (Bothryocepha- 



lidse). Head with a single suctorial pit on each 

 side ; pits simple, or furnished with suckers 

 extended into filaments ; body divided into 

 numerous segments arranged in a linear 

 series. 



3. Family. — Spiny-headed Worms (Acanthocepha- 



lidae). Head with a large suctorial pit on 

 each side, with two or four retractile trunks 

 armed at the end with hooks or simply papil- 

 lary ; body short, sac-like, not divided into 

 distinct segments. 



4. Family. — Strap-Worms (Ligulidse). Mouth a 



little below the larger anterior extremity, 

 armed with moveable hooks on each side. 



5. Family. — Tricuspid- Worms (Tricuspidariidse). 



Mouth sub - terminal, bilabiate, armed on 

 each side with two tricuspid hooks ; body 

 elongate, sub- articulate posteriorly. 



