i8o 



COLLEGE ZOOLOGY 



4. ACANTHOCEPHALA 



The Acanthocephala (Gr. akantha, a spine; hephale, the 

 head) are parasitic worms which are also considered by many 



a class in the Phylum Nemathelminthes. 

 They are spineheaded worms which fasten 

 themselves to the intestinal wall of verte- 

 brates by means of a protrusible proboscis 

 covered with hooks (Fig. 120, R). The 

 presence of this proboscis, and of a com- 

 plex reproductive system, and the absence 

 of an alimentary canal, distinguish the 

 Acanthocephala from the Nematoda 

 and Nematomorpha. 



The adults are most common in nshes, 

 but all vertebrates, including man, are 

 parasitized by them. There is an alterna- 

 tion of hosts during development. For 

 example, the larva of Echinorhynchus 

 gigas lives in the June bug, the adult in 

 the pig. 



ClLETOGNATHA 



Fig. 120. — Echino- 

 rhyncus augnstatus (of 

 the group Acantho- 

 cephala), male. B, re- 



5- 



The Ciletognatha (Gr. chaite, horse- 

 hair ; gnathos, the cheek) are marine 



traded bursa; Be, ejacu- animals which swim about near the sur . 

 latory duct ; G, gang- 



face of the sea. The best-known genus 



is Sagitta, the arrow- worm. Figure 121 



lion ; Li, ligament ; 

 P, penis ; Pr, prostatic 

 sacs; R, proboscis; 



Rs, sheath of proboscis; shows most of the anatomical features of 

 T testes; v d, vasa defer- Sagitta hexaptera. There is a distinct 



entia. (From Sedgwick, ° r 



after Leuckart.) ccelom, an alimentary canal with mouth (a), 



intestine (b), and anus (c), a well-developed 

 nervous system, two eyes, and other sensory organs. The mouth 

 has a lobe on either side provided with bristles (e) which are 

 used in capturing the minute animals and plants that serve as 



