LOWER ANIMALS 



209 



ACANTHOCEPHALA (Spiny-headed Worms) 



Diagnosis: symmetry bilateral; body unseg- 

 mented; body cavity a pseudocoel; all are parasites, 

 larvae in arthropods and adults in the intestines of 

 vertebrates; an anterior, cylindrical snout, or 

 proboscis, can be withdrawn into a sheath; proboscis 

 with hooks or spines; body usually elongate, some- 

 what flattened, and rough in life (rounded and 

 smoother in preserved specimens); 0.06 to 26 inches 

 long (Figure 12.13). 



Rgure 12.13 A spiny-headed worm, Macracanthorhynchus, thot 

 parasitizes pigs. 



The spiny-headed worms do not actually have 

 heads. Their name is derived from the proboscis 

 armed with hooks or spines. The proboscis can be 

 either withdrawn into the body or extended to the 

 outside, but it is not turned inside out upon being 

 withdrawn as is the case with ribbonworms. 



Spiny-headed worms most closely resemble the 

 roundworms; however, acanthocephalans differ in the 

 presence of a proboscis and circular muscles, in the 

 modifications of excretory and reproductive organs, 

 and in the absence of a digestive tract. 



The life cycle shows some complexities but the 

 basic pattern is rather simple. The mature adults 

 are parasites in the digestive tracts of various land, 

 marine, and fresh-water vertebrates. Fertilization of 

 eggs is internal and the eggs develop into a larva 

 within the female. The larva must then leave its 

 parent and the vertebrate host to find a suitable 

 arthropod host, usually an insect, isopod, or amphi- 

 pod. For the life cycle to be completed the inter- 

 mediate arthropod host must be eaten by the host of 

 the adult worm. Modifications of this basic plan can 

 involve hermaphroditic adults, extrusion of eggs 

 rather than larvae from the vertebrate host; and more 

 than one intermediate host. 



ASCHELMINTHES (Cavity Worms) 



Diagnosis: symmetry bilateral; body unseg- 

 mented, worm-like; body cavity a pseudocoel, diges- 

 tive tract lacking a muscular wall but terminating in 

 an anus well behind the mouth; includes several 

 groups of mostly slender, small to microscopic species 

 that usually feed upon microscopic life; readily 

 visible species are earthworm-like, but smooth and 

 without an external ringed or segmented appear- 

 ance (Figure 12.14). 



The cavity worms are, at best, a loosely related 

 taxon of pseudocoelomate, bilateral animals. Many 

 investigators consider this to be a polyphyletic group, 

 or not a true phylum because not all forms can be 

 traced back to a common ancestor that gave rise 

 only to Aschelminthes. However, all are pseudo- 

 coelomate, mostly worm-like, bilateral animals with 

 an unsegmented body clothed with a cuticle. The gut 

 is straight or curved, lacks a definite muscular wall 

 except in the Priapuloidea, but ends in an anus 

 posterior to the mouth and usually at or near the 

 posterior end. 



Only the roundworms and horsehair worms are 

 likely to be seen with the unaided eye. 



CLASS ROTATORIA (Rotifers or Wheel Animalcules) 



Diagnosis: minute to microscopic; body somewhat 

 cylindrical with an anterior group of cilia ("wheel 

 organ") and posterior forked "foot"; most are free- 

 living, with some fixed in protective tubes, a few 

 parasites; mostly marine, a few freshwater species. 



Although the largest rotifers barely reach 1/10 inch 

 long, most are predators and usually upon protists. 

 In predation they generally trap their prey with the 

 wheel organ; movement of the cilia upon this organ is 

 the means by which most rotifers draw prey into their 

 mouths. 



Rotifera are remarkable in that many species ap- 

 parently lack males. Most reproduce parthenogeneti- 

 cally, but in some a situation reminiscent of sex de- 

 termination in bees is found — fertilized eggs develop 

 into females and unfertilized eggs into males. In 

 species where both sexes are known, the males and 

 females usually are distinctly different in appearance. 



CLASS NEMATOMORPHA (Horsehair Worms) 



Diagnosis: about 0.1 to 40 inches long; long and 

 slender with a uniformly cylindrical body, a bluntly 



