542 



FRESH-WATER BIOLOGY 



30(27) Areoles not ebi'ited, usually separated; head usually flattened; 



interareolar groups of fine hairs. 

 5<fcQa Gordius platycephalus 



^Sl Montgomery 1898. 



For male and range of this species consult 21 

 in this key. Female up to 335 mm. long, 1.4 

 mm. broad. Posterior end slightly enlarged. 

 Color brown; tip of head Ughter; dark ring 

 around neck. 



'MS^o) 



Fig. 844. Gordius platycephalus Q ; head in dorsal 

 aspect, tail in ventral aspect, and surface view of 

 cuticula. Magnified. (After Montgomery.) 



ACANTHOCEPHALA 



The Acanthocephala or proboscis roundworms constitute a most 

 remarkable group both in the extreme adaptation to the parasitic 

 habit which they manifest and in the unique structure which pre- 

 sents little or no parallel to any other type of animal. Most of 

 them are small, measuring only a few milUmeters in length although 

 the common parasite of the pig, Gigantorhynchus himdinaceus 

 (commonly called Echinorhynchus gigas), reaches a length of 15 

 cm. in the male and 30 to 50 cm. in the female. 



In form they are elongate, roughly cyKndrical, or spindle-shaped 

 but with several distinct regions that give the body an irregular 

 form. These regions are a retractile proboscis armed with hooks, 

 a neck, and a body proper. When examined living the body is 

 often flattened or slightly bent, and displays a surface irregularly 

 roughened or marked by transverse ridges of var3dng size. When 

 removed to normal salt solution or preserved in other fluids, they 

 tend to assume a smooth rounded form, sometimes with slight 

 regular annulations that suggest segmentation but in fact do not 

 extend beyond the dermal layer. 



At the anterior end the proboscis, which is retractile and in pre- 

 served specimens often partly or wholly withdrawn into the body, 

 presents a variable form being in various species cyHndrical, glo- 

 bose, filiform, spindle-shaped, and even more complex; it may be 

 long or short, straight, oblique, or at right angles to the long axis. 

 The particular form is characteristic of the genus or species and useful 

 in diagnosis. The proboscis bears always a considerable number of 



