290 



ZOOLOGY 



SECT. 



External Characters. The body is cylindrical, and ends in front in a 

 protrusible portion, the ^/o&oxn'.s (A, p.; B, .pr.), which is also cylindrical and is 

 covered with many rows of recurved chitinoid hooks. The worm lies with the 

 proboscis sunk in the wall of the intestine of its host, which is sometimes riddled 

 with holes formed in this way. In some species there is a distinct neck (B. ?;. ) 

 between the proboscis and the trunk, and there may be a globular dilatation at 

 the anterior end of the neck. At the hinder end of the body is a single 

 aperture, the gonopore or reproductive aperture (ynp. ) : connected with this, in 

 the male, is a protrusible, bell-like structure, the bursa (b.), which acts as a 



FIG. 235. A, Echinorb.yncb.us gifiras, female, from the IM<,' (nat. size); 15, E. lesini- 



formis, male, from the edible Frog (magnified), b. bnrsa ; c. ;//. cement glands; gnjA 

 gonopore; lm. lemnisci ; />. neck : p. or /. proboscis; ,-./. retractor muscle <>f proboscis ; 

 s. Ig. suspensory ligament ; t. testis ; <. vessel. 



copulatory organ, like the somewhat similar organ in certain Nematocla. There 

 is no trace of mouth, anus, or excretory pore. 



The body-wall is covered with a stout cuticle, beneath which is a striated 

 protoplasmic layer, probably representing the ectoderm. Then comes a layer of 

 transverse, and then one of longitudinal muscles. The body-wall thus constituted 

 encloses a spacious body-cavity containing a clear fluid. 



In correspondence with the absence of mouth and anivs there is no trace of 

 enteric canal, the Acanthooephala resembling, in this respect, the Cestoda, the only 

 other class of Aletazoa which is entirely anenterous. Food is thus, as in tape- 

 worms, taken entirely by absorption by the general surface of the body. 



The proximal end of the proboscis is contained in a muscular sheath sunk 

 in the anterior end of the trunk, and is provided with four retractor muscles. 



