PHYLUM NEMATHELMINTHES 



313 



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

 encloses ;i spacious body-cavity or ccelome containing a clear fluid. 



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

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

 otlier class of Metazoa 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 prohoacin is contained in a nuiscular sheath sunk 

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

 (Fig. 250, r.m.). The muscles of the sheath are circular and act as protractors. 

 At the sides of the base of the proboscis two club-shaped organs, the Iem7iisci 



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Fig. 250.— a, Echinorbyiiclius (Gigantorhynclius) gigas, female from the Pig (nat. 

 size ; B, E. lesiniformiS; male from the edible Frog (magnified), h. bursa ; c.gl. cement 

 glands ; gnp. gonopore ; Im. leranisei ; n. neck ; 2>. or -pr. proboscis ; r. m. retractor muscle 

 of proboscis ; s. Ig. suspensory ligament ; t. testis ; v. vessel. 



(/»(.), hang down into the body-cavity. Their function is quite unknown, but 

 they have been compared with the cervical glands of Nematodes (p. 306). 



In the body- wall run two longitudinal vessels (v.) containing a granular 

 fluid, and connected with a network of fine canals in the proboscis, bursa, &c. 

 The function of these vessels is not known with certainty : they may have to do 

 with the absorption and circulation of nourishment. 



The central nervous system (Fig, 251, nv.) is represented by a single large 

 ganglion placed at the base of the proboscis, and sending ofl^ nerves in various 

 directions. In the male there are also two ganglia supplying the reproductive 

 organs. Organs of sense are wholly absent. 



A pair of remarkable excretory organs or nephridia (Fig. 253) have been 

 found to occur in Echinorhynchus gigas. These consist of a pair of ramified 



